{"paper_id":"2d352e6f-6ed2-4e22-8429-a7b41d26b072","body_text":"Imaging and modeling crater floor subsidence at Nyiragongo Volcano, DRC | 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 Imaging and modeling crater floor subsidence at Nyiragongo Volcano, DRC Arthur Wan Ki Lo, Christelle Wauthier, Benoît Smets, Nicolas d’Oreye, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9568081/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract Localized crater deformation can shed insight into shallow magma processes and eruption hazards. To study localized crater deformation at Nyiragongo volcano (Democratic Republic of Congo), we processed Interferometric Synthetic Aperture Radar (InSAR) time series using RADARSAT-2 ultra-fine satellite data spanning 2012–2019. We observed persistent crater floor subsidence during August 2013-December 2015 and inverted the corresponding InSAR displacements to model candidate deformation sources with analytic solutions in a homogeneous elastic half-space. We identified a deflating source modeled as a sill at ~ 100 m depth beneath the crater surface, which we interpreted to be the cooling of magma accumulated in the crater. This observation was possible thanks to a quiescent period in 2013–2015 during which the lava lake did not overflow the bottom of the crater. Our study demonstrates the capabilities of imaging localized deformation patterns using high spatial resolution SAR data. InSAR Photogrammetry Magma reservoir Magma intrusion Lava lake Subsidence Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Ground deformation in active volcanic areas is often associated with changes in magma storage and transport conditions and may precede eruptions (Segall 2019 ). Localized crater deformation captures ground deformation in the shallowest portion of the volcanic plumbing system, and may result from pressure changes in shallow reservoirs, crater or fault instabilities, subsidence of lava flows and intrusions, and hydrothermal activity (Anderson et al. 2015 ; Bemelmans et al. 2023 ; Wauthier and Ho 2024). Imaging such spatially localized deformation with satellite geodesy (Interferometric Synthetic Aperture Radar - InSAR) is challenging when using conventional, publicly available SAR sensors with spatial resolutions of a few tens of meters. Monitoring spatially limited ground deformation requires high spatial resolution SAR datasets (on the order of 1m / pixel) to image deformation (Pritchard et al. 2022 ; Salzer et al. 2014 ). Localized crater deformation has been successfully studied with InSAR for example, at Ol Doinyo Lengai, Tanzania (Wauthier and Ho 2024), Kilauea, Hawaii (Richter et al. 2013), and Volcán de Colima, Mexico (Salzer et al. 2014 ) using fine spatial resolution SAR sensors. Localized deformation events have yielded insight into the structural complexity of shallow plumbing systems and can serve as a crucial precursor for eruptions involving shallow magma reservoirs (Salzer et al. 2014 ). In this study, we imaged multi-year crater floor subsidence at Nyiragongo volcano using InSAR time series processed from RADARSAT-2 ultra-fine spatial resolution mode and interpreted it as deflation of a shallow sill-like source at sub-kilometer depths beneath Nyiragongo’s summit crater. This source potentially represents a combination of multiple cooling lava layers which were emplaced onto the crater during lava lake overflows and spattering. Geological Setting of Nyiragongo Nyiragongo, Democratic Republic of Congo, is an active stratovolcano located in the Virunga Volcanic Province, the only active volcanic province along the Western Branch of the East African Rift (Fig. 1 ). The volcanic complex of Nyiragongo is formed by the superposition of three stratovolcanoes, from North to South: the oldest Baruta edifice (3,148 m a.s.l.), the active Nyiragongo vent (3,470 m a.s.l.), and finally the Shaheru (2,600 m a.s.l.). The volcano, characterized by very steep slopes (20° − 50°) and a summit caldera of ~ 1.2 x 1.3 km hosting a semi-permanent lava lake (Fig. 2 ), is located about 15 km north of the highly populated city of Goma and the adjacent Rwandan city of Gisenyi, both situated on the shore of Lake Kivu (Fig. 1 ). The dense population living south of Nyiragongo makes Nyiragongo one of the most dangerous volcanoes in Africa. An aseismic area from 10 to 14 km below the Nyiragongo edifice was interpreted by Tanaka ( 1983 ) as a main magmatic reservoir. However, another aseismic area was identified between 0 and 14 km beneath the edifice when the lava lake was active and was interpreted by Demant et al. ( 1994 ) as a permanent connection between the lava lake and a major magmatic reservoir. As evidenced by the inverse geodetic modeling of InSAR data during the flank eruption of 2002 (Wauthier et al. 2012 ) and 2021 (Smittarello et al. 2022 ; Walwer et al. 2023 ), the lava lake and magma reservoirs have the potential to hydraulically connect and intrude major dike intrusions aligned with the rift axis towards the South and Lake Kivu (Tazieff 1985 ). Three historical eruptions occurred in 1977 (Tazieff 1977 ), 2002 (Komorowski 2002; Wauthier et al. 2012 ), and 2021 (Smittarello et al. 2022 ). They were all fissural flank eruptions associated with the draining of the summit lava lake and southward propagation of a dike. The most recent one in May 2021 led to the opening of eruptive fissures on the southern flank of Nyiragongo with minimal precursors; the resulting lava flows displaced over 400,000 people and caused 32 deaths (Boudoire et al. 2022 ; Murekezi et al. 2026 ; Smittarello et al. 2022 ). Except for these flank eruptions, Nyiragongo activity is limited to the summit lava lake. Meter-scale subsidence of the crater has previously been observed between 2012 and 2014 using photogrammetry (Smets 2016 ). A new eruptive vent opened on the third platform (P3) of the crater (Fig. 2 ) in February 2016 (Burgi et al. 2018 ). Methods Interferometric Synthetic Aperture Radar (InSAR) Time Series We measured ground deformation in Nyiragongo crater using ascending RADARSAT-2 Ultra-Fine SAR data (5.5 cm wavelength). We used the GAMMA software (Werner et al. 2000 ) to process interferograms without multilooking, and removed topographic phase contributions using a digital elevation model (DEM) resampled to 2 m pixel size from the Shuttle Radar Topography Mission (SRTM) and updated with topography derived from photogrammetry in 2014. We resampled the unwrapped phase interferograms to 10 m pixel size, and performed InSAR time series analysis using the Small Baseline Subset algorithm (Berardino et al. 2002 ; Lundgren et al. 2001 ; Samsonov and d’Oreye 2012). Table 1 and Fig. 3 show parameters used in the InSAR time series. To increase the number of pixels available within the crater, we retained pixels with coherence > 0.16 and excluded pixels in the area occupied by the lava lake in December 2014. While utilizing low-coherence pixels can lead to unwrapping errors, our resulting time series displacement map does not exhibit phase jumps or random noise (see Results – InSAR Time Series), which suggests the time series displacements represent real ground deformation signals instead of decorrelation. We set the temporal reference to be the image from 5 April 2015. At Nyiragongo, the steep topography and active edifice present challenges for identifying a stable reference pixel close to the crater due to stratified tropospheric delay and localized crater deformation. We mitigated this by setting the average displacement of each image to be the spatial reference. We acknowledge this spatial reference assumes a net zero displacement across the image area and can lead to underestimation of spatially uniform displacement signals such as tectonic loading or broad-scale inflation which extends across the image. However, our study examines deformation within the crater of Nyiragongo which occupies a small fraction of the total image area. Although our results may underestimate the absolute displacement occurring in the region associated with regional deformation, the spatial average provides a stable reference for localized deformation within the crater. Table 1 RADARSAT-2 SAR dataset information. Satellite Heading Incidence angle Period of data processed # of scenes processed Range pixel spacing (m) Azimuth pixel spacing (m) RADARSAT-2 (ascending orbit) -12.5077935 37.0642 20120701–20181022 62 1.33 2.09 Geodetic Inversions As surface displacements are a non-linear function of the sources’ geometry and location, non-linear inversions of the deformation sources are required. To invert the displacements data and solve for the most likely models, we used the Neighbourhood algorithm (Sambridge 1999a , 1999b ). In the search stage, the model space is explored by iteratively sampling regions where data are well fitted. Multiple minima are determined. The following misfit (cost) function is used which quantifies the discrepancy between observed and modeled displacements: $$\\:{X}^{2}={\\left({u}_{o}-{u}_{m}\\right)}^{T}{C}^{-1}({u}_{o}-{u}_{m})$$ where u o and u m are vectors of observed and modeled displacements, respectively, and C is the data covariance matrix. We used analytic solutions from the dMODELS package (Battaglia et al. 2013 ) for point source (Mogi 1958 ) and penny-shaped crack (Fialko et al. 2001 ) in an elastic half space. We assumed a Poisson’s ratio of 0.25. The Young’s modulus was assumed to be 5 GPa, corresponding to the value inferred from seismic velocities for the upper few kilometers of the crust in the area (Wauthier et al. 2012 ). For comparison between inversions, we calculated root-mean-square errors (RMSE) for each solution. Results InSAR Time Series We generated time series (Fig. 4 ) calculated from the Line-Of-Sight (LOS) displacement of pixels within a 20 m radius on the rim of the lava lake, centered at -1.52099°, 29.24926° (see Fig. 2 for location). We observed persistent negative LOS displacement throughout 2012–2015, corresponding to the ground moving away from the satellite. The negative trend was interrupted in early 2016, which is associated with the formation of a spatter cone next to the lava lake in March 2016 (Barrière et al. 2022 ). Figure 5 shows the cumulative displacement of our InSAR time series for the period 13 August 2013–25 December 2015. We observed negative LOS displacements of up to ~ 94 cm surrounding the lava lake. The negative deformation was localized on P3 in the crater. Geodetic Inversions The full InSAR time series spans 1 July 2012–22 October 2018, and we selected displacements from the period 13 August 2013–25 December 2015 for data inversions given the temporal resolution of the dataset and the formation of a spatter cone within the crater in February 2016, which covered the crater floor with lava and prevented accurate displacement measurements. The inversion with the lowest misfit (RMSE is 5.5 cm) considers a deflating sill-like source with a radius of 220 m, depth of 100 m below the half space elevation, and an underpressure of ~ 5 MPa, representing a volume change of -1.4 x 10 5 m 3 (Fig. 6 and Table 2 ). The lowest-misfit inversion of the point source (Fig. 7 ) yields model parameters (volume change of -1.2 x 10 5 m 3 and depth of 140 m below the half space elevation) that are comparable to those of the sill inversion but with a notably higher RMSE (8.4 cm). Table 2 Search intervals for geodetic inversions using a point source and a sill, and their lowest-misfit inversion results. The volume change for the sill was calculated in dMODELS using the other model parameters and is included for comparison with the point source. Model parameters Pressure change (MPa) Volume change (m 3 ) Radius (m) Depth (m) X_center, UTM (m) Y_center, UTM (m) RMSE (cm) Search interval (point source) - [-1.0 × 10 6 , -1.0 × 10 3 ] - [50, 3000] [748000, 752000] [9830000, 9833000] - Lowest misfit (Point source) - -1.2 × 10 5 - 140 750286 9831678 8.4 Search interval (Sill) [-10, 0.001] - [10, 300] [50, 3000] [748000, 752000] [9830000, 9833000] - Lowest misfit (Sill) -5 -1.4 × 10 5 220 100 750252 9831668 5.5 Discussion Subsidence at volcanic summits can occur through processes including pressure changes in magma reservoirs, hydrothermal activity, and compaction of cooling magma intrusions or lava (Anderson et al. 2015 ; Bemelmans et al. 2023 ; Wauthier and Ho 2024). Here, we identified persistent multi-year subsidence on a platform in the crater of Nyiragongo volcano. The multi-year subsidence contrasts with sudden and large drops in lava lake elevation at Nyiragongo, which have been attributed to pressure decreases due to gas displacement, as well as magma movement at depth (Barrière et al. 2022 , 2019 ; Bobrowski et al. 2017 ). We consider hydrothermal activity to be an unlikely explanation for the observed subsidence. The presence of an active lava lake forms a steam zone with no mobile liquid water around the magma conduit (Hsieh and Ingebritsen 2019 ). Additionally, deformation associated with pressurization of hydrothermal systems typically is on the order of 10 cm (Bemelmans et al. 2023 ; Hutchison et al. 2016 ), significantly smaller than the crater subsidence observed in this study which is ~ 1 m. Our inversions for the InSAR time series displacements identified a shallow deflating sill-like source at ~ 100 m beneath the crater. Magma bodies contributing to crater deformation at shallow depths of < 500 m below volcanic craters have been reported at Ol Doinyo Lengai (Wauthier and Ho 2024) and Kilauea (Sadeghi Chorsi et al. 2025 ). In particular, the shallow inflating sill intrusion at Kilauea was suggested to have occurred within its active lava lake: models suggested a shallow sill intrusion with volume change of 3.4 x 10 4 m 3 at depths of 10–100 m below the lava lake surface (Sadeghi Chorsi et al. 2025 ). Given that the depth of the lava lake at Nyiragongo is ~ 185 m (P3 elevation during 2013–2015 of 3065 +/- 5 m a.s.l. – lava lake bottom after its drainage and collapse in 2002 of 2880 m a.s.l = 185 m) (Barrière et al. 2022 ), our inversion source is located within the crater assuming our half-space elevation is at the elevation of P3 (Fig. 8 ). Our deformation source has a larger magnitude of volume change (-1.4 x 10 5 m 3 ) and involves deflation compared to the inflating sill in Kilauea’s lava lake (3.4 x 10 4 m 3 ). The intrusion at Kilauea occurred over 90 minutes and may represent small but frequent events, whereas we observed persistent subsidence at Nyiragongo over several years. The persistent crater subsidence at Nyiragongo occurred during a period which Barrière et al. ( 2022 ) characterized as having stable lava lake level at ~ 50 m below the lava lake rim, steady state magma budget, and no significant drops of lava lake level nor overflows. While not fully overlapping with our modelled period, (Barrière et al. 2017 ) found low SO 2 degassing associated with the lava lake over April 2014 to February 2017, comparable to low SO 2 emission rates (~ 10 kg s − 1 ) at Nyiragongo during 2004–2011 (Arellano et al., 2017). This suggests that the deformation was unrelated to pressure changes within magma reservoirs. Our deformation source may alternatively represent a shallow cooling layer. A cooling layer with ~ 0.9 m of subsidence over 2.5 years would have a minimum thickness (Turcotte and Schubert 2002): $$\\:{L}_{i}=\\frac{{\\Delta\\:}L}{\\kappa\\:*{\\Delta\\:}T}$$ where \\(\\:{L}_{i}\\) is initial thickness of the layer, \\(\\:{\\Delta\\:}L\\) is the change in layer thickness, \\(\\:\\kappa\\:\\) is the coefficient of thermal expansion, and \\(\\:{\\Delta\\:}T\\) is the temperature change of the layer. Following Wauthier and Ho (2024), we use \\(\\:\\kappa\\:=\\) 5 x 10 − 5 K − 1 (Huppert and Sparks 1988) and a temperature change from molten lava to ambient temperature at the crater surface of \\(\\:{\\Delta\\:}\\:T\\) = 1150°C − 27.5°C = 1122.5°C. The minimum thickness of ~ 17 m is comparable to the typical thickness of mafic sills and laccoliths with a radius of ~ 200 m (Acocella 2021 ). However, we consider the presence of an old injected sill unlikely given Nyiragongo’s plumbing system and the presence of an active lava lake. Furthermore, a temperature decrease of > 1000°C is unlikely to occur on multi-year timescales. Nyiragongo’s nephelinitic lavas have low viscosities and typically form lava flows with sub-meter thickness (Platz et al. 2004 ), in contrast with our proposed 17-m thick cooling compacting layer. However, lava lake spattering can create levees of > 10 m height when the lava lake level is close to the P3 crater surface, as was observed in 2010 and 2011 (Barrière et al. 2019 ; Burgi et al. 2014 ; Smets et al. 2017 ). Successive overflows and spattering from the lava lake could emplace layers of lava on the crater surface which then cool and compact over time. In this hypothesis, our deflating sill-like deformation source represents a composite signal from multiple layers cooling and compacting over time. Our best-fit depth of ~ 100 m below the elevation of P3 is close to the midpoint of the lava lake depth of 185 m as presented by Barrière et al. ( 2022 ). Our best-fit depth could represent an averaged depth of multiple overflow layers. Our best-fit radius coincides with the radius of the crater at the altitude of ~ 3040 m a.s.l, which was the level of P3 around 2008. This supports the hypothesis that deformation is associated with processes occurring on the sides of the lava lake. Although our results suggest that crater subsidence at Nyiragongo in 2013–2015 was associated with a passive cooling process rather than any concerning change in magmatic activity, monitoring crater deformation remains an important component of forecasting volcanic hazards associated with shallow magma bodies. Crater subsidence preceded the 2017 eruption of Shinmoe-dake volcano (Japan) and was interpreted as conduit failure which eventually triggered the eruption (Himematsu et al. 2024). Our observations of crater deformation were also only possible because there was no lava lake overflow between 2012 and 2015. At Nyiragongo, past flank eruptions have involved draining of the lava lake, while crater deformation switched from deflation to inflation (Fig. 4 ) following the formation of a spatter cone in the crater in February 2016. Notably, Barrière et al. ( 2022 ) suggested that ring fissure formation in 2012–2014 (Smets 2016 ; Fig. 2 ) was an evidence for the occurrence of subsidence and thermal contraction, and the resulting cracks in the rock could have facilitated the formation of the spatter cone. While analysis of the 2016 event is beyond the scope of this study, it demonstrates the importance of monitoring localized crater deformation associated with shallow sources to provide insight on eruption risks, whether within the crater or on the flank of the volcano. Conclusions We presented InSAR time series observations of multi-year crater floor subsidence at Nyiragongo volcano. Our model with the lowest misfit suggested the subsidence was caused by a shallow deflating sill-like source. This may be associated with cooling of several layers of lava accumulated by the lava lake overflows surrounding the lava lake rather than pressure changes in magma reservoirs and illustrates the wide range of deformation sources which can generate localized crater deformation. Our observations were possible due to the rare period of no lava lake overflow between 2012 and 2015, and demonstrate the capability of imaging localized crater deformation using high spatial resolution geodetic datasets. While we found that crater subsidence at Nyiragongo was likely a passive cooling process, we suggest monitoring localized crater deformation associated with shallow magma sources as a key component of evaluating eruption risks at Nyiragongo and volcanoes elsewhere. Declarations Funding This work was supported by National Science Foundation EAR 1923943, EAR 2151005, and CAREER EAR 1945417 grants. Photogrammetric products were created in the frame of the NYALHA (FNR Luxembourg, AFR PhD Grant n°3221321), and MUZUBI (BELSPO STEREO-III Programme, project SR/00/324) projects. Author Contribution A.W.K.L: Investigation, Methodology, Writing - original draft, Formal AnalysisC.W: Conceptualization, Investigation, Methodology, Resources, Funding Acquisition, Formal AnalysisB.S: Investigation, Methodology, Resources, Formal AnalysisN.dO: Investigation, Methodology, Resources, Formal AnalysisJ.B: Resources, Formal AnalysisS.S: Resources, Data CurationAll authors: Writing - review and editing Acknowledgement The authors recognize the Penn State Institute for Computational and Data Sciences (ICDS) (RRID:SCR_025154) for providing access to computational research infrastructure (RRID:SCR_026424). 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Smets, Benoît, Nicolas d’Oreye, Matthieu Kervyn, and François Kervyn. 2017. “Gas Piston Activity of the Nyiragongo Lava Lake: First Insights from a Stereographic Time-Lapse Camera System.” Journal of African Earth Sciences 134 (October): 874–87. https://doi.org/10.1016/j.jafrearsci.2016.04.010. Smittarello, D., B. Smets, J. Barrière, et al. 2022. “Precursor-Free Eruption Triggered by Edifice Rupture at Nyiragongo Volcano.” Nature 609 (7925): 83–88. https://doi.org/10.1038/s41586-022-05047-8. Tanaka, K. 1983. “Seismicity and Focal Mechanism of the Volcanic Earthquakes in the Virunga Volcanic Region.” Volcanoes Nyiragongo and Nyamuragira: Geophysical Aspects . Tazieff, H. 1977. “An Exceptional Eruption: Mt. Niragongo, Jan. 10th, 1977.” Bulletin Volcanologique 40 (3): 189–200. https://doi.org/10.1007/BF02596999. Tazieff, H. 1985. “Recent Activity at Nyiragongo and Lava-like Occurrences.” Bulletin of the Geological Society of Finland 57 (1–2): 11–19. https://doi.org/10.17741/bgsf/57.1-2.001. Turcotte, Donald L., and Gerald Schubert. 2002. Geodynamics . 2nd ed. Cambridge University Press. https://doi.org/10.1017/CBO9780511807442. Walwer, D., C. Wauthier, J. Barrière, D. Smittarello, B. Smets, and N. d’Oreye. 2023. “Modeling the Intermittent Lava Lake Drops Occurring Between 2015 and 2021 at Nyiragongo Volcano.” Geophysical Research Letters 50 (8): e2022GL102365. https://doi.org/10.1029/2022GL102365. Wauthier, C., V. Cayol, F. Kervyn, and N. d’Oreye. 2012. “Magma Sources Involved in the 2002 Nyiragongo Eruption, as Inferred from an InSAR Analysis.” Journal of Geophysical Research: Solid Earth 117 (B5): 2011JB008257. https://doi.org/10.1029/2011JB008257. Wauthier, C., and Cristy Ho. 2024. “Satellite Geodesy Unveils a Decade of Summit Subsidence at Ol Doinyo Lengai Volcano, Tanzania.” Geophysical Research Letters 51 (11): e2023GL107673. https://doi.org/10.1029/2023GL107673. Werner, Charles, Urs Wegmüller, Tazio Strozzi, and Andreas Wiesmann. 2000. GAMMA SAR AND INTERFEROMETRIC PROCESSING SOFTWARE . Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 19 May, 2026 Reviewers invited by journal 07 May, 2026 Editor assigned by journal 01 May, 2026 Submission checks completed at journal 30 Apr, 2026 First submitted to journal 29 Apr, 2026 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. 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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-9568081\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":false,\"archivedVersions\":[],\"articleType\":\"Research Article\",\"associatedPublications\":[],\"authors\":[{\"id\":641533849,\"identity\":\"9e995ed8-256e-43a5-a1be-9f00a8bd1088\",\"order_by\":0,\"name\":\"Arthur Wan Ki Lo\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAoElEQVRIiWNgGAWjYDACHuYGhg8QpgGxWhgbGGeQrIWZhyQtBmcONj623VGX2MDevE2COC1nG5uNc88cTmzgOVZGnBaz84xt0rltBxIbJHLMSNBi2QZ0mPwbYrWcbWyTZmxjBtrCQ6QW+zMHmw17zxw2buNJK7YgSotkT/LBBz931Mn2sx/eeIMoLWDA2MDAwEa8cpiWUTAKRsEoGAU4AQDIYC6OS++DUQAAAABJRU5ErkJggg==\",\"orcid\":\"\",\"institution\":\"Pennsylvania State University\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Arthur\",\"middleName\":\"Wan Ki\",\"lastName\":\"Lo\",\"suffix\":\"\"},{\"id\":641533850,\"identity\":\"a02e20a8-fd28-4560-a862-3ac569b024c2\",\"order_by\":1,\"name\":\"Christelle Wauthier\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Pennsylvania State University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Christelle\",\"middleName\":\"\",\"lastName\":\"Wauthier\",\"suffix\":\"\"},{\"id\":641533851,\"identity\":\"5a3659ce-e9ad-457d-812d-038020c657a7\",\"order_by\":2,\"name\":\"Benoît Smets\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Royal Museum for Central Africa\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Benoît\",\"middleName\":\"\",\"lastName\":\"Smets\",\"suffix\":\"\"},{\"id\":641533852,\"identity\":\"ffa2f687-a1e7-4be2-8fbf-a91065e2ae79\",\"order_by\":3,\"name\":\"Nicolas d’Oreye\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"National Museum of Natural History, Luxembourg\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Nicolas\",\"middleName\":\"\",\"lastName\":\"d’Oreye\",\"suffix\":\"\"},{\"id\":641533853,\"identity\":\"0287d693-9bf7-4764-be73-bd031c8c878e\",\"order_by\":4,\"name\":\"Julien Barrière\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"European Center for Geodynamics and Seismology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Julien\",\"middleName\":\"\",\"lastName\":\"Barrière\",\"suffix\":\"\"},{\"id\":641533855,\"identity\":\"7f10b803-f3d8-4ed4-9844-ccea794643cd\",\"order_by\":5,\"name\":\"Sergey Samsonov\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Natural Resources Canada\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Sergey\",\"middleName\":\"\",\"lastName\":\"Samsonov\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2026-04-29 15:56:18\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-9568081/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-9568081/v1\",\"draftVersion\":[],\"editorialEvents\":[],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":109439951,\"identity\":\"721d9482-2f3a-4d94-86db-04a6cd42498c\",\"added_by\":\"auto\",\"created_at\":\"2026-05-18 07:05:58\",\"extension\":\"jpeg\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":1403329,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eNyiragongo volcano, western branch of the East African Rift System, north of Lake Kivu (Democratic Republic of Congo and Rwanda). Eruptive fissures and lava flows (see the legend) were mapped from radar and optical images (Smets et al. 2016; Smittarello et al. 2022). The lava lake area was mapped from ascending RADARSAT-2 Ultra-Fine satellite imagery (6 December 2014). The extent of Figure 5 is shown by the red dashed box. Coordinates are expressed in UTM (UTM zone 35S)\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage1.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/dc9b2a752a0841e391d5c50e.jpeg\"},{\"id\":109759651,\"identity\":\"4c99db6a-4616-4213-b9f5-a3bcea8a580e\",\"added_by\":\"auto\",\"created_at\":\"2026-05-22 07:27:29\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":8403388,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSummit crater of Nyiragongo showing two inner platforms, Platform 2 (P2) and Platform 3 (P3). \\u0026nbsp;Ground displacements in meters inferred from\\u003cstrong\\u003e \\u003c/strong\\u003ephotogrammetry for the period 8\\u003csup\\u003e \\u003c/sup\\u003eMarch 2013 to 5 July 2014 are shown. The black star indicates the location of the MSBAS time-series shown in Figure 4. The figure is adapted from Smets (2016)\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/5db7ba574231545778d11761.png\"},{\"id\":109439957,\"identity\":\"40615e15-c9e5-4e2a-9bec-28e0ca548ba0\",\"added_by\":\"auto\",\"created_at\":\"2026-05-18 07:05:58\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":32577,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eBaseline plot (Y-axis is the perpendicular baseline which is the perpendicular distance between the two SAR positions; X-axis is time) for InSAR time series. Each green dot is a SAR acquisition, and each grey line represents an interferogram\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/4dd3a9111cfffd2730f96e2d.png\"},{\"id\":109439953,\"identity\":\"a11d303f-528d-43f1-94b9-4e060718a6be\",\"added_by\":\"auto\",\"created_at\":\"2026-05-18 07:05:58\",\"extension\":\"jpeg\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":841754,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003e(a) \\u003c/strong\\u003eSAR amplitude shadow lava lake measurements from Barrière et al. (2022) (red) and LOS displacements from InSAR time-series (blue, location shown in Figure 2). The lava lake measurements are relative to the elevation of P3 (3070 m a.s.l.). The period modelled (13 August 2013 – 25 December 2015) is shown by the solid black lines. A spatter cone forms in the crater in February 2016 (dashed black line).\\u003cstrong\\u003e (b) \\u003c/strong\\u003eImages of P3 of Nyiragongo in 2011 where overflows from the lava lake are visible, 2015 where fissures have formed in the lava layer, and 2017 where the spatter cone and new lava flows are visible, adapted from Barrière et al. (2022)\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage4.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/4ee46277377ff4f105c9ec6f.jpeg\"},{\"id\":109439954,\"identity\":\"5bfff451-329a-41e1-891d-b2ea08796c0b\",\"added_by\":\"auto\",\"created_at\":\"2026-05-18 07:05:58\",\"extension\":\"png\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":444770,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eCumulative displacement map for RADARSAT-2 Ultrafine (ascending) InSAR time series spanning 13 August 2013 – 25 December 2015\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage5.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/e26ba62cfecb3482d9c79334.png\"},{\"id\":109439955,\"identity\":\"33220f62-17c9-4b31-8093-a611e0868349\",\"added_by\":\"auto\",\"created_at\":\"2026-05-18 07:05:58\",\"extension\":\"png\",\"order_by\":6,\"title\":\"Figure 6\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":264667,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eModel for lowest-misfit inversion for a sill source. The location of the center of the sill source is shown by the white circle\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage6.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/fbccbbdba21971a3b9eec2e1.png\"},{\"id\":109759927,\"identity\":\"75e6c602-d5b6-4b83-b2b4-feabbad85449\",\"added_by\":\"auto\",\"created_at\":\"2026-05-22 07:27:56\",\"extension\":\"png\",\"order_by\":7,\"title\":\"Figure 7\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":279439,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eModel for lowest-misfit inversion for a point source. The location of the point source is shown by the white circle\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage7.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/33ad8493ac2cfdab91681967.png\"},{\"id\":109759647,\"identity\":\"d398dafd-514d-4b5e-861d-330524aca51b\",\"added_by\":\"auto\",\"created_at\":\"2026-05-22 07:27:29\",\"extension\":\"png\",\"order_by\":8,\"title\":\"Figure 8\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":1476381,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eVisualization of best-fit model on DEM of Nyiragongo’s crater. The lava lake is shown in red, and our modelled cooling layer in blue. The spatter cone on the east side of P3 formed in 2016\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage8.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/bc43b2daa2eb87b493f2910e.png\"},{\"id\":109800157,\"identity\":\"f5afcf3d-bdd4-4322-aa48-0e9b2cb25f59\",\"added_by\":\"auto\",\"created_at\":\"2026-05-22 15:36:31\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":13606201,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9568081/v1/9a3447d7-bc64-4d5d-b76b-50fa229dc45e.pdf\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"Imaging and modeling crater floor subsidence at Nyiragongo Volcano, DRC\",\"fulltext\":[{\"header\":\"Introduction\",\"content\":\"\\u003cp\\u003eGround deformation in active volcanic areas is often associated with changes in magma storage and transport conditions and may precede eruptions (Segall \\u003cspan class=\\\"CitationRef\\\"\\u003e2019\\u003c/span\\u003e). Localized crater deformation captures ground deformation in the shallowest portion of the volcanic plumbing system, and may result from pressure changes in shallow reservoirs, crater or fault instabilities, subsidence of lava flows and intrusions, and hydrothermal activity (Anderson et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2015\\u003c/span\\u003e; Bemelmans et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2023\\u003c/span\\u003e; Wauthier and Ho 2024). Imaging such spatially localized deformation with satellite geodesy (Interferometric Synthetic Aperture Radar - InSAR) is challenging when using conventional, publicly available SAR sensors with spatial resolutions of a few tens of meters. Monitoring spatially limited ground deformation requires high spatial resolution SAR datasets (on the order of 1m / pixel) to image deformation (Pritchard et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e; Salzer et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2014\\u003c/span\\u003e). Localized crater deformation has been successfully studied with InSAR for example, at Ol Doinyo Lengai, Tanzania (Wauthier and Ho 2024), Kilauea, Hawaii (Richter et al. 2013), and Volcán de Colima, Mexico (Salzer et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2014\\u003c/span\\u003e) using fine spatial resolution SAR sensors. Localized deformation events have yielded insight into the structural complexity of shallow plumbing systems and can serve as a crucial precursor for eruptions involving shallow magma reservoirs (Salzer et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2014\\u003c/span\\u003e). In this study, we imaged multi-year crater floor subsidence at Nyiragongo volcano using InSAR time series processed from RADARSAT-2 ultra-fine spatial resolution mode and interpreted it as deflation of a shallow sill-like source at sub-kilometer depths beneath Nyiragongo’s summit crater. This source potentially represents a combination of multiple cooling lava layers which were emplaced onto the crater during lava lake overflows and spattering.\\u003c/p\\u003e\\n\\u003ch3\\u003eGeological Setting of Nyiragongo\\u003c/h3\\u003e\\n\\u003cp\\u003eNyiragongo, Democratic Republic of Congo, is an active stratovolcano located in the Virunga Volcanic Province, the only active volcanic province along the Western Branch of the East African Rift (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e). The volcanic complex of Nyiragongo is formed by the superposition of three stratovolcanoes, from North to South: the oldest Baruta edifice (3,148 m a.s.l.), the active Nyiragongo vent (3,470 m a.s.l.), and finally the Shaheru (2,600 m a.s.l.). The volcano, characterized by very steep slopes (20° − 50°) and a summit caldera of ~ 1.2 x 1.3 km hosting a semi-permanent lava lake (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e), is located about 15 km north of the highly populated city of Goma and the adjacent Rwandan city of Gisenyi, both situated on the shore of Lake Kivu (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e). The dense population living south of Nyiragongo makes Nyiragongo one of the most dangerous volcanoes in Africa.\\u003c/p\\u003e \\u003cp\\u003eAn aseismic area from 10 to 14 km below the Nyiragongo edifice was interpreted by Tanaka (\\u003cspan class=\\\"CitationRef\\\"\\u003e1983\\u003c/span\\u003e) as a main magmatic reservoir. However, another aseismic area was identified between 0 and 14 km beneath the edifice when the lava lake was active and was interpreted by Demant et al. (\\u003cspan class=\\\"CitationRef\\\"\\u003e1994\\u003c/span\\u003e) as a permanent connection between the lava lake and a major magmatic reservoir. As evidenced by the inverse geodetic modeling of InSAR data during the flank eruption of 2002 (Wauthier et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2012\\u003c/span\\u003e) and 2021 (Smittarello et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e; Walwer et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2023\\u003c/span\\u003e), the lava lake and magma reservoirs have the potential to hydraulically connect and intrude major dike intrusions aligned with the rift axis towards the South and Lake Kivu (Tazieff \\u003cspan class=\\\"CitationRef\\\"\\u003e1985\\u003c/span\\u003e). Three historical eruptions occurred in 1977 (Tazieff \\u003cspan class=\\\"CitationRef\\\"\\u003e1977\\u003c/span\\u003e), 2002 (Komorowski 2002; Wauthier et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2012\\u003c/span\\u003e), and 2021 (Smittarello et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e). They were all fissural flank eruptions associated with the draining of the summit lava lake and southward propagation of a dike. The most recent one in May 2021 led to the opening of eruptive fissures on the southern flank of Nyiragongo with minimal precursors; the resulting lava flows displaced over 400,000 people and caused 32 deaths (Boudoire et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e; Murekezi et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2026\\u003c/span\\u003e; Smittarello et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e). Except for these flank eruptions, Nyiragongo activity is limited to the summit lava lake. Meter-scale subsidence of the crater has previously been observed between 2012 and 2014 using photogrammetry (Smets \\u003cspan class=\\\"CitationRef\\\"\\u003e2016\\u003c/span\\u003e). A new eruptive vent opened on the third platform (P3) of the crater (Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e) in February 2016 (Burgi et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2018\\u003c/span\\u003e).\\u003c/p\\u003e\"},{\"header\":\"Methods\",\"content\":\"\\u003ch2\\u003eInterferometric Synthetic Aperture Radar (InSAR) Time Series\\u003c/h2\\u003e\\u003cp\\u003eWe measured ground deformation in Nyiragongo crater using ascending RADARSAT-2 Ultra-Fine SAR data (5.5 cm wavelength). We used the GAMMA software (Werner et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2000\\u003c/span\\u003e) to process interferograms without multilooking, and removed topographic phase contributions using a digital elevation model (DEM) resampled to 2 m pixel size from the Shuttle Radar Topography Mission (SRTM) and updated with topography derived from photogrammetry in 2014. We resampled the unwrapped phase interferograms to 10 m pixel size, and performed InSAR time series analysis using the Small Baseline Subset algorithm (Berardino et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2002\\u003c/span\\u003e; Lundgren et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2001\\u003c/span\\u003e; Samsonov and d’Oreye 2012). Table\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e and Fig.\\u0026nbsp;\\u003cspan class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e show parameters used in the InSAR time series. To increase the number of pixels available within the crater, we retained pixels with coherence \\u0026gt; 0.16 and excluded pixels in the area occupied by the lava lake in December 2014. While utilizing low-coherence pixels can lead to unwrapping errors, our resulting time series displacement map does not exhibit phase jumps or random noise (see Results – InSAR Time Series), which suggests the time series displacements represent real ground deformation signals instead of decorrelation. We set the temporal reference to be the image from 5 April 2015. At Nyiragongo, the steep topography and active edifice present challenges for identifying a stable reference pixel close to the crater due to stratified tropospheric delay and localized crater deformation. We mitigated this by setting the average displacement of each image to be the spatial reference. We acknowledge this spatial reference assumes a net zero displacement across the image area and can lead to underestimation of spatially uniform displacement signals such as tectonic loading or broad-scale inflation which extends across the image. However, our study examines deformation within the crater of Nyiragongo which occupies a small fraction of the total image area. Although our results may underestimate the absolute displacement occurring in the region associated with regional deformation, the spatial average provides a stable reference for localized deformation within the crater.\\u003c/p\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\"\\u003e\\u003c/div\\u003e\\u003ctable id=\\\"Tab1\\\" border=\\\"1\\\"\\u003e \\u003ccaption\\u003e \\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 1\\u003c/div\\u003e \\u003cdiv class=\\\"CaptionContent\\\"\\u003e \\u003cp\\u003eRADARSAT-2 SAR dataset information.\\u003c/p\\u003e \\u003c/div\\u003e \\u003c/caption\\u003e \\u003ccolgroup cols=\\\"7\\\"\\u003e \\u003c/colgroup\\u003e \\u003cthead\\u003e \\u003ctr\\u003e \\u003cth align=\\\"left\\\"\\u003e \\u003cp\\u003eSatellite\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\"\\u003e \\u003cp\\u003eHeading\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\"\\u003e \\u003cp\\u003eIncidence angle\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\"\\u003e \\u003cp\\u003ePeriod of data processed\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\"\\u003e \\u003cp\\u003e# of scenes processed\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\"\\u003e \\u003cp\\u003eRange pixel spacing (m)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\"\\u003e \\u003cp\\u003eAzimuth pixel spacing (m)\\u003c/p\\u003e \\u003c/th\\u003e \\u003c/tr\\u003e \\u003c/thead\\u003e \\u003ctbody\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\"\\u003e \\u003cp\\u003eRADARSAT-2\\u003c/p\\u003e \\u003cp\\u003e(ascending orbit)\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\"\\u003e \\u003cp\\u003e-12.5077935\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\"\\u003e \\u003cp\\u003e37.0642\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\"\\u003e \\u003cp\\u003e20120701–20181022\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\"\\u003e \\u003cp\\u003e62\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\"\\u003e \\u003cp\\u003e1.33\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\"\\u003e \\u003cp\\u003e2.09\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003c/tbody\\u003e \\u003c/table\\u003e\\u003c/div\\u003e\\u003ch3\\u003eGeodetic Inversions\\u003c/h3\\u003e\\u003cp\\u003eAs surface displacements are a non-linear function of the sources’ geometry and location, non-linear inversions of the deformation sources are required. To invert the displacements data and solve for the most likely models, we used the Neighbourhood algorithm (Sambridge \\u003cspan class=\\\"CitationRef\\\"\\u003e1999a\\u003c/span\\u003e, \\u003cspan class=\\\"CitationRef\\\"\\u003e1999b\\u003c/span\\u003e). In the search stage, the model space is explored by iteratively sampling regions where data are well fitted. Multiple minima are determined. The following misfit (cost) function is used which quantifies the discrepancy between observed and modeled displacements:\\u003c/p\\u003e\\u003cdiv id=\\\"Equa\\\" class=\\\"Equation\\\"\\u003e\\u003cdiv class=\\\"mathdisplay\\\" id=\\\"FileID_Equa\\\" name=\\\"EquationSource\\\"\\u003e\\n$$\\\\:{X}^{2}={\\\\left({u}_{o}-{u}_{m}\\\\right)}^{T}{C}^{-1}({u}_{o}-{u}_{m})$$\\u003c/div\\u003e\\u003c/div\\u003e\\u003cp\\u003ewhere \\u003cem\\u003eu\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eo\\u003c/em\\u003e\\u003c/sub\\u003e and \\u003cem\\u003eu\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003em\\u003c/em\\u003e\\u003c/sub\\u003e are vectors of observed and modeled displacements, respectively, and C is the data covariance matrix.\\u003c/p\\u003e\\u003cp\\u003eWe used analytic solutions from the dMODELS package (Battaglia et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2013\\u003c/span\\u003e) for point source (Mogi \\u003cspan class=\\\"CitationRef\\\"\\u003e1958\\u003c/span\\u003e) and penny-shaped crack (Fialko et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2001\\u003c/span\\u003e) in an elastic half space. We assumed a Poisson’s ratio of 0.25. The Young’s modulus was assumed to be 5 GPa, corresponding to the value inferred from seismic velocities for the upper few kilometers of the crust in the area (Wauthier et al. \\u003cspan class=\\\"CitationRef\\\"\\u003e2012\\u003c/span\\u003e). For comparison between inversions, we calculated root-mean-square errors (RMSE) for each solution.\\u003c/p\\u003e\"},{\"header\":\"Results\",\"content\":\"\\u003cdiv id=\\\"Sec7\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eInSAR Time Series\\u003c/h2\\u003e \\u003cp\\u003eWe generated time series (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e) calculated from the Line-Of-Sight (LOS) displacement of pixels within a 20 m radius on the rim of the lava lake, centered at -1.52099\\u0026deg;, 29.24926\\u0026deg; (see Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e for location). We observed persistent negative LOS displacement throughout 2012\\u0026ndash;2015, corresponding to the ground moving away from the satellite. The negative trend was interrupted in early 2016, which is associated with the formation of a spatter cone next to the lava lake in March 2016 (Barri\\u0026egrave;re et al. \\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e).\\u003c/p\\u003e \\u003cp\\u003eFigure \\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e shows the cumulative displacement of our InSAR time series for the period 13 August 2013\\u0026ndash;25 December 2015. We observed negative LOS displacements of up to ~\\u0026thinsp;94 cm surrounding the lava lake. The negative deformation was localized on P3 in the crater.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec8\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eGeodetic Inversions\\u003c/h2\\u003e \\u003cp\\u003eThe full InSAR time series spans 1 July 2012\\u0026ndash;22 October 2018, and we selected displacements from the period 13 August 2013\\u0026ndash;25 December 2015 for data inversions given the temporal resolution of the dataset and the formation of a spatter cone within the crater in February 2016, which covered the crater floor with lava and prevented accurate displacement measurements. The inversion with the lowest misfit (RMSE is 5.5 cm) considers a deflating sill-like source with a radius of 220 m, depth of 100 m below the half space elevation, and an underpressure of ~\\u0026thinsp;5 MPa, representing a volume change of -1.4 x 10\\u003csup\\u003e5\\u003c/sup\\u003e m\\u003csup\\u003e3\\u003c/sup\\u003e (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e and Table\\u0026nbsp;\\u003cspan refid=\\\"Tab2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e). The lowest-misfit inversion of the point source (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003e) yields model parameters (volume change of -1.2 x 10\\u003csup\\u003e5\\u003c/sup\\u003e m\\u003csup\\u003e3\\u003c/sup\\u003e and depth of 140 m below the half space elevation) that are comparable to those of the sill inversion but with a notably higher RMSE (8.4 cm).\\u003c/p\\u003e \\u003cp\\u003e \\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab2\\\" border=\\\"1\\\"\\u003e \\u003ccaption language=\\\"En\\\"\\u003e \\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 2\\u003c/div\\u003e \\u003cdiv class=\\\"CaptionContent\\\"\\u003e \\u003cp\\u003eSearch intervals for geodetic inversions using a point source and a sill, and their lowest-misfit inversion results. The volume change for the sill was calculated in dMODELS using the other model parameters and is included for comparison with the point source.\\u003c/p\\u003e \\u003c/div\\u003e \\u003c/caption\\u003e \\u003ccolgroup cols=\\\"8\\\"\\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=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c6\\\" colnum=\\\"6\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c7\\\" colnum=\\\"7\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c8\\\" colnum=\\\"8\\\"\\u003e\\u003c/div\\u003e \\u003cthead\\u003e \\u003ctr\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eModel parameters\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003ePressure change (MPa)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003eVolume change (m\\u003csup\\u003e3\\u003c/sup\\u003e)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003eRadius (m)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003eDepth (m)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003eX_center, UTM (m)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c7\\\"\\u003e \\u003cp\\u003eY_center, UTM (m)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c8\\\"\\u003e \\u003cp\\u003eRMSE (cm)\\u003c/p\\u003e \\u003c/th\\u003e \\u003c/tr\\u003e \\u003c/thead\\u003e \\u003ctbody\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eSearch interval\\u003c/p\\u003e \\u003cp\\u003e(point source)\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e[-1.0 \\u0026times; 10\\u003csup\\u003e6\\u003c/sup\\u003e,\\u003c/p\\u003e \\u003cp\\u003e-1.0 \\u0026times; 10\\u003csup\\u003e3\\u003c/sup\\u003e]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e[50, 3000]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e[748000, 752000]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c7\\\"\\u003e \\u003cp\\u003e[9830000, 9833000]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c8\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eLowest misfit\\u003c/p\\u003e \\u003cp\\u003e(Point source)\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e-1.2 \\u0026times; 10\\u003csup\\u003e5\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e140\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e750286\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c7\\\"\\u003e \\u003cp\\u003e9831678\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c8\\\"\\u003e \\u003cp\\u003e8.4\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eSearch interval\\u003c/p\\u003e \\u003cp\\u003e(Sill)\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e[-10, 0.001]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e[10, 300]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e[50, 3000]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e[748000, 752000]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c7\\\"\\u003e \\u003cp\\u003e[9830000, 9833000]\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c8\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eLowest misfit\\u003c/p\\u003e \\u003cp\\u003e(Sill)\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e-5\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e-1.4 \\u0026times; 10\\u003csup\\u003e5\\u003c/sup\\u003e\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e220\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e100\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e750252\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c7\\\"\\u003e \\u003cp\\u003e9831668\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c8\\\"\\u003e \\u003cp\\u003e5.5\\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\":\"Discussion\",\"content\":\"\\u003cp\\u003eSubsidence at volcanic summits can occur through processes including pressure changes in magma reservoirs, hydrothermal activity, and compaction of cooling magma intrusions or lava (Anderson et al. \\u003cspan citationid=\\\"CR2\\\" class=\\\"CitationRef\\\"\\u003e2015\\u003c/span\\u003e; Bemelmans et al. \\u003cspan citationid=\\\"CR7\\\" class=\\\"CitationRef\\\"\\u003e2023\\u003c/span\\u003e; Wauthier and Ho 2024). Here, we identified persistent multi-year subsidence on a platform in the crater of Nyiragongo volcano. The multi-year subsidence contrasts with sudden and large drops in lava lake elevation at Nyiragongo, which have been attributed to pressure decreases due to gas displacement, as well as magma movement at depth (Barri\\u0026egrave;re et al. \\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e2019\\u003c/span\\u003e; Bobrowski et al. \\u003cspan citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e2017\\u003c/span\\u003e).\\u003c/p\\u003e \\u003cp\\u003eWe consider hydrothermal activity to be an unlikely explanation for the observed subsidence. The presence of an active lava lake forms a steam zone with no mobile liquid water around the magma conduit (Hsieh and Ingebritsen \\u003cspan citationid=\\\"CR16\\\" class=\\\"CitationRef\\\"\\u003e2019\\u003c/span\\u003e). Additionally, deformation associated with pressurization of hydrothermal systems typically is on the order of 10 cm (Bemelmans et al. \\u003cspan citationid=\\\"CR7\\\" class=\\\"CitationRef\\\"\\u003e2023\\u003c/span\\u003e; Hutchison et al. \\u003cspan citationid=\\\"CR18\\\" class=\\\"CitationRef\\\"\\u003e2016\\u003c/span\\u003e), significantly smaller than the crater subsidence observed in this study which is ~\\u0026thinsp;1 m.\\u003c/p\\u003e \\u003cp\\u003eOur inversions for the InSAR time series displacements identified a shallow deflating sill-like source at ~\\u0026thinsp;100 m beneath the crater. Magma bodies contributing to crater deformation at shallow depths of \\u0026lt;\\u0026thinsp;500 m below volcanic craters have been reported at Ol Doinyo Lengai (Wauthier and Ho 2024) and Kilauea (Sadeghi Chorsi et al. \\u003cspan citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e2025\\u003c/span\\u003e). In particular, the shallow inflating sill intrusion at Kilauea was suggested to have occurred within its active lava lake: models suggested a shallow sill intrusion with volume change of 3.4 x 10\\u003csup\\u003e4\\u003c/sup\\u003e m\\u003csup\\u003e3\\u003c/sup\\u003e at depths of 10\\u0026ndash;100 m below the lava lake surface (Sadeghi Chorsi et al. \\u003cspan citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e2025\\u003c/span\\u003e). Given that the depth of the lava lake at Nyiragongo is ~\\u0026thinsp;185 m (P3 elevation during 2013\\u0026ndash;2015 of 3065 +/- 5 m a.s.l. \\u0026ndash; lava lake bottom after its drainage and collapse in 2002 of 2880 m a.s.l\\u0026thinsp;=\\u0026thinsp;185 m) (Barri\\u0026egrave;re et al. \\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e), our inversion source is located within the crater assuming our half-space elevation is at the elevation of P3 (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003e). Our deformation source has a larger magnitude of volume change (-1.4 x 10\\u003csup\\u003e5\\u003c/sup\\u003e m\\u003csup\\u003e3\\u003c/sup\\u003e) and involves deflation compared to the inflating sill in Kilauea\\u0026rsquo;s lava lake (3.4 x 10\\u003csup\\u003e4\\u003c/sup\\u003e m\\u003csup\\u003e3\\u003c/sup\\u003e). The intrusion at Kilauea occurred over 90 minutes and may represent small but frequent events, whereas we observed persistent subsidence at Nyiragongo over several years.\\u003c/p\\u003e \\u003cp\\u003eThe persistent crater subsidence at Nyiragongo occurred during a period which Barri\\u0026egrave;re et al. (\\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e) characterized as having stable lava lake level at ~\\u0026thinsp;50 m below the lava lake rim, steady state magma budget, and no significant drops of lava lake level nor overflows. While not fully overlapping with our modelled period, (Barri\\u0026egrave;re et al. \\u003cspan citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e2017\\u003c/span\\u003e) found low SO\\u003csub\\u003e2\\u003c/sub\\u003e degassing associated with the lava lake over April 2014 to February 2017, comparable to low SO\\u003csub\\u003e2\\u003c/sub\\u003e emission rates (~\\u0026thinsp;10 kg s\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e) at Nyiragongo during 2004\\u0026ndash;2011 (Arellano et al., 2017). This suggests that the deformation was unrelated to pressure changes within magma reservoirs. Our deformation source may alternatively represent a shallow cooling layer. A cooling layer with ~\\u0026thinsp;0.9 m of subsidence over 2.5 years would have a minimum thickness (Turcotte and Schubert 2002):\\u003cdiv id=\\\"Equb\\\" class=\\\"Equation\\\"\\u003e\\u003cdiv format=\\\"TEX\\\" class=\\\"mathdisplay\\\" id=\\\"FileID_Equb\\\" name=\\\"EquationSource\\\"\\u003e\\n$$\\\\:{L}_{i}=\\\\frac{{\\\\Delta\\\\:}L}{\\\\kappa\\\\:*{\\\\Delta\\\\:}T}$$\\u003c/div\\u003e\\u003c/div\\u003e\\u003c/p\\u003e \\u003cp\\u003ewhere \\u003cspan class=\\\"InlineEquation\\\"\\u003e\\u003cspan class=\\\"mathinline\\\"\\u003e\\\\(\\\\:{L}_{i}\\\\)\\u003c/span\\u003e\\u003c/span\\u003e is initial thickness of the layer, \\u003cspan class=\\\"InlineEquation\\\"\\u003e\\u003cspan class=\\\"mathinline\\\"\\u003e\\\\(\\\\:{\\\\Delta\\\\:}L\\\\)\\u003c/span\\u003e\\u003c/span\\u003e is the change in layer thickness, \\u003cspan class=\\\"InlineEquation\\\"\\u003e\\u003cspan class=\\\"mathinline\\\"\\u003e\\\\(\\\\:\\\\kappa\\\\:\\\\)\\u003c/span\\u003e\\u003c/span\\u003e is the coefficient of thermal expansion, and \\u003cspan class=\\\"InlineEquation\\\"\\u003e\\u003cspan class=\\\"mathinline\\\"\\u003e\\\\(\\\\:{\\\\Delta\\\\:}T\\\\)\\u003c/span\\u003e\\u003c/span\\u003e is the temperature change of the layer.\\u003c/p\\u003e \\u003cp\\u003eFollowing Wauthier and Ho (2024), we use \\u003cspan class=\\\"InlineEquation\\\"\\u003e\\u003cspan class=\\\"mathinline\\\"\\u003e\\\\(\\\\:\\\\kappa\\\\:=\\\\)\\u003c/span\\u003e\\u003c/span\\u003e 5 x 10\\u003csup\\u003e\\u0026minus;\\u0026thinsp;5\\u003c/sup\\u003e K\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e (Huppert and Sparks 1988) and a temperature change from molten lava to ambient temperature at the crater surface of \\u003cspan class=\\\"InlineEquation\\\"\\u003e\\u003cspan class=\\\"mathinline\\\"\\u003e\\\\(\\\\:{\\\\Delta\\\\:}\\\\:T\\\\)\\u003c/span\\u003e\\u003c/span\\u003e = 1150\\u0026deg;C \\u0026minus;\\u0026thinsp;27.5\\u0026deg;C = 1122.5\\u0026deg;C. The minimum thickness of ~\\u0026thinsp;17 m is comparable to the typical thickness of mafic sills and laccoliths with a radius of ~\\u0026thinsp;200 m (Acocella \\u003cspan citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e2021\\u003c/span\\u003e). However, we consider the presence of an old injected sill unlikely given Nyiragongo\\u0026rsquo;s plumbing system and the presence of an active lava lake. Furthermore, a temperature decrease of \\u0026gt; 1000\\u0026deg;C is unlikely to occur on multi-year timescales.\\u003c/p\\u003e \\u003cp\\u003eNyiragongo\\u0026rsquo;s nephelinitic lavas have low viscosities and typically form lava flows with sub-meter thickness (Platz et al. \\u003cspan citationid=\\\"CR23\\\" class=\\\"CitationRef\\\"\\u003e2004\\u003c/span\\u003e), in contrast with our proposed 17-m thick cooling compacting layer. However, lava lake spattering can create levees of \\u0026gt;\\u0026thinsp;10 m height when the lava lake level is close to the P3 crater surface, as was observed in 2010 and 2011 (Barri\\u0026egrave;re et al. \\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e2019\\u003c/span\\u003e; Burgi et al. \\u003cspan citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e2014\\u003c/span\\u003e; Smets et al. \\u003cspan citationid=\\\"CR34\\\" class=\\\"CitationRef\\\"\\u003e2017\\u003c/span\\u003e). Successive overflows and spattering from the lava lake could emplace layers of lava on the crater surface which then cool and compact over time. In this hypothesis, our deflating sill-like deformation source represents a composite signal from multiple layers cooling and compacting over time. Our best-fit depth of ~\\u0026thinsp;100 m below the elevation of P3 is close to the midpoint of the lava lake depth of 185 m as presented by Barri\\u0026egrave;re et al. (\\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e). Our best-fit depth could represent an averaged depth of multiple overflow layers. Our best-fit radius coincides with the radius of the crater at the altitude of ~\\u0026thinsp;3040 m a.s.l, which was the level of P3 around 2008. This supports the hypothesis that deformation is associated with processes occurring on the sides of the lava lake.\\u003c/p\\u003e \\u003cp\\u003eAlthough our results suggest that crater subsidence at Nyiragongo in 2013\\u0026ndash;2015 was associated with a passive cooling process rather than any concerning change in magmatic activity, monitoring crater deformation remains an important component of forecasting volcanic hazards associated with shallow magma bodies. Crater subsidence preceded the 2017 eruption of Shinmoe-dake volcano (Japan) and was interpreted as conduit failure which eventually triggered the eruption (Himematsu et al. 2024). Our observations of crater deformation were also only possible because there was no lava lake overflow between 2012 and 2015. At Nyiragongo, past flank eruptions have involved draining of the lava lake, while crater deformation switched from deflation to inflation (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e) following the formation of a spatter cone in the crater in February 2016. Notably, Barri\\u0026egrave;re et al. (\\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e2022\\u003c/span\\u003e) suggested that ring fissure formation in 2012\\u0026ndash;2014 (Smets \\u003cspan citationid=\\\"CR32\\\" class=\\\"CitationRef\\\"\\u003e2016\\u003c/span\\u003e; Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e) was an evidence for the occurrence of subsidence and thermal contraction, and the resulting cracks in the rock could have facilitated the formation of the spatter cone. While analysis of the 2016 event is beyond the scope of this study, it demonstrates the importance of monitoring localized crater deformation associated with shallow sources to provide insight on eruption risks, whether within the crater or on the flank of the volcano.\\u003c/p\\u003e\"},{\"header\":\"Conclusions\",\"content\":\"\\u003cp\\u003eWe presented InSAR time series observations of multi-year crater floor subsidence at Nyiragongo volcano. Our model with the lowest misfit suggested the subsidence was caused by a shallow deflating sill-like source. This may be associated with cooling of several layers of lava accumulated by the lava lake overflows surrounding the lava lake rather than pressure changes in magma reservoirs and illustrates the wide range of deformation sources which can generate localized crater deformation. Our observations were possible due to the rare period of no lava lake overflow between 2012 and 2015, and demonstrate the capability of imaging localized crater deformation using high spatial resolution geodetic datasets. While we found that crater subsidence at Nyiragongo was likely a passive cooling process, we suggest monitoring localized crater deformation associated with shallow magma sources as a key component of evaluating eruption risks at Nyiragongo and volcanoes elsewhere.\\u003cdiv class=\\\"BlockQuote\\\"\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003ch2\\u003eFunding\\u003c/h2\\u003e \\u003cp\\u003eThis work was supported by National Science Foundation EAR 1923943, EAR 2151005, and CAREER EAR 1945417 grants. Photogrammetric products were created in the frame of the NYALHA (FNR Luxembourg, AFR PhD Grant n\\u0026deg;3221321), and MUZUBI (BELSPO STEREO-III Programme, project SR/00/324) projects.\\u003c/p\\u003e\\u003ch2\\u003eAuthor Contribution\\u003c/h2\\u003e\\u003cp\\u003eA.W.K.L: Investigation, Methodology, Writing - original draft, Formal AnalysisC.W: Conceptualization, Investigation, Methodology, Resources, Funding Acquisition, Formal AnalysisB.S: Investigation, Methodology, Resources, Formal AnalysisN.dO: Investigation, Methodology, Resources, Formal AnalysisJ.B: Resources, Formal AnalysisS.S: Resources, Data CurationAll authors: Writing - review and editing\\u003c/p\\u003e\\u003ch2\\u003eAcknowledgement\\u003c/h2\\u003e\\u003cp\\u003eThe authors recognize the Penn State Institute for Computational and Data Sciences (ICDS) (RRID:SCR_025154) for providing access to computational research infrastructure (RRID:SCR_026424). We would like to thank the Canadian Space Agency for providing RADARSAT-2 SAR data.\\u003c/p\\u003e\\u003ch2\\u003eData Availability\\u003c/h2\\u003e\\u003cp\\u003eData and codes used for geodetic inversions are available through Penn State ScholarSphere (doi:10.26207/djy6-mq63).\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\n\\u003cli\\u003eAcocella, Valerio. 2021. \\u0026ldquo;Magma Emplacement and Accumulation: From Sills to Magma Chambers.\\u0026rdquo; In \\u003cem\\u003eVolcano-Tectonic Processes\\u003c/em\\u003e, by Valerio Acocella. Advances in Volcanology. Springer International Publishing. https://doi.org/10.1007/978-3-030-65968-4_4.\\u003c/li\\u003e\\n\\u003cli\\u003eAnderson, Kyle R., Michael P. Poland, Jessica H. Johnson, and Asta Miklius. 2015. \\u0026ldquo;Episodic Deflation\\u0026ndash;Inflation Events at Kīlauea Volcano and Implications for the Shallow Magma System.\\u0026rdquo; In \\u003cem\\u003eHawaiian Volcanoes\\u003c/em\\u003e. American Geophysical Union (AGU). https://doi.org/10.1002/9781118872079.ch11.\\u003c/li\\u003e\\n\\u003cli\\u003eBarri\\u0026egrave;re, Julien, Nicolas d\\u0026rsquo;Oreye, Adrien Oth, et al. 2019. \\u0026ldquo;Seismicity and Outgassing Dynamics of Nyiragongo Volcano.\\u0026rdquo; \\u003cem\\u003eEarth and Planetary Science Letters\\u003c/em\\u003e 528 (December): 115821. https://doi.org/10.1016/j.epsl.2019.115821.\\u003c/li\\u003e\\n\\u003cli\\u003eBarri\\u0026egrave;re, Julien, Nicolas d\\u0026rsquo;Oreye, Beno\\u0026icirc;t Smets, et al. 2022. \\u0026ldquo;Intra-Crater Eruption Dynamics at Nyiragongo (D.R. 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Niragongo, Jan. 10th, 1977.\\u0026rdquo; \\u003cem\\u003eBulletin Volcanologique\\u003c/em\\u003e 40 (3): 189\\u0026ndash;200. https://doi.org/10.1007/BF02596999.\\u003c/li\\u003e\\n\\u003cli\\u003eTazieff, H. 1985. \\u0026ldquo;Recent Activity at Nyiragongo and Lava-like Occurrences.\\u0026rdquo; \\u003cem\\u003eBulletin of the Geological Society of Finland\\u003c/em\\u003e 57 (1\\u0026ndash;2): 11\\u0026ndash;19. https://doi.org/10.17741/bgsf/57.1-2.001.\\u003c/li\\u003e\\n\\u003cli\\u003eTurcotte, Donald L., and Gerald Schubert. 2002. \\u003cem\\u003eGeodynamics\\u003c/em\\u003e. 2nd ed. Cambridge University Press. https://doi.org/10.1017/CBO9780511807442.\\u003c/li\\u003e\\n\\u003cli\\u003eWalwer, D., C. Wauthier, J. Barri\\u0026egrave;re, D. Smittarello, B. Smets, and N. d\\u0026rsquo;Oreye. 2023. \\u0026ldquo;Modeling the Intermittent Lava Lake Drops Occurring Between 2015 and 2021 at Nyiragongo Volcano.\\u0026rdquo; \\u003cem\\u003eGeophysical Research Letters\\u003c/em\\u003e 50 (8): e2022GL102365. https://doi.org/10.1029/2022GL102365.\\u003c/li\\u003e\\n\\u003cli\\u003eWauthier, C., V. Cayol, F. Kervyn, and N. d\\u0026rsquo;Oreye. 2012. \\u0026ldquo;Magma Sources Involved in the 2002 Nyiragongo Eruption, as Inferred from an InSAR Analysis.\\u0026rdquo; \\u003cem\\u003eJournal of Geophysical Research: Solid Earth\\u003c/em\\u003e 117 (B5): 2011JB008257. https://doi.org/10.1029/2011JB008257.\\u003c/li\\u003e\\n\\u003cli\\u003eWauthier, C., and Cristy Ho. 2024. \\u0026ldquo;Satellite Geodesy Unveils a Decade of Summit Subsidence at Ol Doinyo Lengai Volcano, Tanzania.\\u0026rdquo; \\u003cem\\u003eGeophysical Research Letters\\u003c/em\\u003e 51 (11): e2023GL107673. https://doi.org/10.1029/2023GL107673.\\u003c/li\\u003e\\n\\u003cli\\u003eWerner, Charles, Urs Wegm\\u0026uuml;ller, Tazio Strozzi, and Andreas Wiesmann. 2000. \\u003cem\\u003eGAMMA SAR AND INTERFEROMETRIC PROCESSING SOFTWARE\\u003c/em\\u003e.\\u003c/li\\u003e\\n\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":false,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":false,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"bulletin-of-volcanology\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"buvo\",\"sideBox\":\"Learn more about [Bulletin of Volcanology](http://link.springer.com/journal/445)\",\"snPcode\":\"445\",\"submissionUrl\":\"https://www.editorialmanager.com/buvo/default2.aspx\",\"title\":\"Bulletin of Volcanology\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"em\",\"reportingPortfolio\":\"Springer Hybrid\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false},\"keywords\":\"InSAR, Photogrammetry, Magma reservoir, Magma intrusion, Lava lake, Subsidence\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-9568081/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-9568081/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eLocalized crater deformation can shed insight into shallow magma processes and eruption hazards. To study localized crater deformation at Nyiragongo volcano (Democratic Republic of Congo), we processed Interferometric Synthetic Aperture Radar (InSAR) time series using RADARSAT-2 ultra-fine satellite data spanning 2012\\u0026ndash;2019. We observed persistent crater floor subsidence during August 2013-December 2015 and inverted the corresponding InSAR displacements to model candidate deformation sources with analytic solutions in a homogeneous elastic half-space. We identified a deflating source modeled as a sill at ~\\u0026thinsp;100 m depth beneath the crater surface, which we interpreted to be the cooling of magma accumulated in the crater. This observation was possible thanks to a quiescent period in 2013\\u0026ndash;2015 during which the lava lake did not overflow the bottom of the crater. Our study demonstrates the capabilities of imaging localized deformation patterns using high spatial resolution SAR data.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Imaging and modeling crater floor subsidence at Nyiragongo Volcano, DRC\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2026-05-18 07:05:53\",\"doi\":\"10.21203/rs.3.rs-9568081/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0},{\"type\":\"reviewerAgreed\",\"content\":\"237174178582972664345684485248692526562\",\"date\":\"2026-05-19T07:20:26+00:00\",\"index\":\"hide\",\"fulltext\":\"\"},{\"type\":\"reviewersInvited\",\"content\":\"\",\"date\":\"2026-05-07T13:21:20+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"editorAssigned\",\"content\":\"\",\"date\":\"2026-05-01T09:06:23+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"checksComplete\",\"content\":\"\",\"date\":\"2026-04-30T04:54:00+00:00\",\"index\":\"\",\"fulltext\":\"\"},{\"type\":\"submitted\",\"content\":\"Bulletin of Volcanology\",\"date\":\"2026-04-29T15:42:55+00:00\",\"index\":\"\",\"fulltext\":\"\"}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"bulletin-of-volcanology\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"buvo\",\"sideBox\":\"Learn more about [Bulletin of Volcanology](http://link.springer.com/journal/445)\",\"snPcode\":\"445\",\"submissionUrl\":\"https://www.editorialmanager.com/buvo/default2.aspx\",\"title\":\"Bulletin of Volcanology\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"em\",\"reportingPortfolio\":\"Springer Hybrid\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false}}],\"origin\":\"\",\"ownerIdentity\":\"d84a7a08-34ed-4fe0-ad4c-6dfe7600bc55\",\"owner\":[],\"postedDate\":\"May 18th, 2026\",\"published\":true,\"recentEditorialEvents\":[{\"type\":\"reviewerAgreed\",\"content\":\"237174178582972664345684485248692526562\",\"date\":\"2026-05-19T07:20:26+00:00\",\"index\":14,\"fulltext\":\"\"},{\"type\":\"reviewersInvited\",\"content\":\"7\",\"date\":\"2026-05-07T13:21:20+00:00\",\"index\":\"\",\"fulltext\":\"\"}],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"under-review\",\"subjectAreas\":[],\"tags\":[],\"updatedAt\":\"2026-05-18T07:05:53+00:00\",\"versionOfRecord\":[],\"versionCreatedAt\":\"2026-05-18 07:05:53\",\"video\":\"\",\"vorDoi\":\"\",\"vorDoiUrl\":\"\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-9568081\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-9568081\",\"identity\":\"rs-9568081\",\"version\":[\"v1\"]},\"buildId\":\"8U1c8b4HqxoKbykW_rLl7\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}