Clay-rich fault gouges become frictionally less stable at elevated temperatures

Clay-rich fault gouges become frictionally less stable at elevated temperatures | 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 Article Clay-rich fault gouges become frictionally less stable at elevated temperatures Isabel Ashman, Daniel Faulkner, Elisabetta Mariani This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4612539/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Large earthquakes nucleate on crustal faults that have accumulated significant slip displacement and field observations show that these faults are ubiquitously clay-rich. Earthquake nucleation requires a reduction in shear resistance for instability to develop. Previous laboratory friction measurements indicate that only stable fault creep should occur in clay-rich faults; a result at odds with observations of widespread earthquake behaviour on mature clay-rich faults in nature. Here we show that the frictional stability of synthetic clay-bearing fault gouges decreases systematically with elevated temperatures commensurate with those found at typical earthquake depths. In materials containing ≤50% clay, the stability of slip decreases with increasing temperature so that gouges display unstable slip at temperatures between 100 and 180°C. At room temperature the same materials host only stable slip. This reduction in stability with increasing temperature coincides with a greater degree of localization observed in the gouge microstructure and with progressive loss of water adsorbed on clay surfaces. Our results indicate that a broad compositional range of clay-bearing fault rocks, and therefore mature faults, can nucleate unstable slip at conditions common to the clay-bearing brittle crust; a result that resolves the apparent paradox that mature clay-bearing faults in nature can nucleate and propagate earthquakes. Earth and environmental sciences/Natural hazards Earth and environmental sciences/Solid Earth sciences/Tectonics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction There is a fundamental discrepancy concerning clay-rich fault rocks between the typical frictionally stable slip produced in rock deformation experiments and observations of unstable seismic slip on natural, mature faults. Seismogenic, mature faults, from surface observations and from drilling, typically contain significant proportions of clay and would not be expected to nucleate earthquakes based on laboratory measurements. For earthquake nucleation, the frictional resistance of the fault material must decrease with increased slip velocity to produce run-away unstable slip. In the laboratory, clay-bearing materials almost exclusively strengthen with increasing slip velocity 1–5 and so deform via stable creep, which precludes the possibility of the onset of seismic slip. In contrast, earthquakes of all magnitudes commonly occur on mature faults, in addition to a spectrum of stable and unstable slip behaviours including aseismic creep and slow slip earthquakes 6–9 . Examples of such mature, multi-slip-mode fault zones include the Longitudinal Valley Fault (LVF) in Taiwan with several 10s of kilometres displacement, the Alpine Fault Zone (AFZ) in New Zealand with an estimated 475km displacement, and the Median Tectonic Line (MTL) in Japan, with estimates of displacement between 200 and 1000km (Figure 1). Field observations suggest that the fault core for all these structures, and other similar structures, contains abundant clay minerals, as detailed below. The LVF constitutes a series of locked and creeping fault segments 7,9 with fault rock clay contents from both fault segment types ranging from 20 to 57wt% 7 . Even in low proportions of the bulk material (<10%), a weak phase stabilizes frictional behaviour 2,10–12 . Hence, seismic slip at such clay proportions should be highly improbable, but the LVF has ruptured in earthquakes of M w 6.8 in 2003 6 and twelve M w >6.0 earthquakes in 1951 6,13 . The MTL in southwestern Japan last ruptured in seismic slip in the 1500s 14,15 . Clay contents within the Anko section of the MTL range from 22 to 56wt% 16 in gouges derived from the Ryoke and Sambagawa metamorphic belts (Figure 1A & B). A final example of a clay-rich mature fault zone is the central AFZ (Figure 1C & D), from which the Deep Fault Drilling Project (DFDP-1) recovered cores from a principal fault slip zone of the ‘blue’ and ‘brown’ fault gouges, which contained 16% and 31% clay, respectively 17 . This contrasts with the current ‘locked’ nature of the fault and the significant seismic hazard posed by the AFZ due to the episodic ruptures of M w 8 earthquakes at ~300 year intervals, such as the previous surface-rupturing earthquake in 1717 (± 5) 17,18 . The vast majority of previous velocity step experiments that investigated the rate and state frictional stability of clay minerals were performed at room temperatures (~20°C) 1,2,4,5,19,20 . The seismogenic portion of tectonic faults is subject to elevated temperatures dependent on the local geothermal gradient – temperatures of ~180°C occur at ~9km depth for geothermal gradients of 20°C.km -1 in stable continental regions, compared to ~1.4km depth in the extreme geothermal gradient of 125°C.km -1 in the AFZ 21 . Thus, there is notable lack of friction studies at hydrothermal conditions, particularly involving clays. Elevated temperatures have been shown to affect the frictional behaviour of fault gouges, but studies have focused on gouges from specific localities or a limited range of fault gouge compositions 7,17,22–25 . Boulton et al. 17 and den Hartog & Spiers 23 observed decreases in the frictional stability of three compositions of clay-bearing (muscovite/illite, smectite & chlorite) fault gouges at temperatures up to 300°C (10 to 14km depth). While these studies provide key evidence that the stability of clay-rich gouges changes at elevated temperature, there has not, to date, been a study that systematically documents the evolution of frictional stability as a function of clay content at elevated temperatures. Hence, the aim of this study is to investigate the effect of increasing temperature on frictional stability across the full range of clay proportions (0 to 100 wt%) in fault gouges, providing new evidence that resolves the apparent paradox that mature clay-bearing faults in nature can nucleate and propagate earthquakes. Friction experiments: Kaolinite was the chosen clay for this investigation as it commonly occurs in mature fault zones 7,26 and it has frictional characteristics that are similar to other clays 27 . Kaolinite is a non-swelling clay with a dioctohedral structure and dehydroxylation to metakaolinite occurs at temperatures above 370-400°C 28–30 . It is a clay mineral that does not have the complications of ultra-low permeability and the variable frictional stability characteristics of a swelling clay, such as montmorillonite 4,31 , so kaolinite broadly represents a wide range of non-swelling clay types. The proportion of kaolinite (99% purity) to a pure quartz powder (99% purity; see Methods) was stepped in 25wt% increments to produce five synthetic fault gouge mixtures. The velocity-step experiments were conducted in a triaxial deformation apparatus in a direct shear slider assembly (see Methods). In addition to room temperature experiments (~23°C), temperature was controlled at 60°C, 100°C, 140°C and 180°C (+/-0.4°C) using external band heaters. A confining pressure of 150MPa and a pore fluid pressure (using deionized water as the pore fluid) of 60MPa mimicked the conditions typical of the seismogenic continental crust at approximately 6-7km depth with a hydrostatic pressure gradient ( λ = pore pressure/overburden = 0.4). The experiments included an initial ‘yield phase’ involving 2mm of displacement at a velocity of 0.3μm/s, during which the samples were loaded to yield (Figure 2). Subsequently, in the ‘velocity-step phase’ the slip velocity was stepped between 3.0μm/s and 0.3μm/s every 0.5mm of slip until the maximum displacement of 5.5mm was reached. Rate and state friction (RSF) laws are used to describe the frictional response to such changes in slip velocity and to identify the frictional stability. The rate- and state-dependent constitutive law 32 describes the direct effect ( a ) and evolution effect ( b ) on the initial friction coefficient of a material ( μ 0 ) due to a change from an initial slip velocity ( V 0 ) to a new velocity ( V ) over a critical slip distance ( D RS ). The RSF parameter of (a–b) is used to assess the stability of fault slip in a material (Figure 2F). A material that has negative ( a–b) values is velocity weakening, so that it weakens progressively with increasing slip velocity. Hence, negative ( a–b ) values are considered a prerequisite for seismic slip, although instability still depends on the stiffness of the loading system. In contrast, a material that has positive ( a–b ) values is velocity strengthening, so that it strengthens with increasing slip velocity, leading to the arrest of seismic slip and stable sliding. A notable result from the experiments is the consistent negative trend of the averaged stability parameter (a-b) derived from velocity-step increases with increasing temperature in all the clay-bearing fault gouges tested (Figure 3). In the 25wt% and 50wt% kaolinite gouges, (a-b) decreases from velocity strengthening values at room temperature to become velocity weakening at temperatures ≥100°C. The gouges with higher proportions of kaolinite (75 and 100wt%) show the same negative trend with increasing temperature, but (a-b) does not decrease below 0. The minimum values of (a-b) for all the clay-bearing fault gouges occurs at 140°C, which is then followed by a slight increase as temperature is increased to 180°C. The only material that does not follow this pattern is the 100wt% quartz (clay-absent) gouge, which shows an opposite positive trend of (a-b) values with increasing temperature. The quartz (a-b) values increase to transition from velocity weakening at room temperature to velocity strengthening at temperatures > 60°C. The critical slip weakening distance ( D RS ) needs to be considered when assessing the stability of frictional sliding, as the shorter the critical slip distance, the more likely a velocity weakening material is to show stick-slip behaviour. At room temperatures, all the clay-bearing fault gouges slip by stable sliding, but as the temperature is increased and both (a-b) and D RS decrease (Figure 3), unstable slip becomes more common in the clay-bearing gouges. At 140°C, the 25 and 50wt% clay gouges sometimes experience stick-slip, with significant stress drops occurring immediately following velocity step increases. This unstable slip rapidly transitions to stable slip within the displacement of the velocity step (Figure 2B & 2C). This behaviour is expected, given the combination of RSF parameters and apparatus stiffness, and can be modelled as a system that is on the boundary of unstable slip 26,34,35 . The 100wt% quartz (clay-absent) gouge shows the opposite trend by becoming more stable at higher temperatures. At room temperatures, a velocity-step increase leads to repeated stress drops that repeat throughout the entire displacement range of the velocity-step, with the actual drop in shear stress occurring over less than 0.1 second (our data logging rate). At elevated temperatures, the 100wt% quartz (clay-absent) gouge undergoes a single, audible stress drop upon a velocity-step increase, but the behaviour transitions towards stable sliding. The frictional strength shows a weak positive trend with increasing temperature that is consistent across all the tested clay-bearing fault gouges (Figure 2). The positive trend of frictional strength with increasing temperature is most evident in the gouges with higher proportions of kaolinite, as the 25wt% kaolinite gouge increases by 0.04 whereas the 100wt% kaolinite gouge increases by 0.07 across the temperature range. The only tested material that does not follow this trend is the 100-wt% quartz (clay-absent) gouge, which shows no significant trend with increasing temperature. Microstructures Following the deformation experiments, samples of fault gouge were recovered from the apparatus, dried, and impregnated with epoxy resin. The samples were cut perpendicular to the shear zone boundary, parallel to the slip displacement and polished for analyses using a Zeiss GeminiSEM 450. High resolution images across a wide range of scales were collected using a backscattered electron detector. Localised failure in granular materials is characterised by a high strain gradient within a narrow area, which includes such microstructures as grain comminution, shear or compaction bands, and preferred alignment of clasts 36 . Strain was accommodated in the gouges through shear arrays including shear-parallel Y-shears and low-angle R 1 Riedel shears 36 (Fig. 4 ) that were associated with grain size reduction and were typically less than 50µm thick. The most prominent features in the micrographs are stress-relief (unloading) fractures that open in the sample during depressurisation of the triaxial apparatus. These commonly occur along sites of shear localization that are marked by grain size reduction. Hence these fractures were classified as shear fractures only if grain size reduction of quartz via cataclasis could be identified along their boundaries. The number and lengths of localized shears were quantified using FracPaQ 37 (Fig. 4 ). Fault gouges deformed at room temperature had a greater number of shear localization features and a small average shear localisation length normalised to sample area, at 1.14x10 − 2 mm − 1 in the 25wt% kaolinite gouge. At elevated temperatures, the strain is accommodated in fewer but longer R 1 Riedel and Y shears, as the localized shear length by sample area in the 25wt% kaolinite gouge increased to 2.5x10 − 2 mm − 1 at 140°C (Fig. 4 ). This indicates that localisation of deformation increases with increasing temperature in clay-bearing fault gouges. The total amount of shear-enhanced compaction of the bulk fault gouge in the velocity step tests also increased with increasing temperature across the 5 different fault gouge compositions. By the end of the experiments, the 25wt% kaolinite gouge had compacted relative to the initial volume by 7.6% (71mm 3 ) at 23°C and 11.8% (123mm 3 ) at 180°C. Energy dispersive X-ray spectroscopy (EDS) chemical mapping of the gouge samples in the SEM showed a minor concentration of quartz over kaolinite in the localised shear zones, perhaps due to the EDS resolution limit of 1µm. Discussion The issue of the disparity between field observations and clay mineral ( a-b ) stability data from the laboratory raises questions on the applicability of utilising laboratory measurements in physics-based modelling of earthquake sequences 34 . Many models involve RSF laws 34 , 35 , 38 , 39 and the parameters from rock friction studies conducted at room temperature. Due to a lack of RSF data at elevated temperature, few models incorporate a temperature (or depth) dependence of ( a-b ) 39 , and the relationships that are used are commonly based upon materials such as granite gouge 22 that do not reflect natural fault lithologies present in mature faults. The results of this study show that ≤ 50wt% clay fault gouges can host unstable slip at temperatures typical of depths greater than ~ 4km, assuming a geothermal gradient of 20°C.km − 1 (Fig. 5 ). This indicates that the difference in temperature between ambient lab and subsurface crustal conditions can account for the wide variety of fault slip behaviours seen in nature, including the prominence of seismogenic slip on many major clay-rich faults, which otherwise would be difficult to explain. Segments of the example faults in Fig. 1 , including the LVF in Taiwan and the MTL in Japan have measured clay proportions of 1-69wt% clay 7,16 , which is represented in the range of gouge clay proportions tested in this study, and that acted as conditionally-unstable at elevated temperatures. The shear in the experiments was sometimes accommodated via stable creep until a velocity step increase was imposed, which induced unstable slip – a scenario that may be similar to a propagating slip patch impinging on a clay-rich fault segment in nature. This study has shown that with increasing temperature, shear enhanced compaction of fault gouge increases to produce a denser microstructure, and that grain comminution is pronounced in elongate, discrete shear zones. Increasing localisation of shear and decreasing grain size within a gouge microstructure decreases the stability of slip 40 . The concurrent decrease in the stability parameters of ( a–b ) and D RS with increasing temperature reflect the microstructural change from distributed to localised deformation. The observed dependence of experimental rate and state friction on temperature 17 , 22 – 25 , 41 has led to several attempts at incorporating the effect of temperature into a microphysical rate and state friction framework 41 – 44 . Arrhenius-type relationships for pressure solution 41 – 43 have been used to describe two slip-mechanism-driven regimes: cataclastic flow and dilation at low temperatures (100–300°C for quartz) and solution-precipitation-aided cataclastic flow at higher (> 300°C) temperatures 23 , 44 . Den Hartog & Spiers 23 observed an increase in frictional strength and reduction in ( a-b ) with increasing temperature (at σ n eff = 170MPa), and attributed the cause to the formation of a denser microstructure due to compaction via stress corrosion cracking or pressure solution. However, the high strain rates relative to nature in both Den Hartog & Spiers 23 (~ 1x10 − 3 to 1x10 − 1 s − 1 ) and this study (~ 2x10 − 4 to 2x10 − 3 s − 1 ) do not favour pressure solution as the dominant deformation mechanism 45 . While the bulk pore volume decreased as compaction progressed with increasing temperature 46 , no evidence of pressure solution facilitating cataclasis was observed in this study, such as grain boundary truncation, indentation or mineral overgrowths 45 . The observed increase in shear-enhanced compaction and frictional strength, and the reduction in the ( a-b ) and D RS parameters with increasing temperature in these experiments occur in the expected temperature window for the release of adsorbed water in kaolinite. Therefore, a hypothesis is here proposed that the gradual removal of adsorbed (free) water from kaolinite surfaces with increasing temperatures, and not pressure solution 23 , is the micromechanical mechanism leading to enhanced compaction of the gouge microstructure and subsequent localisation of slip. Increasingly higher energy layers of adsorbed water are removed at progressively higher temperatures to give a temperature range over which the microstructure would evolve, with a peak in rate of weight loss at ~ 70°C and a range of 30–150°C for kaolinite from thermogravimetric (TG) analysis 28 , 47 . The proportion of weight loss ranges from 0.5-2% at 200°C 47 , 48 , but increasing degrees of mechanical deformation of kaolinite have been shown to increase the proportion of mass loss to 5% at 200°C 48 . The relative weakness of clay minerals is often attributed to the lubrication of clay grains by the films of adsorbed water 49 , 50 , therefore the removal of free water from clay surfaces at elevated temperatures would be expected to increase the frictional strength of clay-clay contacts, as was observed in this study. A heterogeneous distribution of stress within a clay-quartz gouge framework may contribute to the localisation of shear by removing adsorbed water preferentially from sites of anomalously high stress 48 . Ripplocations, a nanoscale elastic buckling process in phyllosilicates 51 , may act as an efficient process for fluid transport away from sites of anomalously high stress – in opening, ripplocations generate transient porosity and by reconstituting bonds, porosity is closed. The hypothesis that the loss of adsorbed water leads to the formation of a denser microstructure and, therefore, unstable slip behaviour should be investigated by applying this experimental procedure to a range of clay minerals, which release adsorbed water across characteristic temperature ranges. If earthquake models discount the potential for seismic slip on clay-bearing fault segments due to the use of room temperature RSF measurements, the seismic hazard associated with a clay-rich fault zone could be underestimated. Methods Sample Preparation: The materials used to create the simulated fault gouges in this investigation were the 1:1 layered, dioctahedral clay (Supplementary Fig. 1) kaolinite KGa-1b from the Clay Minerals Society (CMS) and the quartz powder Min-U-Sil 15 from the US Silica Company. Both KGa-1b and Min-U-Sil 15 had purities quoted at > 99% 52 . The kaolinite and quartz powders had similar densities at 2.65g/cm 3 and fine mean grain sizes at < 2 µm and < 15 µm, respectively. To generate the simulated fault gouges, the powders were mixed via dry tumbling as the similar grain sizes and densities promote mixing over segregation. Kaolinite in the fault gouges was stepped in increments of 25 wt% to produce 3 intermediate mixtures that had been tumbled for a 3-hour period at 68 rpm in a 575 cm 3 container. Triaxial Deformation Apparatus: The friction tests were conducted in a triaxial deformation apparatus in the Rock Deformation Laboratory at the University of Liverpool. The apparatus applied a confining pressure ( σ 2 = σ 3 ) using silicon oil at pressures up to 250 MPa. A PVC (room temperature) or Viton (elevated temperatures) jacket isolated the sample from the confining fluid so that a pore pressure of up to 200 MPa could be independently imposed on the sample using deionised water. An axial load ( σ 1 ) was applied to the sample through a motor-driven loading column with an internal force gauge that measured the axial force at a resolution greater than 0.05 kN. In experiments at elevated temperatures, three external knuckle band heaters spaced along the length of the pressure vessel conducted heat through the vessel to the sample (Supplementary Fig. 2). The temperatures of the band heaters and the sample were monitored via external and internal thermocouples with to a resolution of 0.1°C. Calibration tests show the temperature gradient across the length of the sample is < 2˚C. To minimise the conduction of heat through the sample assembly to the force gauge, a cooling plate sat between the base of the pressure vessel and the loading column. The maximum temperature of 180°C in this study was a limit not from the band heaters, but from jacketing, o-rings and direct shear spacers used in the sample assembly. In the experiments, the external furnace and cooling system were activated during the loading of confining and pore pressure to bring the sample to the target temperature ensuring that the target effective normal stress was not exceeded during the heating. Both the confining and pore pressure media were sensitive to the increase in temperature and were only brought to the target pressures once the target temperature had been established. Fault gouge samples were contained within the direct shear assembly for friction experiments. The direct shear sliders moved relative to each other to a maximum displacement of 6mm via the elastic deformation of two rubber spacers. Prior to an experiment, the 1.6 g of fault gouge sample was compacted in a uniaxial press at 50 MPa to improve the cohesion of the sample. The direct shear sliders housed connections to the control systems of the deformation apparatus, including the internal thermocouple and the pore pressure control. Porous 316 stainless steel disks within the direct shear sliders had a permeability of 10 − 13 m 2 , which allowed the exchange of pore fluid between the sample and the servo-controlled pump. The change in volume in the pore pressure control pump during an experiment was used as a proxy for the shear enhanced compaction during a velocity-step experiment (Supplementary Fig. 3). For the experiments in this study, the RSF parameters were calculated using the software RSFit3000 53 , which uses a non-linear least square fit on the experimental data (Supplementary Figs. 4–6). Fitting parameters in the program include the stiffness of the apparatus loading column. Microstructural analysis: All microstructural analyses were performed in a Zeiss GeminiSEM 450 in the Scanning Electron Microscopy Shared Research Facility (SEM SRF) at the University of Liverpool. The techniques used to collect information on the samples microstructure include Backscattered electron (BSE) imaging (Supplementary Fig. 7) and energy dispersive X-Ray spectroscopy (EDS, Supplementary Fig. 8). BSE imaging provides qualitative information on the spatial distribution of different phases present in a sample, based on the atomic number (z). EDS was used to confirm the minerals present based on their elemental composition. Both large area datasets and high-resolution images were acquired, spanning cm to sub-micron scales. Microstructural data acquired for each sample were then used to measure the number, length, distribution, and frequency of shear fractures present in each of the samples tested experimentally, using the software package FracPaQ. Declarations Acknowledgements: We acknowledge financial support from a Natural Environment Research Council grant NE/V011804/1 to DRF and the NERC EAO DTP studentship to IRA at the University of Liverpool. 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Multimechanism friction constitutive model for ultrafine quartz gouge at hypocentral conditions. J. Geophys. Res. 97 , 1859–1870 (1992). Aharonov, E. & Scholz, C. H. A Physics-Based Rock Friction Constitutive Law: Steady State Friction. J. Geophys. Res. Solid Earth 123 , 1591–1614 (2018). van den Ende, M. P. A., Chen, J., Ampuero, J. P. & Niemeijer, A. R. A comparison between rate-and-state friction and microphysical models, based on numerical simulations of fault slip. Tectonophysics 733 , 273–295 (2018). Niemeijer, A. R. & Spiers, C. J. A microphysical model for strong velocity weakening in phyllosilicate-bearing fault gouges. J. Geophys. Res. Solid Earth 112 , (2007). Gratier, J. P., Menegon, L. & Renard, F. Pressure Solution Grain Boundary Sliding as a Large Strain Mechanism of Superplastic Flow in the Upper Crust. J. Geophys. Res. Solid Earth 128 , (2023). Hüpers, A. & Kopf, A. J. The thermal influence on the consolidation state of underthrust sediments from the Nankai margin and its implications for excess pore pressure. Earth Planet. Sci. Lett. 286 , 324–332 (2009). Levy, J. H. Effect of Water Vapor Pressure on the Dehydration and Dehydroxylation of Kaolinite and Smectite Isolated from Australian Tertiary Oil Shales. Energy and Fuels 4 , 146–151 (1990). Mañosa, J., la Rosa, J. C. de, Silvello, A., Maldonado-Alameda, A. & Chimenos, J. M. Kaolinite structural modifications induced by mechanical activation. Appl. Clay Sci. 238 , (2023). Beynon, S. J. & Faulkner, D. R. Dry, damp, or drenched? The effect of water saturation on the frictional properties of clay fault gouges. J. Struct. Geol. 140 , 104094 (2020). Israelachvili, J. N., McGuiggan, P. M. & Homola, A. M. Dynamic properties of molecularly thin liquid films. Science (80-. ). 240 , 189–191 (1988). Aslin, J., Mariani, E., Dawson, K. & Barsoum, M. W. Ripplocations provide a new mechanism for the deformation of phyllosilicates in the lithosphere. Nat. Commun. 10 , (2019). Vogt, C., Lauterjung, J. & Fischer, R. X. Investigation of the clay fraction (<2 μm) of the clay minerals society reference clays. Clays Clay Miner. 50 , 388–400 (2002). Skarbek, R. M. & Savage, H. M. RSFit3000: A MATLAB GUI-based program for determining rate and state frictional parameters from experimental data. Geosphere 15 , 1665–1676 (2019). Additional Declarations There is NO Competing Interest. Supplementary Files NatureGeoscienceSuppInfosubmit.docx Cite Share Download PDF Status: Posted Version 1 posted 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-4612539","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":334935586,"identity":"2228a4ae-e83d-428f-983b-9e54ab1ff98d","order_by":0,"name":"Isabel Ashman","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0002-7491-0678","institution":"University of Liverpool","correspondingAuthor":true,"prefix":"","firstName":"Isabel","middleName":"","lastName":"Ashman","suffix":""},{"id":334935587,"identity":"5232c3cb-ece1-465e-8d06-975170a2a8ef","order_by":1,"name":"Daniel Faulkner","email":"","orcid":"https://orcid.org/0000-0002-6750-3775","institution":"University of Liverpool","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"","lastName":"Faulkner","suffix":""},{"id":334935588,"identity":"852beb8a-8af4-4ae9-9fcb-e8dfc43fa8cc","order_by":2,"name":"Elisabetta Mariani","email":"","orcid":"https://orcid.org/0000-0002-0585-5265","institution":"University of Liverpool","correspondingAuthor":false,"prefix":"","firstName":"Elisabetta","middleName":"","lastName":"Mariani","suffix":""}],"badges":[],"createdAt":"2024-06-20 14:45:39","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4612539/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4612539/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":61747099,"identity":"3ade29ad-4e3a-4f6e-a296-c9e923eaba0b","added_by":"auto","created_at":"2024-08-05 06:56:32","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2817764,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003ePhotos of example exposures of clay-rich, mature tectonic faults. Photos (A) \u0026amp; (B): the Median Tectonic Line, Japan with example clay contents of 35 – 73 wt% in the Anko Section\u003c/em\u003e\u003csup\u003e16\u003c/sup\u003e\u003cem\u003e. Photos (C) \u0026amp; (D): the Alpine Fault Zone (Stony Creek) with clay contents in ‘brown’ and ‘blue’ gouges between 16 and 31 wt%\u003c/em\u003e\u003csup\u003e17\u003c/sup\u003e\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4612539/v1/cf4cc531496a8b472f232e26.png"},{"id":61747103,"identity":"be94e553-8a40-4d6b-b2db-616b2a108263","added_by":"auto","created_at":"2024-08-05 06:56:32","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1159153,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eShear stress curves against load point displacement for all 5 gouge compositions at 5 experimental conditions (25 experiments shown). The panels are separated by clay proportion from (A) 0 wt% to (E) 100 wt%. Conditions in all experiments were 150 MPa total normal stress and a pore fluid pressure (deionized water) of 60 MPa.\u003c/em\u003e \u003cem\u003eThe insert (F) summarises the typical rate and state friction response to a velocity-step change for both a velocity strengthening material and a velocity weakening material.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4612539/v1/20ad70b52a77e5485e4fa916.png"},{"id":61747098,"identity":"344e94d6-9959-4679-8bae-552fd98f5971","added_by":"auto","created_at":"2024-08-05 06:56:32","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":273112,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eChanges in rate and state friction parameters with temperature for the range of gouge clay contents. (A) (a-b) stability parameter shown as a function of temperature. The mean for each test is shown by the symbol, with the error bars indicating the range of the data for each condition. (B) critical slip distance (D\u003c/em\u003e\u003csub\u003e\u003cem\u003eRS\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e) for velocity step increases in all synthetic fault gouges across the increasing temperature conditions. The mean is indicated by the symbol and the range by the error bars.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4612539/v1/3a435b50e69f30035ce9ba20.png"},{"id":61747102,"identity":"f213e514-4d39-4653-9e99-ad41da026910","added_by":"auto","created_at":"2024-08-05 06:56:32","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1705793,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eMicrostructural features of the deformed kaolinite-quartz fault gouges. (A) and (B) Backscattered electron (BSE) images of 25wt% kaolinite gouge deformed at room temperature with shears highlighted; (C) fracture trace map of the 25wt% kaolinite sample deformed at room temperature as the input for the FracPaQ analysis software; (D) fracture trace map of the 25wt% kaolinite sample deformed at 140°C as the input for the FracPaQ analysis software; (E) BSE image montage of 25wt% kaolinite gouge deformed at 140°C with shears highlighted.\u003c/em\u003e \u003cem\u003eAll images have top to the left sense of shear.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4612539/v1/d392ae46ee406d770e2f88be.png"},{"id":61747795,"identity":"d8cb8ed6-4752-4edb-b141-ae337b4c4b44","added_by":"auto","created_at":"2024-08-05 07:04:32","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":219040,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eSummary schematic diagram of the conditions within a tectonic fault. The left-hand graph shows the change in temperature with depth – the line is an example geothermal gradient of 20°C.km\u003c/em\u003e\u003csup\u003e\u003cem\u003e-1\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e, with the shaded area representing a range of geothermal gradients. The right-hand panel shows the measured temperature controlled variations in the (a-b) parameter with the expected crustal depth according to a 20°C.km\u003c/em\u003e\u003csup\u003e\u003cem\u003e-1\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e geothermal gradient.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4612539/v1/eb36674cb1af4ae8503dd3db.png"},{"id":66417990,"identity":"1c1a65e0-f5b5-4178-8e66-cc8bcb868c0a","added_by":"auto","created_at":"2024-10-11 15:04:56","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8027241,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4612539/v1/8de9531e-cd40-4372-9e45-099aabf14b0b.pdf"},{"id":61747100,"identity":"c754e7f5-d5e2-4c10-8206-6953fbc927a3","added_by":"auto","created_at":"2024-08-05 06:56:32","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":7240357,"visible":true,"origin":"","legend":"","description":"","filename":"NatureGeoscienceSuppInfosubmit.docx","url":"https://assets-eu.researchsquare.com/files/rs-4612539/v1/a5a20f7fe39b8817b278713f.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Clay-rich fault gouges become frictionally less stable at elevated temperatures","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThere is a fundamental discrepancy concerning clay-rich fault rocks between the typical frictionally stable slip produced in rock deformation experiments and observations of unstable seismic slip on natural, mature faults. Seismogenic, mature faults, from surface observations and from drilling, typically contain significant proportions of clay and would not be expected to nucleate earthquakes based on laboratory measurements. For earthquake nucleation, the frictional resistance of the fault material must decrease with increased slip velocity to produce run-away unstable slip. In the laboratory, clay-bearing materials almost exclusively strengthen with increasing slip velocity\u003csup\u003e1\u0026ndash;5\u003c/sup\u003e and so deform via stable creep, which precludes the possibility of the onset of seismic slip.\u003c/p\u003e\n\u003cp\u003eIn contrast, earthquakes of all magnitudes commonly occur on mature faults, in addition to a spectrum of stable and unstable slip behaviours including aseismic creep and slow slip earthquakes\u003csup\u003e6\u0026ndash;9\u003c/sup\u003e. Examples of such mature, multi-slip-mode fault zones include the Longitudinal Valley Fault (LVF) in Taiwan with several 10s of kilometres displacement, the Alpine Fault Zone (AFZ) in New Zealand with an estimated 475km displacement, and the Median Tectonic Line (MTL) in Japan, with estimates of displacement between 200 and 1000km (Figure 1). Field observations suggest that the fault core for all these structures, and other similar structures, contains abundant clay minerals, as detailed below.\u003c/p\u003e\n\u003cp\u003eThe LVF constitutes a series of locked and creeping fault segments\u003csup\u003e7,9\u003c/sup\u003e with fault rock clay contents from both fault segment types ranging from 20 to 57wt%\u003csup\u003e7\u003c/sup\u003e.\u0026nbsp;Even in low proportions of the bulk material (\u0026lt;10%), a weak phase stabilizes frictional behaviour\u003csup\u003e2,10\u0026ndash;12\u003c/sup\u003e. Hence, seismic slip at such clay proportions should be highly improbable, but the LVF has ruptured in earthquakes of M\u003csub\u003ew\u003c/sub\u003e 6.8 in 2003\u003csup\u003e6\u003c/sup\u003e and twelve M\u003csub\u003ew\u003c/sub\u003e\u0026gt;6.0 earthquakes in 1951\u003csup\u003e6,13\u003c/sup\u003e. The MTL in southwestern Japan last ruptured in seismic slip in the 1500s\u003csup\u003e14,15\u003c/sup\u003e. Clay contents within the Anko section of the MTL range from 22 to 56wt%\u003csup\u003e16\u003c/sup\u003e in gouges derived from the Ryoke and Sambagawa metamorphic belts (Figure 1A \u0026amp; B). A final example of a clay-rich mature fault zone is the central AFZ (Figure 1C \u0026amp; D), from which the Deep Fault Drilling Project (DFDP-1) recovered cores from a principal fault slip zone of the \u0026lsquo;blue\u0026rsquo; and \u0026lsquo;brown\u0026rsquo; fault gouges, which contained 16% and 31% clay, respectively\u003csup\u003e17\u003c/sup\u003e. This contrasts with the current \u0026lsquo;locked\u0026rsquo; nature of the fault and the significant seismic hazard posed by the AFZ due to the episodic ruptures of M\u003csub\u003ew\u003c/sub\u003e 8 earthquakes at ~300 year intervals, such as the previous surface-rupturing earthquake in 1717 (\u0026plusmn; 5)\u003csup\u003e17,18\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe vast majority of previous velocity step experiments that investigated the rate and state frictional stability of clay minerals were performed at room temperatures (~20\u0026deg;C)\u003csup\u003e1,2,4,5,19,20\u003c/sup\u003e. The seismogenic portion of tectonic faults is subject to elevated temperatures dependent on the local geothermal gradient \u0026ndash; temperatures of ~180\u0026deg;C occur at ~9km depth for geothermal gradients of 20\u0026deg;C.km\u003csup\u003e-1\u003c/sup\u003e in stable continental regions, compared to ~1.4km depth in the extreme geothermal gradient of 125\u0026deg;C.km\u003csup\u003e-1\u003c/sup\u003e in the AFZ\u003csup\u003e21\u003c/sup\u003e. Thus, there is notable lack of friction studies at hydrothermal conditions, particularly involving clays. Elevated temperatures have been shown to affect the frictional behaviour of fault gouges, but studies have focused on gouges from specific localities or a limited range of fault gouge compositions\u003csup\u003e7,17,22\u0026ndash;25\u003c/sup\u003e. Boulton et al.\u003csup\u003e17\u003c/sup\u003e and den Hartog \u0026amp; Spiers\u003csup\u003e23\u003c/sup\u003e observed decreases in the frictional stability of three compositions of clay-bearing (muscovite/illite, smectite \u0026amp; chlorite) fault gouges at temperatures up to 300\u0026deg;C (10 to 14km depth). While these studies provide key evidence that the stability of clay-rich gouges changes at elevated temperature, there has not, to date, been a study that systematically documents the evolution of frictional stability as a function of clay content at elevated temperatures. Hence, the aim of this study is to investigate the effect of increasing temperature on frictional stability across the full range of clay proportions (0 to 100 wt%) in fault gouges, providing new evidence that resolves the apparent paradox that mature clay-bearing faults in nature can nucleate and propagate earthquakes.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFriction experiments:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eKaolinite was the chosen clay for this investigation as it commonly occurs in mature fault zones\u003csup\u003e7,26\u003c/sup\u003e and it has frictional characteristics that are similar to other clays\u003csup\u003e27\u003c/sup\u003e. Kaolinite is a non-swelling clay with a dioctohedral structure and dehydroxylation to metakaolinite occurs at temperatures above 370-400\u0026deg;C\u003csup\u003e28\u0026ndash;30\u003c/sup\u003e. It is a clay mineral that does not have the complications of ultra-low permeability and the variable frictional stability characteristics of a swelling clay, such as montmorillonite\u003csup\u003e4,31\u003c/sup\u003e, so kaolinite broadly represents a wide range of non-swelling clay types. The proportion of kaolinite (\u0026lt;2\u0026mu;m grain size, \u0026gt;99% purity) to a pure quartz powder (\u0026lt;15mm grain size, \u0026gt;99% purity; see Methods) was stepped in 25wt% increments to produce five synthetic fault gouge mixtures. The velocity-step experiments were conducted in a triaxial deformation apparatus in a direct shear slider assembly (see Methods). In addition to room temperature experiments (~23\u0026deg;C), temperature was controlled at 60\u0026deg;C, 100\u0026deg;C, 140\u0026deg;C and 180\u0026deg;C (+/-0.4\u0026deg;C) using external band heaters. A confining pressure of 150MPa and a pore fluid pressure (using deionized water as the pore fluid) of 60MPa mimicked the conditions typical of the seismogenic continental crust at approximately 6-7km depth with a hydrostatic pressure gradient (\u003cem\u003e\u0026lambda;\u003c/em\u003e = pore pressure/overburden = 0.4). The experiments included an initial \u0026lsquo;yield phase\u0026rsquo; involving 2mm of displacement at a velocity of 0.3\u0026mu;m/s, during which the samples were loaded to yield (Figure 2). Subsequently, in the \u0026lsquo;velocity-step phase\u0026rsquo; the slip velocity was stepped between 3.0\u0026mu;m/s and 0.3\u0026mu;m/s every 0.5mm of slip until the maximum displacement of 5.5mm was reached.\u003c/p\u003e\n\u003cp\u003eRate and state friction (RSF) laws are used to describe the frictional response to such changes in slip velocity and to identify the frictional stability. The rate- and state-dependent constitutive law\u003csup\u003e32\u003c/sup\u003e describes the direct effect (\u003cem\u003ea\u003c/em\u003e) and evolution effect (\u003cem\u003eb\u003c/em\u003e) on the initial friction coefficient of a material (\u003cem\u003e\u0026mu;\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e) due to a change from an initial slip velocity (\u003cem\u003eV\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e) to a new velocity (\u003cem\u003eV\u003c/em\u003e) over a critical slip distance (\u003cem\u003eD\u003csub\u003eRS\u003c/sub\u003e\u003c/em\u003e). The RSF parameter of\u003cem\u003e\u0026nbsp;(a\u0026ndash;b)\u003c/em\u003e is used to assess the stability of fault slip in a material (Figure 2F). A material that has negative (\u003cem\u003ea\u0026ndash;b)\u003c/em\u003e values is velocity weakening, so that it weakens progressively with increasing slip velocity. Hence, negative (\u003cem\u003ea\u0026ndash;b\u003c/em\u003e) values are considered a prerequisite for seismic slip, although instability still depends on the stiffness of the loading system. In contrast, a material that has positive (\u003cem\u003ea\u0026ndash;b\u003c/em\u003e) values is velocity strengthening, so that it strengthens with increasing slip velocity, leading to the arrest of seismic slip and stable sliding.\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\"\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eA notable result from the experiments is the consistent negative trend of the averaged stability parameter \u003cem\u003e(a-b)\u003c/em\u003e derived from velocity-step increases with increasing temperature in all the clay-bearing fault gouges tested (Figure 3). In the 25wt% and 50wt% kaolinite gouges, \u003cem\u003e(a-b)\u003c/em\u003e decreases from velocity strengthening values at room temperature to become velocity weakening at temperatures \u0026ge;100\u0026deg;C. The gouges with higher proportions of kaolinite (75 and 100wt%) show the same negative trend with increasing temperature, but \u003cem\u003e(a-b)\u003c/em\u003e does not decrease below 0. The minimum values of \u003cem\u003e(a-b)\u003c/em\u003e for all the clay-bearing fault gouges occurs at 140\u0026deg;C, which is then followed by a slight increase as temperature is increased to 180\u0026deg;C. The only material that does not follow this pattern is the 100wt% quartz (clay-absent) gouge, which shows an opposite positive trend of \u003cem\u003e(a-b)\u003c/em\u003e values with increasing temperature. The quartz \u003cem\u003e(a-b)\u003c/em\u003e values increase to transition from velocity weakening at room temperature to velocity strengthening at temperatures \u0026gt; 60\u0026deg;C.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe critical slip weakening distance (\u003cem\u003eD\u003csub\u003eRS\u003c/sub\u003e\u003c/em\u003e) needs to be considered when assessing the stability of frictional sliding, as the shorter the critical slip distance, the more likely a velocity weakening material is to show stick-slip behaviour. At room temperatures, all the clay-bearing fault gouges slip by stable sliding, but as the temperature is increased and both \u003cem\u003e(a-b)\u003c/em\u003e and\u003cem\u003e\u0026nbsp;D\u003csub\u003eRS\u003c/sub\u003e\u003c/em\u003e decrease (Figure 3), unstable slip becomes more common in the clay-bearing gouges. At 140\u0026deg;C, the 25 and 50wt% clay gouges sometimes experience stick-slip, with significant stress drops occurring immediately following velocity step increases. This unstable slip rapidly transitions to stable slip within the displacement of the velocity step (Figure 2B \u0026amp; 2C). This behaviour is expected, given the combination of RSF parameters and apparatus stiffness, and can be modelled as a system that is on the boundary of unstable slip\u0026nbsp;\u003csup\u003e26,34,35\u003c/sup\u003e. The 100wt% quartz (clay-absent) gouge shows the opposite trend by becoming more stable at higher temperatures. At room temperatures, a velocity-step increase leads to repeated stress drops that repeat throughout the entire displacement range of the velocity-step, with the actual drop in shear stress occurring over less than 0.1 second (our data logging rate).\u0026nbsp;At elevated temperatures, the 100wt% quartz (clay-absent) gouge undergoes a single, audible stress drop upon a velocity-step increase, but the behaviour transitions towards stable sliding.\u003c/p\u003e\n\u003cp\u003eThe frictional strength shows a weak positive trend with increasing temperature that is consistent across all the tested clay-bearing fault gouges (Figure 2). The positive trend of frictional strength with increasing temperature is most evident in the gouges with higher proportions of kaolinite, as the 25wt% kaolinite gouge increases by 0.04 whereas the 100wt% kaolinite gouge increases by 0.07 across the temperature range. The only tested material that does not follow this trend is the 100-wt% quartz (clay-absent) gouge, which shows no significant trend with increasing temperature.\u0026nbsp;\u003c/p\u003e\n\u003ch3\u003eMicrostructures\u003c/h3\u003e\n\u003cp\u003eFollowing the deformation experiments, samples of fault gouge were recovered from the apparatus, dried, and impregnated with epoxy resin. The samples were cut perpendicular to the shear zone boundary, parallel to the slip displacement and polished for analyses using a Zeiss GeminiSEM 450. High resolution images across a wide range of scales were collected using a backscattered electron detector. Localised failure in granular materials is characterised by a high strain gradient within a narrow area, which includes such microstructures as grain comminution, shear or compaction bands, and preferred alignment of clasts\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Strain was accommodated in the gouges through shear arrays including shear-parallel Y-shears and low-angle R\u003csub\u003e1\u003c/sub\u003e Riedel shears\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) that were associated with grain size reduction and were typically less than 50\u0026micro;m thick. The most prominent features in the micrographs are stress-relief (unloading) fractures that open in the sample during depressurisation of the triaxial apparatus. These commonly occur along sites of shear localization that are marked by grain size reduction. Hence these fractures were classified as shear fractures only if grain size reduction of quartz via cataclasis could be identified along their boundaries. The number and lengths of localized shears were quantified using FracPaQ\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFault gouges deformed at room temperature had a greater number of shear localization features and a small average shear localisation length normalised to sample area, at 1.14x10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003emm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the 25wt% kaolinite gouge. At elevated temperatures, the strain is accommodated in fewer but longer R\u003csub\u003e1\u003c/sub\u003e Riedel and Y shears, as the localized shear length by sample area in the 25wt% kaolinite gouge increased to 2.5x10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003emm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 140\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). This indicates that localisation of deformation increases with increasing temperature in clay-bearing fault gouges. The total amount of shear-enhanced compaction of the bulk fault gouge in the velocity step tests also increased with increasing temperature across the 5 different fault gouge compositions. By the end of the experiments, the 25wt% kaolinite gouge had compacted relative to the initial volume by 7.6% (71mm\u003csup\u003e3\u003c/sup\u003e) at 23\u0026deg;C and 11.8% (123mm\u003csup\u003e3\u003c/sup\u003e) at 180\u0026deg;C. Energy dispersive X-ray spectroscopy (EDS) chemical mapping of the gouge samples in the SEM showed a minor concentration of quartz over kaolinite in the localised shear zones, perhaps due to the EDS resolution limit of 1\u0026micro;m.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe issue of the disparity between field observations and clay mineral (\u003cem\u003ea-b\u003c/em\u003e) stability data from the laboratory raises questions on the applicability of utilising laboratory measurements in physics-based modelling of earthquake sequences\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. Many models involve RSF laws\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e and the parameters from rock friction studies conducted at room temperature. Due to a lack of RSF data at elevated temperature, few models incorporate a temperature (or depth) dependence of (\u003cem\u003ea-b\u003c/em\u003e)\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e, and the relationships that are used are commonly based upon materials such as granite gouge\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e that do not reflect natural fault lithologies present in mature faults. The results of this study show that \u0026le;\u0026thinsp;50wt% clay fault gouges can host unstable slip at temperatures typical of depths greater than ~\u0026thinsp;4km, assuming a geothermal gradient of 20\u0026deg;C.km\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). This indicates that the difference in temperature between ambient lab and subsurface crustal conditions can account for the wide variety of fault slip behaviours seen in nature, including the prominence of seismogenic slip on many major clay-rich faults, which otherwise would be difficult to explain. Segments of the example faults in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, including the LVF in Taiwan and the MTL in Japan have measured clay proportions of 1-69wt% clay\u003csup\u003e7,16\u003c/sup\u003e, which is represented in the range of gouge clay proportions tested in this study, and that acted as conditionally-unstable at elevated temperatures. The shear in the experiments was sometimes accommodated via stable creep until a velocity step increase was imposed, which induced unstable slip \u0026ndash; a scenario that may be similar to a propagating slip patch impinging on a clay-rich fault segment in nature.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis study has shown that with increasing temperature, shear enhanced compaction of fault gouge increases to produce a denser microstructure, and that grain comminution is pronounced in elongate, discrete shear zones. Increasing localisation of shear and decreasing grain size within a gouge microstructure decreases the stability of slip\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. The concurrent decrease in the stability parameters of (\u003cem\u003ea\u0026ndash;b\u003c/em\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003eRS\u003c/em\u003e\u003c/sub\u003e with increasing temperature reflect the microstructural change from distributed to localised deformation.\u003c/p\u003e \u003cp\u003eThe observed dependence of experimental rate and state friction on temperature\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e has led to several attempts at incorporating the effect of temperature into a microphysical rate and state friction framework\u003csup\u003e\u003cspan additionalcitationids=\"CR42 CR43\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Arrhenius-type relationships for pressure solution\u003csup\u003e\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e have been used to describe two slip-mechanism-driven regimes: cataclastic flow and dilation at low temperatures (100\u0026ndash;300\u0026deg;C for quartz) and solution-precipitation-aided cataclastic flow at higher (\u0026gt;\u0026thinsp;300\u0026deg;C) temperatures\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Den Hartog \u0026amp; Spiers\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e observed an increase in frictional strength and reduction in (\u003cem\u003ea-b\u003c/em\u003e) with increasing temperature (at \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003csup\u003e\u003cem\u003eeff\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;170MPa), and attributed the cause to the formation of a denser microstructure due to compaction via stress corrosion cracking or pressure solution. However, the high strain rates relative to nature in both Den Hartog \u0026amp; Spiers\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e (~\u0026thinsp;1x10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e to 1x10\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and this study (~\u0026thinsp;2x10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e to 2x10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003es\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) do not favour pressure solution as the dominant deformation mechanism\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. While the bulk pore volume decreased as compaction progressed with increasing temperature\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e, no evidence of pressure solution facilitating cataclasis was observed in this study, such as grain boundary truncation, indentation or mineral overgrowths\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe observed increase in shear-enhanced compaction and frictional strength, and the reduction in the (\u003cem\u003ea-b\u003c/em\u003e) and D\u003csub\u003eRS\u003c/sub\u003e parameters with increasing temperature in these experiments occur in the expected temperature window for the release of adsorbed water in kaolinite. Therefore, a hypothesis is here proposed that the gradual removal of adsorbed (free) water from kaolinite surfaces with increasing temperatures, and not pressure solution\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, is the micromechanical mechanism leading to enhanced compaction of the gouge microstructure and subsequent localisation of slip. Increasingly higher energy layers of adsorbed water are removed at progressively higher temperatures to give a temperature range over which the microstructure would evolve, with a peak in rate of weight loss at ~\u0026thinsp;70\u0026deg;C and a range of 30\u0026ndash;150\u0026deg;C for kaolinite from thermogravimetric (TG) analysis\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. The proportion of weight loss ranges from 0.5-2% at 200\u0026deg;C\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e,\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e, but increasing degrees of mechanical deformation of kaolinite have been shown to increase the proportion of mass loss to 5% at 200\u0026deg;C\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. The relative weakness of clay minerals is often attributed to the lubrication of clay grains by the films of adsorbed water\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e,\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e, therefore the removal of free water from clay surfaces at elevated temperatures would be expected to increase the frictional strength of clay-clay contacts, as was observed in this study. A heterogeneous distribution of stress within a clay-quartz gouge framework may contribute to the localisation of shear by removing adsorbed water preferentially from sites of anomalously high stress\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. Ripplocations, a nanoscale elastic buckling process in phyllosilicates\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e, may act as an efficient process for fluid transport away from sites of anomalously high stress \u0026ndash; in opening, ripplocations generate transient porosity and by reconstituting bonds, porosity is closed.\u003c/p\u003e \u003cp\u003eThe hypothesis that the loss of adsorbed water leads to the formation of a denser microstructure and, therefore, unstable slip behaviour should be investigated by applying this experimental procedure to a range of clay minerals, which release adsorbed water across characteristic temperature ranges. If earthquake models discount the potential for seismic slip on clay-bearing fault segments due to the use of room temperature RSF measurements, the seismic hazard associated with a clay-rich fault zone could be underestimated.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eSample Preparation:\u003c/h2\u003e \u003cp\u003eThe materials used to create the simulated fault gouges in this investigation were the 1:1 layered, dioctahedral clay (Supplementary Fig.\u0026nbsp;1) kaolinite KGa-1b from the Clay Minerals Society (CMS) and the quartz powder Min-U-Sil 15 from the US Silica Company. Both KGa-1b and Min-U-Sil 15 had purities quoted at \u0026gt;\u0026thinsp;99%\u003csup\u003e52\u003c/sup\u003e. The kaolinite and quartz powders had similar densities at 2.65g/cm\u003csup\u003e3\u003c/sup\u003e and fine mean grain sizes at \u0026lt;\u0026thinsp;2 \u0026micro;m and \u0026lt;\u0026thinsp;15 \u0026micro;m, respectively. To generate the simulated fault gouges, the powders were mixed via dry tumbling as the similar grain sizes and densities promote mixing over segregation. Kaolinite in the fault gouges was stepped in increments of 25 wt% to produce 3 intermediate mixtures that had been tumbled for a 3-hour period at 68 rpm in a 575 cm\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e container.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eTriaxial Deformation Apparatus:\u003c/h2\u003e \u003cp\u003eThe friction tests were conducted in a triaxial deformation apparatus in the Rock Deformation Laboratory at the University of Liverpool. The apparatus applied a confining pressure (\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e) using silicon oil at pressures up to 250 MPa. A PVC (room temperature) or Viton (elevated temperatures) jacket isolated the sample from the confining fluid so that a pore pressure of up to 200 MPa could be independently imposed on the sample using deionised water. An axial load (\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e) was applied to the sample through a motor-driven loading column with an internal force gauge that measured the axial force at a resolution greater than 0.05 kN.\u003c/p\u003e \u003cp\u003eIn experiments at elevated temperatures, three external knuckle band heaters spaced along the length of the pressure vessel conducted heat through the vessel to the sample (Supplementary Fig.\u0026nbsp;2). The temperatures of the band heaters and the sample were monitored via external and internal thermocouples with to a resolution of 0.1\u0026deg;C. Calibration tests show the temperature gradient across the length of the sample is \u0026lt;\u0026thinsp;2˚C. To minimise the conduction of heat through the sample assembly to the force gauge, a cooling plate sat between the base of the pressure vessel and the loading column. The maximum temperature of 180\u0026deg;C in this study was a limit not from the band heaters, but from jacketing, o-rings and direct shear spacers used in the sample assembly. In the experiments, the external furnace and cooling system were activated during the loading of confining and pore pressure to bring the sample to the target temperature ensuring that the target effective normal stress was not exceeded during the heating. Both the confining and pore pressure media were sensitive to the increase in temperature and were only brought to the target pressures once the target temperature had been established.\u003c/p\u003e \u003cp\u003eFault gouge samples were contained within the direct shear assembly for friction experiments. The direct shear sliders moved relative to each other to a maximum displacement of 6mm via the elastic deformation of two rubber spacers. Prior to an experiment, the 1.6 g of fault gouge sample was compacted in a uniaxial press at 50 MPa to improve the cohesion of the sample. The direct shear sliders housed connections to the control systems of the deformation apparatus, including the internal thermocouple and the pore pressure control. Porous 316 stainless steel disks within the direct shear sliders had a permeability of 10\u003csup\u003e\u0026minus;\u0026thinsp;13\u003c/sup\u003e m\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, which allowed the exchange of pore fluid between the sample and the servo-controlled pump. The change in volume in the pore pressure control pump during an experiment was used as a proxy for the shear enhanced compaction during a velocity-step experiment (Supplementary Fig.\u0026nbsp;3).\u003c/p\u003e \u003cp\u003eFor the experiments in this study, the RSF parameters were calculated using the software RSFit3000\u003csup\u003e53\u003c/sup\u003e, which uses a non-linear least square fit on the experimental data (Supplementary Figs.\u0026nbsp;4\u0026ndash;6). Fitting parameters in the program include the stiffness of the apparatus loading column.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eMicrostructural analysis:\u003c/h2\u003e \u003cp\u003eAll microstructural analyses were performed in a Zeiss GeminiSEM 450 in the Scanning Electron Microscopy Shared Research Facility (SEM SRF) at the University of Liverpool. The techniques used to collect information on the samples microstructure include Backscattered electron (BSE) imaging (Supplementary Fig.\u0026nbsp;7) and energy dispersive X-Ray spectroscopy (EDS, Supplementary Fig.\u0026nbsp;8). BSE imaging provides qualitative information on the spatial distribution of different phases present in a sample, based on the atomic number (z). EDS was used to confirm the minerals present based on their elemental composition. Both large area datasets and high-resolution images were acquired, spanning cm to sub-micron scales. Microstructural data acquired for each sample were then used to measure the number, length, distribution, and frequency of shear fractures present in each of the samples tested experimentally, using the software package FracPaQ.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements:\u003c/h2\u003e \u003cp\u003eWe acknowledge financial support from a Natural Environment Research Council grant NE/V011804/1 to DRF and the NERC EAO DTP studentship to IRA at the University of Liverpool. Michael Allen provided essential technical support in the maintenance and development of the experimental apparatus. We gratefully acknowledge the Scanning Electron Microscopy Shared Research Facility (SEM SRF), in particular Joe Gardner, for their technical support, training \u0026amp; assistance with microstructural analysis.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eGiorgetti, C., Carpenter, B. M. \u0026amp; Collettini, C. Frictional behavior of talc-calcite mixtures. \u003cem\u003eJ. Geophys. Res. Solid Earth\u003c/em\u003e\u003cstrong\u003e120\u003c/strong\u003e, 6614\u0026ndash;6633 (2015).\u003c/li\u003e\n \u003cli\u003eTembe, S., Lockner, D. A. \u0026amp; Wong, T. F. Effect of clay content and mineralogy on frictional sliding behavior of simulated gouges: Binary and ternary mixtures of quartz, illite, and montmorillonite. \u003cem\u003eJ. Geophys. Res. Solid Earth\u003c/em\u003e\u003cstrong\u003e115\u003c/strong\u003e, (2010).\u003c/li\u003e\n \u003cli\u003eIkari, M. J., Saffer, D. M. \u0026amp; Marone, C. Frictional and hydrologic properties of clay-rich fault gouge. \u003cem\u003eJ. Geophys. Res. Solid Earth\u003c/em\u003e\u003cstrong\u003e114\u003c/strong\u003e, 1\u0026ndash;18 (2009).\u003c/li\u003e\n \u003cli\u003eSaffer, D. M. \u0026amp; Marone, C. 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RSFit3000: A MATLAB GUI-based program for determining rate and state frictional parameters from experimental data. \u003cem\u003eGeosphere\u003c/em\u003e\u003cstrong\u003e15\u003c/strong\u003e, 1665\u0026ndash;1676 (2019).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4612539/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4612539/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLarge earthquakes nucleate on crustal faults that have accumulated significant slip displacement and field observations show that these faults are ubiquitously clay-rich. Earthquake nucleation requires a reduction in shear resistance for instability to develop. Previous laboratory friction measurements indicate that only stable fault creep should occur in clay-rich faults; a result at odds with observations of widespread earthquake behaviour on mature clay-rich faults in nature.\u003c/p\u003e\n\u003cp\u003eHere we show that the frictional stability of synthetic clay-bearing fault gouges decreases systematically with elevated temperatures commensurate with those found at typical earthquake depths. In materials containing ≤50% clay, the stability of slip decreases with increasing temperature so that gouges display unstable slip at temperatures between 100 and 180°C. At room temperature the same materials host only stable slip. This reduction in stability with increasing temperature coincides with a greater degree of localization observed in the gouge microstructure and with progressive loss of water adsorbed on clay surfaces.\u003c/p\u003e\n\u003cp\u003eOur results indicate that a broad compositional range of clay-bearing fault rocks, and therefore mature faults, can nucleate unstable slip at conditions common to the clay-bearing brittle crust; a result that resolves the apparent paradox that mature clay-bearing faults in nature can nucleate and propagate earthquakes.\u003c/p\u003e","manuscriptTitle":"Clay-rich fault gouges become frictionally less stable at elevated temperatures","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-08-05 06:56:27","doi":"10.21203/rs.3.rs-4612539/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a560fb6a-06a9-4f03-a8c5-9a8804c849b5","owner":[],"postedDate":"August 5th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":35472843,"name":"Earth and environmental sciences/Natural hazards"},{"id":35472844,"name":"Earth and environmental sciences/Solid Earth sciences/Tectonics"}],"tags":[],"updatedAt":"2024-10-11T14:56:45+00:00","versionOfRecord":[],"versionCreatedAt":"2024-08-05 06:56:27","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4612539","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4612539","identity":"rs-4612539","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}