Pitfalls and opportunities in estimating the volumes of small, young submarine landslides

Pitfalls and opportunities in estimating the volumes of small, young submarine landslides | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Pitfalls and opportunities in estimating the volumes of small, young submarine landslides Morelia Urlaub, Thore Falk Sager, Christian Berndt This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7380838/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 03 Nov, 2025 Read the published version in Geo-Marine Letters → Version 1 posted 8 You are reading this latest preprint version Abstract Submarine landslides have the potential to be major geohazards as they can destroy seafloor infrastructure such as communication cables and cause tsunamis. The volume of material displaced during the landslide is one factor that determines its hazard and is typically estimated using bathymetric and/or seismic data. Here, we use various established methods to determine the initial failed volume based on a well-constrained case study, the Ana Slide, a small slope failure in the Eivissa Channel off the eastern Iberian Peninsula. We find that, not only, the availability and quality of marine-geophysical data, but also the emplacement mechanism affects how precisely the volume can be estimated. In general, the volume estimation based on comparison of recent and reconstructed pre-failure seafloor topographies yields conservative, yet robust estimates for the volume mobilized. In contrast, volumes estimated from seismic data may be overestimated if the nature of the chaotic, transparent, or disrupted seismic facies commonly used to identify landslide material is unknown. volume assessment pre-failure seafloor reconstruction landslide volume emplacement mechanism Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Submarine landslides may have the potential to be a significant geohazard to coastal populations worldwide (Bondevik et al., 2005 ; Haugen et al., 2005 ; Løvholt et al., 2017 ; Prior et al., 1984 ; Synolakis et al., 2002 ; Talling et al., 2014 ; Watt et al., 2012 ). They can destroy offshore infrastructure such as platforms and telecommunication cables (e.g., Løvholt et al., 2019 ; Vanneste et al., 2014 ) and release large quantities of methane and other greenhouse gases from the seafloor (e.g., Talling et al., 2014 ). Submarine landslides occur on submerged slopes in all geological settings and cover a vast range of sizes, from surficial sediment removal (Kioka et al., 2019 ) to several orders of magnitude larger than terrestrial landslides (Korup et al. 2007). The terms submarine landslide and slope failure describe the active processes of landsliding while their remnants left on the seafloor are termed mass-transport deposits (MTDs). The inclination of the seafloor, water depth, duration of the slide event, its acceleration, related landslide mechanisms, run-out velocity, the timing between multiple stages of failure, the volume of mobilized material, and its density and cohesion are all factors that control the impact of a landslide (e.g., Harbitz et al., 2014 ; Murty, 2003 ). Constraining these factors requires multibeam echosounder bathymetric and seismic data, as well as seafloor samples and age datings. Marine-geophysical data are becoming more available, yet the samples and ages are rarely available (Tappin, 2021 ). Critical factors in tsunami generation and essential to numerical tsunami modelling, such as the initial acceleration and slide velocity, cannot be quantified at all from MTDs and thus rely heavily on assumptions. However, the volume of a submarine landslide can be estimated relatively easily from bathymetry and/or a few seismic lines. Therefore, the hazard of submarine landslides is oftentimes assessed from their volume alone (e.g. Grilli et al., 2009; Urgeles & Camerlenghi, 2013). The volume of a landslide is not constant as it may change during its evolution. The volume of the resulting deposit may thus exceed the initial failed volume due to processes such as basal erosion and sediment entrainment (e.g., Sobiesiak et al., 2018 ; Watt et al., 2012 ). On the other hand, the deposit may be distributed over large areas by highly mobile sediment flows (Talling et al., 2007 ), which are beyond the resolution of the imaging systems and the extent of mapping. Here, we focus particularly on estimating the initial failed volume of a landslide as it is one of the key input parameters for tsunami models (Tappin, 2021 ) and it is quantifiable based on marine-geophysical data. There are various ways to estimate the volume of submarine landslides from bathymetric or reflection seismic data. McAdoo et al. ( 2000 ) measured the area of the source area (A) and height of the headscarp (H) from bathymetric data to estimate the evacuated volume through volume = 1/2 * A * H. On the contrary, Völker ( 2010 ) estimated the evacuated volume by subtracting a pre-failure seafloor that was reconstructed by fitting slope functions into the landslide scar from the present-day seafloor. Here, the interpreted evacuated region in the source area provides an estimate of the initial failed volume and the deposit in the sink area provides an estimate of the deposited and accumulated volume. Wilson et al. ( 2004 ) calculated the volume of a debris lobe from its depth or thickness (D), width (W), and length (L) through the relationship of volume = 1/6 * π * D * W * L. For landslides imaged from sub-seafloor echo-sounder profiles or 2D and 3D reflection seismic data, the average thickness of chaotic, transparent, and/or disrupted seismic facies representing the mobilized material can be measured and multiplied by the landslide area. This provides the ‘bulk volume’ of material involved in and affected by the landslide (referred to as Vd by Nugraha et al. ( 2022 )) that accumulated inside the sink area. This method was used by Lastras et al. ( 2004 ) to estimate the landslide volume or total affected volume of Ana Slide, located in the Eivissa Channel, western Mediterranean Sea. These authors used the average thickness of Ana Slide at 23 m inside the landslide scar (with an area of 6 km 2 ) to propose a volume or Vd (sensu Nugraha et al., 2022 ) of 0.14 km 3 . This study aims to provide an exact initial volume of the Ana Slide in the Eivissa Channel, Mediterranean Sea, making use of recently published detailed knowledge on its emplacement processes and a 3D seismic dataset covering the entire landslide area. We compare the resulting ‘exact’ volume to volumes estimated by applying the approaches outlined above, which are typically used in the absence of extensive coverage of seismic data and/or geological sampling. The comparison allows us to identify potential pitfalls and provide recommendations on suitable approaches to determine the initial landslide volume. This work is timely because submarine landslides have now been mapped around many ocean margins and their failure and emplacement mechanisms identified, allowing us to determine their volumes. 2. Emplacement of Ana Slide The Ana Slide is a relatively small landslide located on the eastern slopes of the Eivissa Channel, western Mediterranean Sea between 635 and 790 m water depth (Berndt et al., 2012 ; Lastras et al., 2004 , 2006 ; Sager et al., 2022 ) (Fig. 1 ). The landslide affected an area of 4.5 km² with a headscarp height of 30 m (Lastras et al., 2004 and 2006 ; Berndt et al., 2012 ; Sager et al., 2022 ) (Fig. 1 b). Previous volume estimates based on two 2D seismic lines suggest 0.14 km³ (Lastras et al., 2004 ). The average slope angle is 1–2° (Sager et al., 2022 ). Here, we use the interpretation of development and emplacement processes of Ana Slide by Sager et al. ( 2022 ). The authors use high-resolution bathymetry, 3D seismic data and re-processed 2D reflection seismic profiles over this landslide. Based on 3D seismic imaging, Sager et al. ( 2022 ) show that Ana Slide developed in two stages referred to as the primary (300 ka) and secondary failures (61.5 ka after Cattaneo et al., 2011 ). Landslide material mobilized during the primary failure attained a thickness of approximately 15 m in the sink area. The secondary failure involved slope material between two shallow reflectors R1 and the present day seafloor (SFR). The deposit of this secondary failure was much smaller and is not resolved even on high-resolution seismic data (~ 5 m vertical resolution, Berndt et al., 2012 ). For this reason, the volumes of the primary and secondary failures of Ana Slide are combined for the purposes of this study (Fig. 2 ) and the top reflector of Ana Slide, referred to as reflector SFR, corresponds to the upper bound of the secondary failure. The landslide involved slope material between the basal shear surface (represented by the reference reflector Ref) and the reflector R1 above (Fig. 2 ). A small fraction of the mobilized landslide material remained inside the source area (undifferentiated landslide material, Fig. 2 ). This is observed for submarine landslides elsewhere, e.g. the Storegga Slide (Micallef et al., 2009 ) and the Sahara Slide (Krastel et al., 2018). The much larger fraction of the landslide material at the base of the evacuation scar emerged frontally, travelled across a 500 m long by-pass zone, and over-rode the pre-failure seafloor at the time of the primary failure (represented by the ‘pre-failure R1 reflector’) inside the sink area (actual deposit, Fig. 2 ). The accumulation of this deposit induced in situ deformation of the underlying sediments reaching a depth of up to 30 m below the pre-failure R1 reflector that marked the seafloor at the time of the primary failure. Deformed material underlying a submarine landslide is not unique to Ana Slide. Recent studies based on onshore outcrops, high resolution and 3D seismic data have shown that interaction with the substrate is a common feature for submarine landslides (Sobiesiak et al., 2019 and references therein; Kühn et al., 2021 , Lenz et al., 2022; Nugraha et al., 2022 ; Ogata et al., 2019 ). Ana Slide’s sink area is affected by three active faults vertically offsetting reflectors (Berndt et al., 2012 , Sager et al., 2022 ). These faults characterize an en-echelon fault system that strikes SSW and NNE and extends into the 3D reflection seismic data from the south with an unknown extent. This fault system dips in the opposite direction compared to the seafloor and therefore is termed ‘seafloor antithetic’ in this study. Primarily the northern but also the central fault affected the seafloor morphology before the primary failure of Ana Slide by vertical fault movement, thereby generating a local depression and accumulation space for the landslide deposit (Sager et al., 2022 ). The undisturbed area north and south of Ana Slide shows homogenous, well-stratified, seafloor sub-parallel reflectors without any evidence for bottom current activity(e.g., Lastras et al., 2004 ). The interval between reflector Ref and SFR becomes steadily larger in downslope direction (Sager et al., 2022 their Fig. 4 c and 5 a). 3. Data and methods This study uses bathymetric and 3D reflection seismic data acquired in 2006 with the P-Cable system of the National Oceanographic Centre (NOC) in Southampton, UK equipped with two sleeve guns and 11 streamers during cruise 178 onboard RSS Charles Darwin (CD178). Data were processed including time migration with water velocity (1500 m s − 1 ). For further information about acquisition and processing workflows, the reader is referred to Berndt et al. ( 2012 ) and Sager et al. ( 2022 ). The bathymetric data have a horizontal resolution of 5 m, and the vertical resolution is approximately 0.5% of the water depth while the 3D reflection seismic data have a vertical resolution of 5–6 m and a horizontal resolution of 10–15 m (Berndt et al., 2012 ). The 2D reflection seismic profile presented in Fig. 2 a shows a re-processed profile presented previously by Sager et al. ( 2022 ). Volume calculations of evacuated and accumulated landslide material of Ana Slide are performed in Kingdom Suite using the Volumetric tool that uses one bounding polygon and two depth-converted grids (calculated from horizons in seconds two-way travel time or seconds TWTT). For depth conversion of seismic horizons, a seismic velocity of 1500 m s − 1 is used. This velocity is consistent with seismic velocity measurements from the shallow 8 m long Kullenberg gravity core PSM-KS18 (0° 50.453’ E 38° 38.184’ N) presented by Lafuerza et al. (2012) obtained during the PRISM cruise with the R/V L’Atalante in 2007 led by IFREMER, France. The 3D reflection seismic data are presented in the time domain (seconds TWTT) and volumes are calculated in the upper 50 m beneath the seafloor. Sediments are water-rich (Lafuerza et al., 2012; Lastras et al., 2004 ; Panieri et al., 2012 ) and seismic P-wave velocities of such sediments typically vary between 1500 to 1640 m s − 1 (Hamilton, 1979 ). Thus, a seismic velocity of 1500 m s − 1 is appropriate to use for depth conversion of seismic reflectors Ref and SFR and the reconstructed pre-failure seafloors for the source and sink areas. In this study, three horizontal bounding polygons are defined: the source area that covers an area of 1.9 km 2 , the by-pass zone with an area of 0.45 km 2 , and the sink area covering an area of 2.45 km 2 (Figs. 1 b). In total, Ana Slide covers an area of 4.8 km 2 referred to as the landslide scar. We define several landslide volumes (Figure. 2), the values of which are calculated independently and with different approaches: Apparent evacuated volume (Vm void ) the void space in the source area. Vm void is calculated following the approach of Völker ( 2010 ) where the volume of evacuated landslide material is calculated by comparing the present-day with the reconstructed pre-failure seafloors (e.g, Omira et al., 2022 ; Sun et al., 2018 ; Webster et al., 2016 ). The pre-failure seafloor of the source area of Ana Slide is reconstructed by manually interpolating the local and regional contour lines from outside the landslide scar into the inside using the contour-line approach. To test the sensitivity of the applied pre-failure seafloor reconstructions, we reconstructed the seafloor inside the source area assuming a simple yet unrealistic pre-failure seafloor morphology (referred to as the straight-line approach hereafter). This was done by reconstructing straight pre-failure contour lines between the intersection of the landslide scar with local 10 m contour lines. The resulting calculated volume of evacuated landslide material from the source area serves as the maximum value for Vm void . Remaining volume (Vm r ) the volume of ‘undifferentiated landslide material’ (sensu Sager et al., 2022 ) that was mobilized but remained within the seabed scar. It is calculated from the 3D reflection seismic data and is the difference between the present-day seafloor (SFR) and the basal shear surface (Ref) in the source area. Turbidite volume (Vt) the volume of material which potentially was transported into the deeper basin and out of the study area by turbidity currents. This material accumulated over a potentially vast area approaching zero thickness. With the available geophysical datasets limited to the proximal area of Ana Slide, it is impossible to determine whether a turbidity current was caused nor to estimate the volume of the turbidite deposit because of the lack of appropriate distal geological sampling. In the following, we, therefore, assume Vt = 0 for Ana Slide. Mobilized volume (Vm) the initial failed (or mobilized) volume. It is the sum of Vm void , Vm r , and Vt (if a turbidite deposit was generated in the distal part): Vm = Vm void + Vm r + Vt. Accumulated volume (Va) the amount of material that accumulated between the pre-failure seafloor (R1) and the present-day seafloor SFR inside the sink area. It represents the difference between the apparent evacuated volume of Vm void and the volume of a potential turbidite deposit and therefore: Va = Vm void – Vt. The approach of Völker ( 2010 ) could be used to estimate Va. However, for Ana Slide, the morphology of the sink area was modified by a local seafloor antithetic en-echelon fault system prior to landslide occurrence(Sager et al., 2022 ). Thus, the pre-failure seafloor inside the sink area cannot be reconstructed using the contour-line approach previously used for the source area. Therefore, to account for vertical fault movement in the pre-failure seafloor reconstruction, the predictable thickness of sedimentary sequences (between Ref and SFR) throughout the study area is used to constrain the course of the pre-failure seafloor inside the sink area (Fig. 2 ). For the horizon-flattening approach (Fig. 3 ), firstly, reflectors Ref and SFR are picked in the 3D reflection seismic data. Then, reflector SFR is removed from inside the sink area. For pre-failure seafloor reconstruction, reflector Ref is horizontally flattened and SFR is reconstructed inside the sink area using the predictable thickness of the stratigraphic sequence between reflectors Ref and SFR by manually picking straight lines that represent this thickness between the upslope and downslope extent of the source area on individual inlines (E-W). The workflow is presented in Figure. 3). After reconstruction is completed, reflector Ref is de-flattened with the resulting reconstructed pre-failure seafloor accounting for vertical tectonic movement of the seafloor antithetic en-echelon fault system with activity before failure occurrence of the primary failure of Ana Slide (Sager et al., 2022 ). The resulting surface is called the ‘pre-failure seafloor following the horizon-flattening approach’ (Fig. 3 ). 3.1. Uncertainties of volume estimations General uncertainties for volume estimation of submarine landslides are related to unknown seismic velocities, lateral changes in seismic velocities, and reflector picking errors (related to the vertical resolution) of seismic data. Additional uncertainties unique to Ana Slide are related to the localization of bounding polygons of the source and sink areas related to the horizontal resolution of reflection seismic data and those related to issues with ghost artefacts unique to the 3D seismic data of Ana Slide previously discussed by Sager et al. ( 2022 ). Overall, the picking errors for volume estimation of submarine landslides based on 3D seismic data are small (< 5% of the total volume) and the uncertainty due to unknown seismic velocity is small for the uppermost sediments (< 5%), while the uncertainty related to polygons of the source and sink areas is neglectable (< 2%). Dugan ( 2012 ) and Sun & Alves ( 2020 ) demonstrate that MTD material has higher density and lower porosity compared to background sediment, which would impact the seismic velocity of the landslide interval. A comparison of several cores from inside and outside the landslide area of Ana Slide shows that no notable differences in P-wave velocities in background sediment and deposit exist for at least the upper 8 m below seafloor (Lafuerza et al., 2012). Therefore, we assume no uncertainties resulting from lateral variations in seismic velocities. The above uncertainties affect the different volume estimations in distinct ways. In sum, the above uncertainties of the volume estimation of Ana Slide add up to 12%. Uncertainties related to the approach to pre-failure seafloor reconstruction are significantly larger but difficult to quantify in percentages (Table 1 ). Table 1 Results from volume assessment of Ana Slide. Names of volumes calculated, and bounding surfaces (Fig. 1 b) are presented. The volume of Vm void (evacuated volume from the source area) and Va (volume accumulated above the pre-failure seafloor inside the sink area) is the same at 0.016 km 3 (b and d). Name Upper surface (depth-converted horizon) Lower surface (depth-converted horizon) Bounding polygon Areal extent (km 2 ) Volume (km 3 ) Vm r present-day seafloor (SFR) Reference reflector (Ref) source area 1.90 0.024 Vm void reconstructed pre-failure seafloor using the contour-line approach present-day seafloor (SFR) source area 1.90 0.016 Vm Vm r + Vm void Vm r + Va 0.040 0.040 Va present-day seafloor (SFR) reconstructed pre-failure seafloor using the horizon-flattening approach sink area 2.45 0.016 Vm void (straight-line approach) reconstructed pre-failure seafloor using the straight-line approach present-day seafloor (SFR) source area 1.90 0.027 Va (straight-line approach) present-day seafloor (SFR) reconstructed pre-failure seafloor using the straight-line approach sink area 2.45 0.006 4. Results from volume assessment of Ana Slide The volume of evacuated landslide material that remained inside the source area of Ana Slide called Vm r is 0.024 km 3 (Table 1 a). Together with the volume of Vm void of 0.016 km 3 (Table 1 b), the amount of all mobilized and involved landslide material from inside the source area called Vm is calculated by: Vm = Vm r + Vm void + (Vt), thus 0.024 km 3 + 0.016 km 3 = 0.040 km 3 (Table 1 b and d). The amount of material that was transported and deposited as a turbidite Vt could not be determined and is assumed to be zero.Thus, Va corresponds to Vm void through (Vt is assumed to be zero): Va = Vm void – Vt, thus 0.016 km 3 = 0.016 km 3 , and therefore Va = Vm void for Ana Slide. From the unrealistic pre-failure seafloor reconstruction using the straight-line approach applied inside the landslide scar of Ana Slide, we have estimated the amounts of Vm void (straight-line approach) between the reconstructed and present-day seafloors inside the source area at 0.027 km 3 (Table 1 e). The amount of Va estimated between the pre-failure seafloor using the straight-line approach for pre-failure seafloor reconstruction and the present-day seafloor inside the sink area yields a volume of 0.006 km 3 for Va (Table 1 f). 5. Discussion 5.1. Ana Slide volume assessment The initial total failed volume of Ana Slide Vm is the sum of the actually evacuated volume Vm void from the source area and the volume of the remaining material Vm r inside the source area (Fig. 4 a). Vm is the total failed landslide volume but only the Vm void volume was deposited in the sink area. Vm void equals Va (accumulated volume) at 0.016 km 3 . This supports that the Ana Slide did not generate a turbidity current, because otherwise Vm estimated from Vm void and Vm r should exceed the Vm estimated from Va and Vm r . Our here estimated volume is much smaller than the previous estimate of 0.14 km³ by Lastras et al. ( 2004 ). The previously detailed investigations for Ana Slide by Sager et al. ( 2022 ) offer an explanation: The deposit of Ana Slide induced in-situ deformation that penetrated to a depth of reflector Ref in the sink area so that this deformed slope sediment appears chaotic, transparent, or disrupted in the reflection seismic data. However, the sediment beneath the deposit inside the sink area is not related to the initial failed volume mobilized from the source area Vm as the sediment deformed in-situ and only very limited displacement or transport occurred above reflector Ref inside the sink area of Ana Slide (sensu Lastras et al., 2004 ). Lastras et al. ( 2004 ) thus provide the sum of the volumes of mobilised and affected material. Hence, volume estimation based on sparse 2D seismic data considering seismically chaotic facies without further constraints on its origin would overestimate the initial failed volume of Ana Slide by almost one order of magnitude. To obtain the initial failed volume Vm, Vm r and either Vm void or Va need to be known. While Vm void can be estimated from bathymetry data alone, for example through comparing the modern and reconstructed pre-failure seafloors, in the case of Ana Slide, reflection seismic data was necessary to estimate the amount of Va using the horizon-flattening approach to pre-failure seafloor reconstruction because the geometry of the sink area was affected by vertical tectonic movement. Furthermore, Vm r can only be identified and quantified using seismic data. While Vm r is interesting from a landslide mechanism perspective, it is not essential for numerical tsunami modelling. If no reflection seismic data had been available for Ana Slide, only Vm void could have been estimated. This would have underestimated the initial failed volume by 40% (because Vm = 0.040 km 3 while Vm void = 0.016 km 3 ). The value of Vm void using the straight-line approach represents the upper limit of Vm void on a convex downslope profile as that in the study area. More realistic, e.g. polygonal fits of the contour lines, pre-failure seafloor reconstructions would lead to smaller volume of Vm void , (Fig. 1 b). The maximum Vm void is 0.027 km³, which exceeds the actual value of Vm void of 0.016 km³ by far. Similarly, the amount of Va estimated using the straight-line approach represents the lower limit of Va because more realistic contour lines will again diverge further upslope than those reconstructed with this approach. This value is much lower than Va estimated from the ‘realistic’ seafloor reconstruction. Ana Slide is known to have formed during two stages of failure (Sager et al., 2022 ). Although the volume of material mobilized during the secondary failure is not resolved in the seismic data due to limitation of the vertical seismic resolution, a potential volume may be estimated. Assuming the deposit uniformly covers the entire sink area with a thickness of 4 m (just below the vertical seismic resolution of 5 m), the deposit’s volume is < 0.010 km 3 , hence about a fourth of the volume of Vm at 0.040 km 3 (Table 1 ). 5.2. Volume assessment of submarine landslides In this section, we generalize our findings for Ana Slide to be applicable to other events. With increasing resolution of geophysical imaging and a growing number of case studies it becomes evident that submarine landslides are highly complex and inherently different from one another. Nevertheless, what might appear as peculiarities of Ana Slide has recently been recognised as common mechanisms, such as in-situ substrate deformation (compare section 2). Moreover, Ana Slide’s quantitative morphometric parameters align well with global compilations of submarine landslides (Hühnerbach and Masson, 2004 ; Moscardelli and Wood, 2015 ; Clare et al., 2019 ). For these reasons, we suggest that Ana Slide can be assumed representative for many submarine landslides and that our conclusions regarding volume estimations can be used to make inferences on generic volume calculations. For the discussion, it is useful to categorize submarine landslides according to their emplacement mechanisms. Here, we distinguish between two endmembers following the terminology and definition by Moscardelli and Wood ( 2008 ). Attached landslides remain connected to the slope by a portion of their original material, while detached landslides become completely separated, leaving a distinct scar or headwall behind. In the case a landslide is detached, the source area is fully evacuated and devoid of all mobilized material, so that Vm = Vm void (Fig. 4 b). Vm can also be validated by adding Va and Vt. Consequently, to assess the volume of young detached submarine landslides, bathymetric data are ideal to estimate the volumes of Vm void (and potentially Va) by comparing the modern and reconstructed pre-failure seafloor topographies. Here, any post-slide modifications of the seafloor by external factors, such as vertical tectonic movement or bottom currents, need to be excluded. The bathymetry-based approach will yield more robust values for Vm than approaches based on 2D profiles of sub-seafloor reflection seismic data for three reasons. First, it is more feasible to cover the entire landslide area. Second, estimates for both Vm (difference in the sink area) and Va (difference in the source area) can be made independently and a comparison of the results provides an additional quality control. Ideally, Vm equals Va and certainly, Va should not exceed Vm. Third, when using sub-seafloor reflection seismic data, a potential flaw could be introduced by estimating Va bulk instead of Va. This misinterpretation can happen as the internal structure of a submarine landslide may be imaged as chaotic, disrupted, or transparent seismic facies (e.g. Tripsanas et al., 2008 ). Similarly, sediments that were disturbed through internal deformation for example by rapid loading (e.g., Sager et al., 2022 ) or shearing induced by passing landslide material (e.g., Sobiesiak et al., 2018 ) also display as chaotic, disrupted, or transparent seismic facies. Hence, seismic facies of Va and Va bulk are similar if not identical and therefore it is difficult if not impossible to distinguish material that moved, was translated, or was deformed in-situ (Sobiesiak et al., 2018 , Sager et al., 2022 ). This is problematic because Va bulk may largely exceed Vm (and Va). In the case of an attachedlandslide (Fig. 4 c), the initial failed volume Vm equals the sum of Vm void and Vm r , where Vm r > > Vm void . In this case, it is impossible to differentiate between Vm r and Va. The amount of Vm r can be calculated between the present-day seafloor and the basal shear surface and thus this approach requires seismic data. Using only bathymetric data will correctly determine Va, but it will underestimate the initial failed volume by the value of Vm r (Vm = Vm void + Vm r ). In the analysis of reflection seismic data, care must be taken in the identification of the basal shear surface. There is a risk estimating the sum of mobilized and affected material instead of Va due to the reasons outlined above. 5.3. How to determine the initial failed volume of submarine landslides? Based on the above considerations we here provide a framework and recommendations for assessing the initial failed volume of submarine landslides taking into account available datasets and emplacement mechanism (Fig. 5 ). The framework applies only to landslide scars outcropping at the seafloor in areas, which have not experienced modification of the seafloor since the occurrence of the landslide. This excludes large multi-retrogressive landslide such as the Storegga Slide with a complex development and overlapping MTDs. Further, vertical tectonic movement, deposition by sediment transport processes, or ocean currents must be excluded, or these influences need to be accounted for in the pre-failure seafloor reconstruction. For instance, this is demonstrated by the horizon-flattening approach applied for pre-failure seafloor reconstruction performed inside the sink area of Ana Slide. First, the pre-failure seafloor in the entire landslide area must be reconstructed using bathymetric data. Then, this reconstructed pre-failure seafloor may act as the upper surface in calculating Vm void inside the source area using the present-day seafloor as the lower surface. Va is calculated in the sink area between the present-day and reconstructed pre-failure seafloor inside the sink area. Now, the suggested workflow deviates according to the emplacement mode (detached or attached). In the case the given submarine landslide developed detached (left branch in Fig. 5 ), it is beneficial to have reflection seismic data covering the source area so that the amount of remaining landslide material (Vm r ) can be calculated in order to estimate the complete initial failed volume by Vm = Vm void + Vm r . Vm can also be estimated from Vm = Va + Vt. In the case no reflection seismic data are available, the rule that Vm void > = Va can serve as an additional constraint. Va cannot be larger than Vm void because the material deposited in the sink area (estimated based on bathymetry) cannot exceed what was evacuated from the source area while Va can decrease if landslide material is transported as a turbidity current (Vt). If this condition, nevertheless, is not fulfilled one should revise the pre-failure seafloor reconstruction until the condition is fulfilled. In both cases, if reflection seismic data are available or not, it is robust to calculate Vm void inside the sink area, because it either represents the amount of initial failed landslide material and represents the amount of Va, or the pre-failure seafloor reconstruction is incorrect assuming that no turbidite transported landslide material (not resolvable in the bathymetric data) and that no erosion and incorporation of seafloor sediment occurred. In the case the given submarine landslide developed as an attached landslide (right branch in Fig. 5 ) if reflection seismic data are available, the amount of all landslide material involved in or affected by the slope failure can be calculated as the sum of the mobilized and affected material. For attached submarine landslides the sum of the mobilized and affected material is equal to Vm only in the case where no deep deformation has occurred. In that case, the sum of the mobilised and affected material is larger than Vm. Vm void underestimates Vm because the source area is only partly evacuated. In case no reflection seismic data is available, the only means to approximate the initial failed volume of a submarine landslide is by Vm void and Va, both of which will underestimate Vm. In case the given landslide developed as a mixed system placed kinematically between the attached or detached cases, or if the emplacement mode is unknown, we suggest estimating Vm void and Va. If the amount of landslide material transported and deposited as a turbidite is neglectable, the amount of Vm void and Va should be the same (Vm void = Va). If Vm r is unknown, one should consider that Vm void and Va likely underestimate Vm by the unknown amount of Vm r . We showed that for Ana Slide, the sum of the mobilized and affected material estimated on seismic data overestimates Vm by almost one order of magnitude, whereas estimating Vm from V void and/or Va based on bathymetric data alone underestimates Vm by 40%. Due to the risk of excessive overestimation, we suggest that the Vm = Vm void + Ve r approach should always be preferred, even if Vm r is unknown. 5.4. Limitations and assumption of submarine landslide volume assessments Estimation of Vm relies on the approach to pre-failure seafloor reconstruction. This may be challenging for submarine landslides in morphologically complex settings and pre-failure seafloor reconstruction might require comprehensive knowledge of the geological, sedimentary, and tectonic setting. For instance, the seafloor morphology of the source area before the failure of Ana Slide was affected by the earlier pre-Ana Slide (Berndt et al., 2012 ; Sager et al., 2022 ). It is clear that Vm and Vm void must be equal or larger than Va (if Vt > zero). This constraint can help assess the quality of the seafloor reconstruction at least in one direction. Volume estimation based on bathymetry and pre-failure seafloor reconstructions also relies on the assumption that the seafloor has not changed significantly since the occurrence of the landslide. Any modifications of the seafloor, for instance by vertical tectonic movements, erosion, deposition by sediment transport, or ocean currents will result in wrong volume estimates. Another assumption is that the volume has been evacuated during one event. If the volume was evacuated during multiple stages with significant time gaps in between the potential hazard will likely be overestimated, although controlled by many other factors such as landslide mechanisms, angle of the slide, water depth, density and cohesion of the landslide material, duration of the slide event, its acceleration, and run-out velocity (e.g., Harbitz et al., 2014 ; Tappin et al., 2008). Va is prone to underestimation because the resolution of bathymetric data and reflection seismic data is typically too low to resolve thin and far travelled turbidites approaching zero thickness in the distal parts, making a clear distinction between Va and Vt difficult. It is only with gravity cores recovered from the distal lobes that underestimation of Va of turbidity current-producing landslides can be prevented. Recent studies have shown that processes of basal erosion and incorporation of seafloor material can lead to a significant increase in Va (Nugraha et al., 2022 ; Sobiesiak et al., 2018 ), but this volume does not represent the volume of initial failed landslide material Ve. The deposit in the sink area might therefore not be a good representation and therefore we advocate to consider Vm, Vm r , and Vm void to estimate the initial failed volume. 6. Conclusions The volume of a submarine landslide is critical for the hazard it poses. For tsunami generation, in particular, the initial failed volume is important (Murty, 2003 ). We identify the most robust method to estimate the initial failed volume depending on the data set available as well as the landslide`s emplacement mechanism. If no reflection seismic data from the source area of the landslide is available, the initial failed landslide volume Ve can relatively reliable be determined only for fully detached landslides. For attached or mixed systems, this approach will underestimate the true initial failed volume because the amount of landslide material that was mobilized but remained inside the source area is neglected. Seismic data are required to estimate the volume of landslide material that remained inside the source area. If such data are not available, we find that the most robust approximation for the initial failed volume of a detached or mixed-system submarine landslide also is to determine the amount of evacuated landslide material between the pre-failure and present-day seafloors inside the source area from the void space between both surfaces using bathymetric data. The initial failed volume has previously been estimated using the seismically identified deposit. It is important to acknowledge that this may be prone to extreme overestimation (more than 200% in this case) because of in-situ deformation of sediments underlying the pre-failure seafloor in the sink area that may result from rapid deposition or shearing of the accumulating landslide deposits. When estimating the volume from the amount of seismically chaotic, transparent, or disrupted seismic facies, we advocate for balancing this against the initial failed volume estimated from bathymetric data, which yields a more conservative estimate. Declarations Acknowledgements Major thanks and appreciation for the patience of reviewer Prof David Tappin. Funding The research leading to results presented in this study received funding from Deutsche Forschungsgemeinschaft (DFG) under Grant Agreement number UR 226/3-1. MU acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 948797). Conflicts of Interest/Competing Interests The authors have no relevant financial or non-financial interests to disclose. These authors have no competing interests to declare that are relevant to the content of this article. Availability of Datasets and Material The 3D reflection seismic data from Ana Slide analyzed in the current study are available at the World Data Centre Pangaea repository (https://doi.pangaea.de/10.1594/PANGAEA.943506), the 2D reflection seismic data are available under (https://doi.pangaea.de/10.1594/PANGAEA.943523), and the multibeam bathymetry data of the study area are available under (https://doi.pangaea.de/10.1594/PANGAEA.953762). Code Availability (Not applicable) Author Contribution Conceptualization: TFS, MU, CB; Methodology: MU, TFS; Formal analysis and investigation: TFS; Writing original draft preparation: MU, TFS; Writing – review and editing: TFS, MU, CB; Visualizations: TFS, MU; Funding acquisition: MU; Resources: CB; Supervision: MU, CB. All authors reviewed the manuscript. Additional Declarations for Articles in Life Science Journals that Report the Results of Studies Involving Humans and/or Animals (not applicable) Ethics Approval (not applicable) Consent to Participate (not applicable) Consent for publication (not applicable) References Berndt C, Costa S, Canals M, Camerlenghi A, de Mol B, Saunders M (2012) Repeated slope failure linked to fluid migration: The Ana submarine landslide complex, Eivissa Channel, Western Mediterranean Sea. Earth Planet Sci Lett 319–320:65–74. https://doi.org/10.1016/j.epsl.2011.11.045 Bondevik S, Inge Svendsen J, Mangerud J (1997) Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, western Norway. 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Mar Geol 213(1–4):149–167. https://doi.org/10.1016/j.margeo.2004.10.005 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 03 Nov, 2025 Read the published version in Geo-Marine Letters → Version 1 posted Editorial decision: Revision requested 01 Oct, 2025 Reviews received at journal 25 Sep, 2025 Reviewers agreed at journal 30 Aug, 2025 Reviewers agreed at journal 30 Aug, 2025 Reviewers invited by journal 21 Aug, 2025 Editor assigned by journal 21 Aug, 2025 Submission checks completed at journal 21 Aug, 2025 First submitted to journal 15 Aug, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7380838","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":503726868,"identity":"eed17526-603d-42e0-9973-1bb795921f26","order_by":0,"name":"Morelia Urlaub","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAy0lEQVRIiWNgGAWjYHACxgMMDAfk4LwGYvSAtBiTriURppKwFt0ZuQ8O/Gy7kz7fvfnohp97GGT7CWkxu5FucLC37VnuxjPH0m72PGMwnknIGrMbaQwHeNsO526ckWN2g+cAQ+KGA0RoOfi37XC64fw3Zjf/ALXsJ0bLYaAtCfISPGa3wbYQ9MuZZwyHZc49M9zAk5Z2W+aAhPEMgrYcT2N8+Kbsjrx8++FjN98csJHtbyBkDQwYQAyXIFY9EMgTbfgoGAWjYBSMOAAA06tOx0gwVtAAAAAASUVORK5CYII=","orcid":"","institution":"GEOMAR Helmholtz Centre for Ocean Research Kiel","correspondingAuthor":true,"prefix":"","firstName":"Morelia","middleName":"","lastName":"Urlaub","suffix":""},{"id":503726869,"identity":"c1d530d7-4b37-4dba-8540-d98273308a24","order_by":1,"name":"Thore Falk Sager","email":"","orcid":"","institution":"GEOMAR Helmholtz Centre for Ocean Research Kiel","correspondingAuthor":false,"prefix":"","firstName":"Thore","middleName":"Falk","lastName":"Sager","suffix":""},{"id":503726870,"identity":"f2dd15da-a145-4953-856b-294982ad76d3","order_by":2,"name":"Christian Berndt","email":"","orcid":"","institution":"GEOMAR Helmholtz Centre for Ocean Research Kiel","correspondingAuthor":false,"prefix":"","firstName":"Christian","middleName":"","lastName":"Berndt","suffix":""}],"badges":[],"createdAt":"2025-08-15 11:08:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7380838/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7380838/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00367-025-00826-4","type":"published","date":"2025-11-03T15:56:54+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":90314626,"identity":"a4c92ed5-d1dc-42a9-9381-e886fe4571c5","added_by":"auto","created_at":"2025-09-01 10:09:29","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":432834,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea)\u003c/strong\u003e Regional map of the western Mediterranean Sea showing the study area in the Eivissa Channel located on the Balearic Promontory between the Iberian Peninsula and the island of Eivissa. \u003cstrong\u003eb)\u003c/strong\u003e Hillshaded bathymetry map of Ana Slide at water depths between 635 and 790 m. The southern, central, and northern faults comprise a local seafloor antithetic en-echelon fault system that controlled the development of Ana Slide (Sager et al., 2022). The location of Kullenberg gravity core PSM-KS18 is indicated by a blue symbol (0° 50.453’ E 38° 38.184’ N) presented by Lafuerza et al. (2012). Landslide material involved in Ana Slide was evacuated from the source area and accumulated inside the sink area.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-7380838/v1/7313580cfb494fd7f2892dc7.png"},{"id":90314625,"identity":"984e34ad-9e41-4d49-a263-24033c6828ec","added_by":"auto","created_at":"2025-09-01 10:09:29","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":350590,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea)\u003c/strong\u003e Uninterpreted 2D reflection seismic profile (Channel 01 – Line 37) of Ana Slide (for location see Figure 1). \u003cstrong\u003eb)\u003c/strong\u003e Interpreted profile including main seismic horizons associated to the landslide: Ref (green), R1 (blue), and SFR (red). The different volumes defined in this study are shown for the Ana Slide: Va describes accumulated landslide material inside the sink area above the pre-failure seafloor. Vm\u003csub\u003er\u003c/sub\u003e represents mobilized landslide material (of the primary and secondary failures) which was unable to overcome frontal confinement and ponded downslope inside the source area (also referred to as undifferentiated landslide material). Va represents the volume of accumulated landslide material above the pre-failure seafloor inside the sink area.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-7380838/v1/1da2cb873f20589109764dd5.png"},{"id":90314629,"identity":"e55e2873-d900-4a79-bf5c-3f3c045e5720","added_by":"auto","created_at":"2025-09-01 10:09:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":370954,"visible":true,"origin":"","legend":"\u003cp\u003eSteps of the horizon-flattening approach for reconstructing the pre-failure seafloor in the sink area of Ana Slide exemplarily shown for one profile out of the 3D seismic dataset. a) The profile as it was recorded shows the Ref reflector bends downward in response to normal faulting at a profile distance of 300 m. b) The Ref reflector is horizontally flattened to correct for any vertical tectonic movement. C) The pre-failure seafloor (black) is reconstructed and Va outlined (hatched area). D) The Ref reflector is de-flattened.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-7380838/v1/9c827f8df30073ed352b920f.png"},{"id":90315982,"identity":"2fc72c85-2243-47d6-bbfb-e15e47c43e4b","added_by":"auto","created_at":"2025-09-01 10:17:29","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":614343,"visible":true,"origin":"","legend":"\u003cp\u003eConceptual models for assessing submarine landslide volumes with implications for the total initial failed volume Vm.. \u003cstrong\u003ea)\u003c/strong\u003e Model of a mixed system like the Ana Slide, with detached landslide material above slope sediment that comprises the by-pass zone (in profile) and a small attached fraction of the mobilised material remaining in the scar (Vm\u003csub\u003er\u003c/sub\u003e). \u003cstrong\u003eb)\u003c/strong\u003e Model of a detached landslide (sensu Moscardelli and Wood, 2008) with free propagation of landslide material over the seafloor inside of the extensively evacuated source area. \u003cstrong\u003ec)\u003c/strong\u003e Model of a detached landslide (sensu Moscardelli and Wood, 2008) with restricted propagation of landslide material inside the sink area and limited evacuation of the source area.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-7380838/v1/a91c172653bceb3241baaa90.png"},{"id":90314636,"identity":"9e5caddb-5a99-470b-a6dc-b55af8026dc7","added_by":"auto","created_at":"2025-09-01 10:09:29","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":325124,"visible":true,"origin":"","legend":"\u003cp\u003eWorkflow for assessing volumes submarine landslides.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-7380838/v1/d36bfc51d295b1357cb88c46.png"},{"id":95563938,"identity":"095694c6-700a-40ee-b39a-f9b8cef21d90","added_by":"auto","created_at":"2025-11-10 16:04:46","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2932204,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7380838/v1/5bb523bb-a036-44c3-a0d5-461d58e97978.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Pitfalls and opportunities in estimating the volumes of small, young submarine landslides","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSubmarine landslides may have the potential to be a significant geohazard to coastal populations worldwide (Bondevik et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Haugen et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; L\u0026oslash;vholt et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Prior et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e1984\u003c/span\u003e; Synolakis et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Talling et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Watt et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). They can destroy offshore infrastructure such as platforms and telecommunication cables (e.g., L\u0026oslash;vholt et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Vanneste et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) and release large quantities of methane and other greenhouse gases from the seafloor (e.g., Talling et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Submarine landslides occur on submerged slopes in all geological settings and cover a vast range of sizes, from surficial sediment removal (Kioka et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) to several orders of magnitude larger than terrestrial landslides (Korup et al. 2007). The terms submarine landslide and slope failure describe the active processes of landsliding while their remnants left on the seafloor are termed mass-transport deposits (MTDs).\u003c/p\u003e\u003cp\u003eThe inclination of the seafloor, water depth, duration of the slide event, its acceleration, related landslide mechanisms, run-out velocity, the timing between multiple stages of failure, the volume of mobilized material, and its density and cohesion are all factors that control the impact of a landslide (e.g., Harbitz et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Murty, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Constraining these factors requires multibeam echosounder bathymetric and seismic data, as well as seafloor samples and age datings. Marine-geophysical data are becoming more available, yet the samples and ages are rarely available (Tappin, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Critical factors in tsunami generation and essential to numerical tsunami modelling, such as the initial acceleration and slide velocity, cannot be quantified at all from MTDs and thus rely heavily on assumptions. However, the volume of a submarine landslide can be estimated relatively easily from bathymetry and/or a few seismic lines. Therefore, the hazard of submarine landslides is oftentimes assessed from their volume alone (e.g. Grilli et al., 2009; Urgeles \u0026amp; Camerlenghi, 2013).\u003c/p\u003e\u003cp\u003eThe volume of a landslide is not constant as it may change during its evolution. The volume of the resulting deposit may thus exceed the initial failed volume due to processes such as basal erosion and sediment entrainment (e.g., Sobiesiak et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Watt et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). On the other hand, the deposit may be distributed over large areas by highly mobile sediment flows (Talling et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), which are beyond the resolution of the imaging systems and the extent of mapping. Here, we focus particularly on estimating the initial failed volume of a landslide as it is one of the key input parameters for tsunami models (Tappin, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) and it is quantifiable based on marine-geophysical data.\u003c/p\u003e\u003cp\u003eThere are various ways to estimate the volume of submarine landslides from bathymetric or reflection seismic data. McAdoo et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) measured the area of the source area (A) and height of the headscarp (H) from bathymetric data to estimate the evacuated volume through volume\u0026thinsp;=\u0026thinsp;1/2 * A * H. On the contrary, V\u0026ouml;lker (\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) estimated the evacuated volume by subtracting a pre-failure seafloor that was reconstructed by fitting slope functions into the landslide scar from the present-day seafloor. Here, the interpreted evacuated region in the source area provides an estimate of the initial failed volume and the deposit in the sink area provides an estimate of the deposited and accumulated volume. Wilson et al. (\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) calculated the volume of a debris lobe from its depth or thickness (D), width (W), and length (L) through the relationship of volume\u0026thinsp;=\u0026thinsp;1/6 * π * D * W * L. For landslides imaged from sub-seafloor echo-sounder profiles or 2D and 3D reflection seismic data, the average thickness of chaotic, transparent, and/or disrupted seismic facies representing the mobilized material can be measured and multiplied by the landslide area. This provides the \u0026lsquo;bulk volume\u0026rsquo; of material involved in and affected by the landslide (referred to as Vd by Nugraha et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)) that accumulated inside the sink area. This method was used by Lastras et al. (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) to estimate the landslide volume or total affected volume of Ana Slide, located in the Eivissa Channel, western Mediterranean Sea. These authors used the average thickness of Ana Slide at 23 m inside the landslide scar (with an area of 6 km\u003csup\u003e2\u003c/sup\u003e) to propose a volume or Vd (sensu Nugraha et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) of 0.14 km\u003csup\u003e3\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThis study aims to provide an exact initial volume of the Ana Slide in the Eivissa Channel, Mediterranean Sea, making use of recently published detailed knowledge on its emplacement processes and a 3D seismic dataset covering the entire landslide area. We compare the resulting \u0026lsquo;exact\u0026rsquo; volume to volumes estimated by applying the approaches outlined above, which are typically used in the absence of extensive coverage of seismic data and/or geological sampling. The comparison allows us to identify potential pitfalls and provide recommendations on suitable approaches to determine the initial landslide volume. This work is timely because submarine landslides have now been mapped around many ocean margins and their failure and emplacement mechanisms identified, allowing us to determine their volumes.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2. Emplacement of Ana Slide","content":"\u003cp\u003eThe Ana Slide is a relatively small landslide located on the eastern slopes of the Eivissa Channel, western Mediterranean Sea between 635 and 790 m water depth (Berndt et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Lastras et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The landslide affected an area of 4.5 km\u0026sup2; with a headscarp height of 30 m (Lastras et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e and \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Berndt et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Previous volume estimates based on two 2D seismic lines suggest 0.14 km\u0026sup3; (Lastras et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The average slope angle is 1\u0026ndash;2\u0026deg; (Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eHere, we use the interpretation of development and emplacement processes of Ana Slide by Sager et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The authors use high-resolution bathymetry, 3D seismic data and re-processed 2D reflection seismic profiles over this landslide. Based on 3D seismic imaging, Sager et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) show that Ana Slide developed in two stages referred to as the primary (300 ka) and secondary failures (61.5 ka after Cattaneo et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Landslide material mobilized during the primary failure attained a thickness of approximately 15 m in the sink area. The secondary failure involved slope material between two shallow reflectors R1 and the present day seafloor (SFR). The deposit of this secondary failure was much smaller and is not resolved even on high-resolution seismic data (~\u0026thinsp;5 m vertical resolution, Berndt et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). For this reason, the volumes of the primary and secondary failures of Ana Slide are combined for the purposes of this study (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and the top reflector of Ana Slide, referred to as reflector SFR, corresponds to the upper bound of the secondary failure.\u003c/p\u003e\u003cp\u003eThe landslide involved slope material between the basal shear surface (represented by the reference reflector Ref) and the reflector R1 above (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). A small fraction of the mobilized landslide material remained inside the source area (undifferentiated landslide material, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This is observed for submarine landslides elsewhere, e.g. the Storegga Slide (Micallef et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) and the Sahara Slide (Krastel et al., 2018). The much larger fraction of the landslide material at the base of the evacuation scar emerged frontally, travelled across a 500 m long by-pass zone, and over-rode the pre-failure seafloor at the time of the primary failure (represented by the \u0026lsquo;pre-failure R1 reflector\u0026rsquo;) inside the sink area (actual deposit, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The accumulation of this deposit induced \u003cem\u003ein situ\u003c/em\u003e deformation of the underlying sediments reaching a depth of up to 30 m below the pre-failure R1 reflector that marked the seafloor at the time of the primary failure. Deformed material underlying a submarine landslide is not unique to Ana Slide. Recent studies based on onshore outcrops, high resolution and 3D seismic data have shown that interaction with the substrate is a common feature for submarine landslides (Sobiesiak et al., 2019 and references therein; K\u0026uuml;hn et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, Lenz et al., 2022; Nugraha et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Ogata et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAna Slide\u0026rsquo;s sink area is affected by three active faults vertically offsetting reflectors (Berndt et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2012\u003c/span\u003e, Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). These faults characterize an en-echelon fault system that strikes SSW and NNE and extends into the 3D reflection seismic data from the south with an unknown extent. This fault system dips in the opposite direction compared to the seafloor and therefore is termed \u0026lsquo;seafloor antithetic\u0026rsquo; in this study. Primarily the northern but also the central fault affected the seafloor morphology before the primary failure of Ana Slide by vertical fault movement, thereby generating a local depression and accumulation space for the landslide deposit (Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe undisturbed area north and south of Ana Slide shows homogenous, well-stratified, seafloor sub-parallel reflectors without any evidence for bottom current activity(e.g., Lastras et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The interval between reflector Ref and SFR becomes steadily larger in downslope direction (Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e their Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea).\u003c/p\u003e"},{"header":"3. Data and methods","content":"\u003cp\u003eThis study uses bathymetric and 3D reflection seismic data acquired in 2006 with the P-Cable system of the National Oceanographic Centre (NOC) in Southampton, UK equipped with two sleeve guns and 11 streamers during cruise 178 onboard RSS Charles Darwin (CD178). Data were processed including time migration with water velocity (1500 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). For further information about acquisition and processing workflows, the reader is referred to Berndt et al. (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and Sager et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The bathymetric data have a horizontal resolution of 5 m, and the vertical resolution is approximately 0.5% of the water depth while the 3D reflection seismic data have a vertical resolution of 5\u0026ndash;6 m and a horizontal resolution of 10\u0026ndash;15 m (Berndt et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The 2D reflection seismic profile presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea shows a re-processed profile presented previously by Sager et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eVolume calculations of evacuated and accumulated landslide material of Ana Slide are performed in Kingdom Suite using the Volumetric tool that uses one bounding polygon and two depth-converted grids (calculated from horizons in seconds two-way travel time or seconds TWTT). For depth conversion of seismic horizons, a seismic velocity of 1500 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is used. This velocity is consistent with seismic velocity measurements from the shallow 8 m long Kullenberg gravity core PSM-KS18 (0\u0026deg; 50.453\u0026rsquo; E 38\u0026deg; 38.184\u0026rsquo; N) presented by Lafuerza et al. (2012) obtained during the PRISM cruise with the R/V L\u0026rsquo;Atalante in 2007 led by IFREMER, France. The 3D reflection seismic data are presented in the time domain (seconds TWTT) and volumes are calculated in the upper 50 m beneath the seafloor. Sediments are water-rich (Lafuerza et al., 2012; Lastras et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Panieri et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and seismic P-wave velocities of such sediments typically vary between 1500 to 1640 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Hamilton, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1979\u003c/span\u003e). Thus, a seismic velocity of 1500 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is appropriate to use for depth conversion of seismic reflectors Ref and SFR and the reconstructed pre-failure seafloors for the source and sink areas.\u003c/p\u003e\u003cp\u003eIn this study, three horizontal bounding polygons are defined: the source area that covers an area of 1.9 km\u003csup\u003e2\u003c/sup\u003e, the by-pass zone with an area of 0.45 km\u003csup\u003e2\u003c/sup\u003e, and the sink area covering an area of 2.45 km\u003csup\u003e2\u003c/sup\u003e (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). In total, Ana Slide covers an area of 4.8 km\u003csup\u003e2\u003c/sup\u003e referred to as the landslide scar.\u003c/p\u003e\u003cp\u003eWe define several landslide volumes (Figure. 2), the values of which are calculated independently and with different approaches:\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eApparent evacuated volume (Vm\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003evoid\u003c/span\u003e\u003c/sub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e)\u003c/span\u003e\u003c/p\u003e\u003cp\u003ethe void space in the source area. Vm\u003csub\u003evoid\u003c/sub\u003e is calculated following the approach of V\u0026ouml;lker (\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) where the volume of evacuated landslide material is calculated by comparing the present-day with the reconstructed pre-failure seafloors (e.g, Omira et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Sun et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Webster et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). The pre-failure seafloor of the source area of Ana Slide is reconstructed by manually interpolating the local and regional contour lines from outside the landslide scar into the inside using the contour-line approach. To test the sensitivity of the applied pre-failure seafloor reconstructions, we reconstructed the seafloor inside the source area assuming a simple yet unrealistic pre-failure seafloor morphology (referred to as the straight-line approach hereafter). This was done by reconstructing straight pre-failure contour lines between the intersection of the landslide scar with local 10 m contour lines. The resulting calculated volume of evacuated landslide material from the source area serves as the maximum value for Vm\u003csub\u003evoid\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eRemaining volume (Vm\u003c/span\u003e\u003csub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003er\u003c/span\u003e\u003c/sub\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e)\u003c/span\u003e\u003c/p\u003e\u003cp\u003ethe volume of \u0026lsquo;undifferentiated landslide material\u0026rsquo; (sensu Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) that was mobilized but remained within the seabed scar. It is calculated from the 3D reflection seismic data and is the difference between the present-day seafloor (SFR) and the basal shear surface (Ref) in the source area.\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eTurbidite volume (Vt)\u003c/span\u003e\u003c/p\u003e\u003cp\u003ethe volume of material which potentially was transported into the deeper basin and out of the study area by turbidity currents. This material accumulated over a potentially vast area approaching zero thickness. With the available geophysical datasets limited to the proximal area of Ana Slide, it is impossible to determine whether a turbidity current was caused nor to estimate the volume of the turbidite deposit because of the lack of appropriate distal geological sampling. In the following, we, therefore, assume Vt\u0026thinsp;=\u0026thinsp;0 for Ana Slide.\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eMobilized volume (Vm)\u003c/span\u003e\u003c/p\u003e\u003cp\u003ethe initial failed (or mobilized) volume. It is the sum of Vm\u003csub\u003evoid\u003c/sub\u003e, Vm\u003csub\u003er\u003c/sub\u003e, and Vt (if a turbidite deposit was generated in the distal part):\u003c/p\u003e\u003cp\u003eVm\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e + Vm\u003csub\u003er\u003c/sub\u003e + Vt.\u003c/p\u003e\u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eAccumulated volume (Va)\u003c/span\u003e\u003c/p\u003e\u003cp\u003ethe amount of material that accumulated between the pre-failure seafloor (R1) and the present-day seafloor SFR inside the sink area. It represents the difference between the apparent evacuated volume of Vm\u003csub\u003evoid\u003c/sub\u003e and the volume of a potential turbidite deposit and therefore:\u003c/p\u003e\u003cp\u003eVa\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e \u0026ndash; Vt.\u003c/p\u003e\u003cp\u003eThe approach of V\u0026ouml;lker (\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) could be used to estimate Va. However, for Ana Slide, the morphology of the sink area was modified by a local seafloor antithetic en-echelon fault system prior to landslide occurrence(Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Thus, the pre-failure seafloor inside the sink area cannot be reconstructed using the contour-line approach previously used for the source area. Therefore, to account for vertical fault movement in the pre-failure seafloor reconstruction, the predictable thickness of sedimentary sequences (between Ref and SFR) throughout the study area is used to constrain the course of the pre-failure seafloor inside the sink area (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). For the horizon-flattening approach (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), firstly, reflectors Ref and SFR are picked in the 3D reflection seismic data. Then, reflector SFR is removed from inside the sink area. For pre-failure seafloor reconstruction, reflector Ref is horizontally flattened and SFR is reconstructed inside the sink area using the predictable thickness of the stratigraphic sequence between reflectors Ref and SFR by manually picking straight lines that represent this thickness between the upslope and downslope extent of the source area on individual inlines (E-W). The workflow is presented in Figure. 3). After reconstruction is completed, reflector Ref is de-flattened with the resulting reconstructed pre-failure seafloor accounting for vertical tectonic movement of the seafloor antithetic en-echelon fault system with activity before failure occurrence of the primary failure of Ana Slide (Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The resulting surface is called the \u0026lsquo;pre-failure seafloor following the horizon-flattening approach\u0026rsquo; (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Uncertainties of volume estimations\u003c/h2\u003e\u003cp\u003eGeneral uncertainties for volume estimation of submarine landslides are related to unknown seismic velocities, lateral changes in seismic velocities, and reflector picking errors (related to the vertical resolution) of seismic data. Additional uncertainties unique to Ana Slide are related to the localization of bounding polygons of the source and sink areas related to the horizontal resolution of reflection seismic data and those related to issues with ghost artefacts unique to the 3D seismic data of Ana Slide previously discussed by Sager et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Overall, the picking errors for volume estimation of submarine landslides based on 3D seismic data are small (\u0026lt;\u0026thinsp;5% of the total volume) and the uncertainty due to unknown seismic velocity is small for the uppermost sediments (\u0026lt;\u0026thinsp;5%), while the uncertainty related to polygons of the source and sink areas is neglectable (\u0026lt;\u0026thinsp;2%).\u003c/p\u003e\u003cp\u003eDugan (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and Sun \u0026amp; Alves (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) demonstrate that MTD material has higher density and lower porosity compared to background sediment, which would impact the seismic velocity of the landslide interval. A comparison of several cores from inside and outside the landslide area of Ana Slide shows that no notable differences in P-wave velocities in background sediment and deposit exist for at least the upper 8 m below seafloor (Lafuerza et al., 2012). Therefore, we assume no uncertainties resulting from lateral variations in seismic velocities.\u003c/p\u003e\u003cp\u003eThe above uncertainties affect the different volume estimations in distinct ways. In sum, the above uncertainties of the volume estimation of Ana Slide add up to 12%. Uncertainties related to the approach to pre-failure seafloor reconstruction are significantly larger but difficult to quantify in percentages (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eResults from volume assessment of Ana Slide. Names of volumes calculated, and bounding surfaces (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) are presented. The volume of Vm\u003csub\u003evoid\u003c/sub\u003e (evacuated volume from the source area) and Va (volume accumulated above the pre-failure seafloor inside the sink area) is the same at 0.016 km\u003csup\u003e3\u003c/sup\u003e (b and d).\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eName\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUpper surface (depth-converted horizon)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLower surface (depth-converted horizon)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eBounding polygon\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eAreal extent (km\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eVolume (km\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eVm\u003c/b\u003e\u003csub\u003e\u003cb\u003er\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003epresent-day seafloor (SFR)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eReference reflector (Ref)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003esource area\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e0.024\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eVm\u003c/b\u003e\u003csub\u003e\u003cb\u003evoid\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ereconstructed pre-failure seafloor using the contour-line approach\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003epresent-day seafloor (SFR)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003esource area\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e0.016\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eVm\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cb\u003eVm\u003c/b\u003e\u003csub\u003e\u003cb\u003er\u003c/b\u003e\u003c/sub\u003e \u003cb\u003e+ Vm\u003c/b\u003e\u003csub\u003e\u003cb\u003evoid\u003c/b\u003e\u003c/sub\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eVm\u003c/b\u003e\u003csub\u003e\u003cb\u003er\u003c/b\u003e\u003c/sub\u003e \u003cb\u003e+ Va\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e0.040\u003c/b\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003e0.040\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eVa\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003epresent-day seafloor (SFR)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ereconstructed pre-failure seafloor using the horizon-flattening approach\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003esink area\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e0.016\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eVm\u003c/b\u003e\u003csub\u003e\u003cb\u003evoid\u003c/b\u003e\u003c/sub\u003e (straight-line approach)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ereconstructed pre-failure seafloor using the straight-line approach\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003epresent-day seafloor (SFR)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003esource area\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e0.027\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eVa\u003c/b\u003e (straight-line approach)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003epresent-day seafloor (SFR)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ereconstructed pre-failure seafloor using the straight-line approach\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003esink area\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e2.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e0.006\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Results from volume assessment of Ana Slide","content":"\u003cp\u003eThe volume of evacuated landslide material that remained inside the source area of Ana Slide called Vm\u003csub\u003er\u003c/sub\u003e is 0.024 km\u003csup\u003e3\u003c/sup\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Together with the volume of Vm\u003csub\u003evoid\u003c/sub\u003e of 0.016 km\u003csup\u003e3\u003c/sup\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), the amount of all mobilized and involved landslide material from inside the source area called Vm is calculated by:\u003c/p\u003e\u003cp\u003eVm\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003er\u003c/sub\u003e + Vm\u003csub\u003evoid\u003c/sub\u003e + (Vt), thus 0.024 km\u003csup\u003e3\u003c/sup\u003e\u0026thinsp;+\u0026thinsp;0.016 km\u003csup\u003e3\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.040 km\u003csup\u003e3\u003c/sup\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and d).\u003c/p\u003e\u003cp\u003eThe amount of material that was transported and deposited as a turbidite Vt could not be determined and is assumed to be zero.Thus, Va corresponds to Vm\u003csub\u003evoid\u003c/sub\u003e through (Vt is assumed to be zero):\u003c/p\u003e\u003cp\u003eVa\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e \u0026ndash; Vt, thus 0.016 km\u003csup\u003e3\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.016 km\u003csup\u003e3\u003c/sup\u003e, and therefore Va\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e for Ana Slide.\u003c/p\u003e\u003cp\u003eFrom the unrealistic pre-failure seafloor reconstruction using the straight-line approach applied inside the landslide scar of Ana Slide, we have estimated the amounts of Vm\u003csub\u003evoid\u003c/sub\u003e (straight-line approach) between the reconstructed and present-day seafloors inside the source area at 0.027 km\u003csup\u003e3\u003c/sup\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). The amount of Va estimated between the pre-failure seafloor using the straight-line approach for pre-failure seafloor reconstruction and the present-day seafloor inside the sink area yields a volume of 0.006 km\u003csup\u003e3\u003c/sup\u003e for Va (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"5. Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e5.1. Ana Slide volume assessment\u003c/h2\u003e\u003cp\u003eThe initial total failed volume of Ana Slide Vm is the sum of the actually evacuated volume Vm\u003csub\u003evoid\u003c/sub\u003e from the source area and the volume of the remaining material Vm\u003csub\u003er\u003c/sub\u003e inside the source area (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). Vm is the total failed landslide volume but only the Vm\u003csub\u003evoid\u003c/sub\u003e volume was deposited in the sink area. Vm\u003csub\u003evoid\u003c/sub\u003e equals Va (accumulated volume) at 0.016 km\u003csup\u003e3\u003c/sup\u003e. This supports that the Ana Slide did not generate a turbidity current, because otherwise Vm estimated from Vm\u003csub\u003evoid\u003c/sub\u003e and Vm\u003csub\u003er\u003c/sub\u003e should exceed the Vm estimated from Va and Vm\u003csub\u003er\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003eOur here estimated volume is much smaller than the previous estimate of 0.14 km\u0026sup3; by Lastras et al. (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The previously detailed investigations for Ana Slide by Sager et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) offer an explanation: The deposit of Ana Slide induced \u003cem\u003ein-situ\u003c/em\u003e deformation that penetrated to a depth of reflector Ref in the sink area so that this deformed slope sediment appears chaotic, transparent, or disrupted in the reflection seismic data. However, the sediment beneath the deposit inside the sink area is not related to the initial failed volume mobilized from the source area Vm as the sediment deformed \u003cem\u003ein-situ\u003c/em\u003e and only very limited displacement or transport occurred above reflector Ref inside the sink area of Ana Slide (sensu Lastras et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Lastras et al. (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) thus provide the sum of the volumes of mobilised and affected material. Hence, volume estimation based on sparse 2D seismic data considering seismically chaotic facies without further constraints on its origin would overestimate the initial failed volume of Ana Slide by almost one order of magnitude.\u003c/p\u003e\u003cp\u003eTo obtain the initial failed volume Vm, Vm\u003csub\u003er\u003c/sub\u003e and either Vm\u003csub\u003evoid\u003c/sub\u003e or Va need to be known. While Vm\u003csub\u003evoid\u003c/sub\u003e can be estimated from bathymetry data alone, for example through comparing the modern and reconstructed pre-failure seafloors, in the case of Ana Slide, reflection seismic data was necessary to estimate the amount of Va using the horizon-flattening approach to pre-failure seafloor reconstruction because the geometry of the sink area was affected by vertical tectonic movement. Furthermore, Vm\u003csub\u003er\u003c/sub\u003e can only be identified and quantified using seismic data. While Vm\u003csub\u003er\u003c/sub\u003e is interesting from a landslide mechanism perspective, it is not essential for numerical tsunami modelling. If no reflection seismic data had been available for Ana Slide, only Vm\u003csub\u003evoid\u003c/sub\u003e could have been estimated. This would have underestimated the initial failed volume by 40% (because Vm\u0026thinsp;=\u0026thinsp;0.040 km\u003csup\u003e3\u003c/sup\u003e while Vm\u003csub\u003evoid\u003c/sub\u003e = 0.016 km\u003csup\u003e3\u003c/sup\u003e).\u003c/p\u003e\u003cp\u003eThe value of Vm\u003csub\u003evoid\u003c/sub\u003e using the straight-line approach represents the upper limit of Vm\u003csub\u003evoid\u003c/sub\u003e on a convex downslope profile as that in the study area. More realistic, e.g. polygonal fits of the contour lines, pre-failure seafloor reconstructions would lead to smaller volume of Vm\u003csub\u003evoid\u003c/sub\u003e, (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). The maximum Vm\u003csub\u003evoid\u003c/sub\u003e is 0.027 km\u0026sup3;, which exceeds the actual value of Vm\u003csub\u003evoid\u003c/sub\u003e of 0.016 km\u0026sup3; by far. Similarly, the amount of Va estimated using the straight-line approach represents the lower limit of Va because more realistic contour lines will again diverge further upslope than those reconstructed with this approach. This value is much lower than Va estimated from the \u0026lsquo;realistic\u0026rsquo; seafloor reconstruction.\u003c/p\u003e\u003cp\u003eAna Slide is known to have formed during two stages of failure (Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Although the volume of material mobilized during the secondary failure is not resolved in the seismic data due to limitation of the vertical seismic resolution, a potential volume may be estimated. Assuming the deposit uniformly covers the entire sink area with a thickness of 4 m (just below the vertical seismic resolution of 5 m), the deposit\u0026rsquo;s volume is \u0026lt;\u0026thinsp;0.010 km\u003csup\u003e3\u003c/sup\u003e, hence about a fourth of the volume of Vm at 0.040 km\u003csup\u003e3\u003c/sup\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e5.2. Volume assessment of submarine landslides\u003c/h2\u003e\u003cp\u003eIn this section, we generalize our findings for Ana Slide to be applicable to other events. With increasing resolution of geophysical imaging and a growing number of case studies it becomes evident that submarine landslides are highly complex and inherently different from one another. Nevertheless, what might appear as peculiarities of Ana Slide has recently been recognised as common mechanisms, such as \u003cem\u003ein-situ\u003c/em\u003e substrate deformation (compare section 2). Moreover, Ana Slide\u0026rsquo;s quantitative morphometric parameters align well with global compilations of submarine landslides (H\u0026uuml;hnerbach and Masson, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Moscardelli and Wood, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Clare et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). For these reasons, we suggest that Ana Slide can be assumed representative for many submarine landslides and that our conclusions regarding volume estimations can be used to make inferences on generic volume calculations.\u003c/p\u003e\u003cp\u003eFor the discussion, it is useful to categorize submarine landslides according to their emplacement mechanisms. Here, we distinguish between two endmembers following the terminology and definition by Moscardelli and Wood (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Attached landslides remain connected to the slope by a portion of their original material, while detached landslides become completely separated, leaving a distinct scar or headwall behind.\u003c/p\u003e\u003cp\u003eIn the case a landslide is detached, the source area is fully evacuated and devoid of all mobilized material, so that Vm\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). Vm can also be validated by adding Va and Vt. Consequently, to assess the volume of young detached submarine landslides, bathymetric data are ideal to estimate the volumes of Vm\u003csub\u003evoid\u003c/sub\u003e (and potentially Va) by comparing the modern and reconstructed pre-failure seafloor topographies. Here, any post-slide modifications of the seafloor by external factors, such as vertical tectonic movement or bottom currents, need to be excluded. The bathymetry-based approach will yield more robust values for Vm than approaches based on 2D profiles of sub-seafloor reflection seismic data for three reasons. First, it is more feasible to cover the entire landslide area. Second, estimates for both Vm (difference in the sink area) and Va (difference in the source area) can be made independently and a comparison of the results provides an additional quality control. Ideally, Vm equals Va and certainly, Va should not exceed Vm. Third, when using sub-seafloor reflection seismic data, a potential flaw could be introduced by estimating Va\u003csub\u003ebulk\u003c/sub\u003e instead of Va. This misinterpretation can happen as the internal structure of a submarine landslide may be imaged as chaotic, disrupted, or transparent seismic facies (e.g. Tripsanas et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Similarly, sediments that were disturbed through internal deformation for example by rapid loading (e.g., Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) or shearing induced by passing landslide material (e.g., Sobiesiak et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) also display as chaotic, disrupted, or transparent seismic facies. Hence, seismic facies of Va and Va\u003csub\u003ebulk\u003c/sub\u003e are similar if not identical and therefore it is difficult if not impossible to distinguish material that moved, was translated, or was deformed \u003cem\u003ein-situ\u003c/em\u003e (Sobiesiak et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This is problematic because Va\u003csub\u003ebulk\u003c/sub\u003e may largely exceed Vm (and Va).\u003c/p\u003e\u003cp\u003eIn the case of an attachedlandslide (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), the initial failed volume Vm equals the sum of Vm\u003csub\u003evoid\u003c/sub\u003e and Vm\u003csub\u003er\u003c/sub\u003e, where Vm\u003csub\u003er\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;\u0026gt;\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e. In this case, it is impossible to differentiate between Vm\u003csub\u003er\u003c/sub\u003e and Va. The amount of Vm\u003csub\u003er\u003c/sub\u003e can be calculated between the present-day seafloor and the basal shear surface and thus this approach requires seismic data. Using only bathymetric data will correctly determine Va, but it will underestimate the initial failed volume by the value of Vm\u003csub\u003er\u003c/sub\u003e (Vm\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e + Vm\u003csub\u003er\u003c/sub\u003e). In the analysis of reflection seismic data, care must be taken in the identification of the basal shear surface. There is a risk estimating the sum of mobilized and affected material instead of Va due to the reasons outlined above.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e5.3. How to determine the initial failed volume of submarine landslides?\u003c/h2\u003e\u003cp\u003eBased on the above considerations we here provide a framework and recommendations for assessing the initial failed volume of submarine landslides taking into account available datasets and emplacement mechanism (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The framework applies only to landslide scars outcropping at the seafloor in areas, which have not experienced modification of the seafloor since the occurrence of the landslide. This excludes large multi-retrogressive landslide such as the Storegga Slide with a complex development and overlapping MTDs. Further, vertical tectonic movement, deposition by sediment transport processes, or ocean currents must be excluded, or these influences need to be accounted for in the pre-failure seafloor reconstruction. For instance, this is demonstrated by the horizon-flattening approach applied for pre-failure seafloor reconstruction performed inside the sink area of Ana Slide.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFirst, the pre-failure seafloor in the entire landslide area must be reconstructed using bathymetric data. Then, this reconstructed pre-failure seafloor may act as the upper surface in calculating Vm\u003csub\u003evoid\u003c/sub\u003e inside the source area using the present-day seafloor as the lower surface. Va is calculated in the sink area between the present-day and reconstructed pre-failure seafloor inside the sink area. Now, the suggested workflow deviates according to the emplacement mode (detached or attached).\u003c/p\u003e\u003cp\u003eIn the case the given submarine landslide developed detached (left branch in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), it is beneficial to have reflection seismic data covering the source area so that the amount of remaining landslide material (Vm\u003csub\u003er\u003c/sub\u003e) can be calculated in order to estimate the complete initial failed volume by Vm\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e + Vm\u003csub\u003er\u003c/sub\u003e. Vm can also be estimated from Vm\u0026thinsp;=\u0026thinsp;Va\u0026thinsp;+\u0026thinsp;Vt. In the case no reflection seismic data are available, the rule that Vm\u003csub\u003evoid\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;=\u0026thinsp;Va can serve as an additional constraint. Va cannot be larger than Vm\u003csub\u003evoid\u003c/sub\u003e because the material deposited in the sink area (estimated based on bathymetry) cannot exceed what was evacuated from the source area while Va can decrease if landslide material is transported as a turbidity current (Vt). If this condition, nevertheless, is not fulfilled one should revise the pre-failure seafloor reconstruction until the condition is fulfilled. In both cases, if reflection seismic data are available or not, it is robust to calculate Vm\u003csub\u003evoid\u003c/sub\u003e inside the sink area, because it either represents the amount of initial failed landslide material and represents the amount of Va, or the pre-failure seafloor reconstruction is incorrect assuming that no turbidite transported landslide material (not resolvable in the bathymetric data) and that no erosion and incorporation of seafloor sediment occurred.\u003c/p\u003e\u003cp\u003eIn the case the given submarine landslide developed as an attached landslide (right branch in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) if reflection seismic data are available, the amount of all landslide material involved in or affected by the slope failure can be calculated as the sum of the mobilized and affected material. For attached submarine landslides the sum of the mobilized and affected material is equal to Vm only in the case where no deep deformation has occurred. In that case, the sum of the mobilised and affected material is larger than Vm. Vm\u003csub\u003evoid\u003c/sub\u003e underestimates Vm because the source area is only partly evacuated. In case no reflection seismic data is available, the only means to approximate the initial failed volume of a submarine landslide is by Vm\u003csub\u003evoid\u003c/sub\u003e and Va, both of which will underestimate Vm.\u003c/p\u003e\u003cp\u003eIn case the given landslide developed as a mixed system placed kinematically between the attached or detached cases, or if the emplacement mode is unknown, we suggest estimating Vm\u003csub\u003evoid\u003c/sub\u003e and Va. If the amount of landslide material transported and deposited as a turbidite is neglectable, the amount of Vm\u003csub\u003evoid\u003c/sub\u003e and Va should be the same (Vm\u003csub\u003evoid\u003c/sub\u003e = Va). If Vm\u003csub\u003er\u003c/sub\u003e is unknown, one should consider that Vm\u003csub\u003evoid\u003c/sub\u003e and Va likely underestimate Vm by the unknown amount of Vm\u003csub\u003er\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003eWe showed that for Ana Slide, the sum of the mobilized and affected material estimated on seismic data overestimates Vm by almost one order of magnitude, whereas estimating Vm from V\u003csub\u003evoid\u003c/sub\u003e and/or Va based on bathymetric data alone underestimates Vm by 40%. Due to the risk of excessive overestimation, we suggest that the Vm\u0026thinsp;=\u0026thinsp;Vm\u003csub\u003evoid\u003c/sub\u003e + Ve\u003csub\u003er\u003c/sub\u003e approach should always be preferred, even if Vm\u003csub\u003er\u003c/sub\u003e is unknown.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e5.4. Limitations and assumption of submarine landslide volume assessments\u003c/h2\u003e\u003cp\u003eEstimation of Vm relies on the approach to pre-failure seafloor reconstruction. This may be challenging for submarine landslides in morphologically complex settings and pre-failure seafloor reconstruction might require comprehensive knowledge of the geological, sedimentary, and tectonic setting. For instance, the seafloor morphology of the source area before the failure of Ana Slide was affected by the earlier pre-Ana Slide (Berndt et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Sager et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). It is clear that Vm and Vm\u003csub\u003evoid\u003c/sub\u003e must be equal or larger than Va (if Vt\u0026thinsp;\u0026gt;\u0026thinsp;zero). This constraint can help assess the quality of the seafloor reconstruction at least in one direction.\u003c/p\u003e\u003cp\u003eVolume estimation based on bathymetry and pre-failure seafloor reconstructions also relies on the assumption that the seafloor has not changed significantly since the occurrence of the landslide. Any modifications of the seafloor, for instance by vertical tectonic movements, erosion, deposition by sediment transport, or ocean currents will result in wrong volume estimates. Another assumption is that the volume has been evacuated during one event. If the volume was evacuated during multiple stages with significant time gaps in between the potential hazard will likely be overestimated, although controlled by many other factors such as landslide mechanisms, angle of the slide, water depth, density and cohesion of the landslide material, duration of the slide event, its acceleration, and run-out velocity (e.g., Harbitz et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Tappin et al., 2008).\u003c/p\u003e\u003cp\u003eVa is prone to underestimation because the resolution of bathymetric data and reflection seismic data is typically too low to resolve thin and far travelled turbidites approaching zero thickness in the distal parts, making a clear distinction between Va and Vt difficult. It is only with gravity cores recovered from the distal lobes that underestimation of Va of turbidity current-producing landslides can be prevented.\u003c/p\u003e\u003cp\u003eRecent studies have shown that processes of basal erosion and incorporation of seafloor material can lead to a significant increase in Va (Nugraha et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Sobiesiak et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), but this volume does not represent the volume of initial failed landslide material Ve. The deposit in the sink area might therefore not be a good representation and therefore we advocate to consider Vm, Vm\u003csub\u003er\u003c/sub\u003e, and Vm\u003csub\u003evoid\u003c/sub\u003e to estimate the initial failed volume.\u003c/p\u003e\u003c/div\u003e"},{"header":"6. Conclusions","content":"\u003cp\u003eThe volume of a submarine landslide is critical for the hazard it poses. For tsunami generation, in particular, the initial failed volume is important (Murty, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). We identify the most robust method to estimate the initial failed volume depending on the data set available as well as the landslide`s emplacement mechanism. If no reflection seismic data from the source area of the landslide is available, the initial failed landslide volume Ve can relatively reliable be determined only for fully detached landslides. For attached or mixed systems, this approach will underestimate the true initial failed volume because the amount of landslide material that was mobilized but remained inside the source area is neglected. Seismic data are required to estimate the volume of landslide material that remained inside the source area. If such data are not available, we find that the most robust approximation for the initial failed volume of a detached or mixed-system submarine landslide also is to determine the amount of evacuated landslide material between the pre-failure and present-day seafloors inside the source area from the void space between both surfaces using bathymetric data.\u003c/p\u003e\u003cp\u003eThe initial failed volume has previously been estimated using the seismically identified deposit. It is important to acknowledge that this may be prone to extreme overestimation (more than 200% in this case) because of \u003cem\u003ein-situ\u003c/em\u003e deformation of sediments underlying the pre-failure seafloor in the sink area that may result from rapid deposition or shearing of the accumulating landslide deposits. When estimating the volume from the amount of seismically chaotic, transparent, or disrupted seismic facies, we advocate for balancing this against the initial failed volume estimated from bathymetric data, which yields a more conservative estimate.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMajor thanks and appreciation for the patience of reviewer Prof David Tappin.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe research leading to results presented in this study received funding from Deutsche Forschungsgemeinschaft (DFG) under Grant Agreement number UR 226/3-1.\u0026nbsp;MU acknowledges funding from the European Research Council (ERC) under the European Union\u0026rsquo;s Horizon 2020 research and innovation programme (grant agreement No. 948797).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest/Competing Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose. These authors have no competing interests to declare that are relevant to the content of this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of Datasets and Material\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe 3D reflection seismic data from Ana Slide analyzed in the current study are available at the World Data Centre Pangaea repository\u0026nbsp;(https://doi.pangaea.de/10.1594/PANGAEA.943506), the 2D reflection seismic data are available under (https://doi.pangaea.de/10.1594/PANGAEA.943523), and the multibeam bathymetry data of the study area are available under (https://doi.pangaea.de/10.1594/PANGAEA.953762).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode Availability (Not applicable)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization: TFS, MU, CB; Methodology: MU, TFS; Formal analysis and investigation: TFS; Writing original draft preparation: MU, TFS; Writing \u0026ndash; review and editing: TFS, MU, CB;\u0026nbsp;Visualizations: TFS, MU;\u0026nbsp;Funding acquisition: MU; Resources: CB; Supervision: MU, CB. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003eAdditional Declarations for Articles in Life Science Journals that Report the Results of Studies Involving Humans and/or Animals (not applicable)\u003c/p\u003e\n\u003cp\u003eEthics Approval (not applicable)\u003c/p\u003e\n\u003cp\u003eConsent to Participate (not applicable)\u003c/p\u003e\n\u003cp\u003eConsent for publication (not applicable) \u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBerndt C, Costa S, Canals M, Camerlenghi A, de Mol B, Saunders M (2012) Repeated slope failure linked to fluid migration: The Ana submarine landslide complex, Eivissa Channel, Western Mediterranean Sea. 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Mar Geol 213(1\u0026ndash;4):149\u0026ndash;167. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.margeo.2004.10.005\u003c/span\u003e\u003cspan address=\"10.1016/j.margeo.2004.10.005\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"geo-marine-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"gmle","sideBox":"Learn more about [Geo-Marine Letters](http://link.springer.com/journal/367)","snPcode":"367","submissionUrl":"https://submission.nature.com/new-submission/367/3","title":"Geo-Marine Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"volume assessment, pre-failure seafloor reconstruction, landslide volume, emplacement mechanism","lastPublishedDoi":"10.21203/rs.3.rs-7380838/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7380838/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSubmarine landslides have the potential to be major geohazards as they can destroy seafloor infrastructure such as communication cables and cause tsunamis. The volume of material displaced during the landslide is one factor that determines its hazard and is typically estimated using bathymetric and/or seismic data. Here, we use various established methods to determine the initial failed volume based on a well-constrained case study, the Ana Slide, a small slope failure in the Eivissa Channel off the eastern Iberian Peninsula. We find that, not only, the availability and quality of marine-geophysical data, but also the emplacement mechanism affects how precisely the volume can be estimated. In general, the volume estimation based on comparison of recent and reconstructed pre-failure seafloor topographies yields conservative, yet robust estimates for the volume mobilized. In contrast, volumes estimated from seismic data may be overestimated if the nature of the chaotic, transparent, or disrupted seismic facies commonly used to identify landslide material is unknown.\u003c/p\u003e","manuscriptTitle":"Pitfalls and opportunities in estimating the volumes of small, young submarine landslides","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-01 10:09:24","doi":"10.21203/rs.3.rs-7380838/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-10-02T02:50:54+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-25T07:00:43+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"121269072441048659086450685686227171542","date":"2025-08-31T03:57:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"330438554540318490886666456589951878360","date":"2025-08-30T22:30:52+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-08-21T14:19:33+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-21T13:59:08+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-21T06:21:58+00:00","index":"","fulltext":""},{"type":"submitted","content":"Geo-Marine Letters","date":"2025-08-15T11:00:44+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"geo-marine-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"gmle","sideBox":"Learn more about [Geo-Marine Letters](http://link.springer.com/journal/367)","snPcode":"367","submissionUrl":"https://submission.nature.com/new-submission/367/3","title":"Geo-Marine Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"042413b0-43f3-4be3-8f06-2239efb8e1fb","owner":[],"postedDate":"September 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-11-10T15:59:20+00:00","versionOfRecord":{"articleIdentity":"rs-7380838","link":"https://doi.org/10.1007/s00367-025-00826-4","journal":{"identity":"geo-marine-letters","isVorOnly":false,"title":"Geo-Marine Letters"},"publishedOn":"2025-11-03 15:56:54","publishedOnDateReadable":"November 3rd, 2025"},"versionCreatedAt":"2025-09-01 10:09:24","video":"","vorDoi":"10.1007/s00367-025-00826-4","vorDoiUrl":"https://doi.org/10.1007/s00367-025-00826-4","workflowStages":[]},"version":"v1","identity":"rs-7380838","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7380838","identity":"rs-7380838","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}