TS4.5

Repeating earthquakes are thought to represent the repeated rupture of loaded patches surrounded by regions that are slipping aseismically; they provide a natural laboratory to study interactions between seismic and aseismic processes. These events occur less often than one would expect if these earthquakes accommodate all of the long-term slip. Recent crack models using rate-and-state friction (Cattania and Segall, 2019; Chen and Lapusta, 2009 ) suggest a possible explanation: for small events, a larger amount of the slip budget on the patch being taken up by aseismic slip. For larger events where most of the slip budget is seismic, the patch experiences partial ruptures, also leading to the deviation from expected scaling. We aim to test the predictions of this model of repeating ruptures by searching for the proposed partial ruptures. We choose to search using the Northern California earthquakes catalogue, which contains many well-located repeating earthquake sequences. Preliminary results suggest that partial ruptures in the Parkfield region are not common. If preliminary results pass additional tests, it may suggest that partial ruptures do not make up a significant proportion of the slip budget of larger repeating earthquakes in this region. 

How to cite: Turner, A. and Hawthorne, J.: Searching for partial ruptures of repeating earthquakes in Parkfield, California, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2559, https://doi.org/10.5194/egusphere-egu22-2559, 2022.

Crustal deformation comprises a combination of seismic energy release occurring by earthquakes and aseismic unloading through creeping or frictional sliding. Efficient segregation of the seismic component attributed to faults is critical in evaluating Seismic Activity Rate (SAR) or Magnitude Frequency Distribution (MFD) of earthquakes in fault-based hazard assessment. The MFDs are routinely calculated by utilizing fault geometry and slip rate, evaluated from geodetic (or geological) data. The slip rates as an integrated representation of elastic and anelastic loadings overestimate the MFDs since earthquakes release only the elastic strain. To work around this problem, the seismic/geodetic moment-rate ratio defined as Seismic Coupling Coefficient (SCC) is incorporated in this study to account for the seismic portion of the total moment rate to calculate MFDs. The parameter has been studied for different tectonic regions worldwide, including the USA, Canada, Iran, Greece, and Italy. We modify the Moment Budget (MB) algorithm introduced by Pace et al. (2016) to weight the total moment rate corresponding to the maximum magnitude (M_max) generated by the modeled faults by incorporating the SCC. An updated mean recurrence time (T_mean) for the M_max and its corresponding uncertainty is computed when SCC is incorporated in the calculation. Then, the seismic moment, the SCC weighted T_mean, and its uncertainty are utilized to compute MFDs by balancing the modeled seismic moment rates (by Doubly Truncated Gutenberg-Richter (DTGR) or Characteristic Gaussian (CHG)) and the SCC weighted moment rates. This process is implemented by the Activity Rates (AR) tool of FiSH codes. Fault data of 89 fault segments in Zagros, Iran, are introduced into the algorithm to compute the SCC incorporated MFDs. The acquired fault-based hazard maps are in harmony with the history of seismicity and tectonics of the region, while the total moment rates exaggerate the calculated hazard. Future work involves implementing the processing algorithm on hazard assessment in the Gulf of Guinea. This research contributes to the FCT-funded SHAZAM (Ref. PTDC/CTA-GEO/31475/2017), IDL (Ref. FCT/UIDB/50019/2020), and SIGHT (Ref. PTDC/CTA-GEF/30264/2017) projects. It also uses computational resources provided by C4G (Collaboratory for Geosciences) (Ref. PINFRA/22151/2016).

How to cite: Tavakolizadeh, N., Mohammadigheymasi, H., Matias, L., Silveira, G., Fernandes, R., and Dolatabadi, N.: To what extent do slip rates contribute to the seismic activity of faults?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12893, https://doi.org/10.5194/egusphere-egu22-12893, 2022.

The short-term slow slip event (SSE), a class of slow earthquakes which has duration of a few to tens of days, is typically detected and modeled from daily static Global Positioning System (GPS) data. However, the daily GPS data cannot image the sub-daily SSE processes, so underlying mechanisms of SSEs have been still elusive. By processing the raw GPS observables in the kinematic analysis approach, we can obtain surface deformation field at the subdaily interval, which has great potential to overcome the time resolution issue present in the daily static GPS data. Although the kinematic GPS coordinates are known much noisier (~ cm) than the daily static coordinates (~ a few mm), recent applications to postseismic deformation studies achieved identifying sub-cm deformation. Motivated by them, we for the first time applied the kinematic GPS coordinates to model the short-term SSE.

We chose one Cascadia SSE in March – April 2017, which has been already reported from daily GNSS data, and performed the kinematic GPS analysis at a 30-second interval for observations during the event occurrence. Although the obtained raw coordinate series were quite noisy, we were able to discern the transient motion of a few mm during the event after carefully removing non-tectonic position fluctuation such as multipath effects, common mode errors and outliers.

Then, we inverted the cleaned data at a 30-minute interval using a Kalman-filter based method to infer spatiotemporal evolution of slip. The obtained spatiotemporal slip distribution exhibits a multi-stage evolution consisting of an isotropic growth of SSE and subsequent along-strike migration and termination. The transition of the slip growth mode occurs when the slip area fills the rheologically permitted down-dip width for the SSE occurrence. As conceptualized by Gomberg et al. (2016, GRL), this is analogous to the rupture growth of regular great earthquakes, so it implies the presence of common mechanical factors behind the two distinct slip phenomena. The inferred moment rate has two peaks, which are consistent with the daily tremor counts in this region.

We carried out another slip inversion using the daily static GPS data recorded during the same period and the same inversion method to investigate the performance and limitation of our kinematic GPS data. A moment rate inferred from the daily data has also two peaks, so our 30-minute inversion result has the comparable time resolution to that derived from the widely-used daily data. This is an astonishing result given the long-believed low signal-to-noise ratio of the kinematic GPS. Our results strongly highlight the importance of better understanding of the non-tectonic noise in the kinematic GPS analysis, which will further improve the temporal resolution of SSE.

How to cite: Itoh, Y., Aoki, Y., and Fukuda, J.: Imaging evolution of Cascadia slow-slip event using high-rate GPS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-879, https://doi.org/10.5194/egusphere-egu22-879, 2022.

Large earthquakes impose differential stresses in the crust and upper mantle that are transiently relaxed during the postseismic phase mostly due to afterslip on the fault interface, viscoelastic relaxation in the lower crust and upper mantle, and poroelastic rebound in the upper crust. During the last years, the wealth of geophysical and geodetic observations, as well as great effort in forward and inverse modelling have allowed a better comprehension of the role of these mechanisms during the postseismic period. However, it is still an open question to what extent postseismic processes contribute to the surface deformation signal, especially during the early postseismic period. In this study, we use GNSS and InSAR observations collected in the first 48 days following the 2010 Maule earthquake in Chile along with a model approach that integrates afterslip, poroelasticity, and temperature-controlled power-law (non-linear viscosity) rheology. The afterslip distribution is obtained from a geodetic data inversion after removing the poro-viscoelastic component by forward modelling to the geodetic data. We find that our model approach explains the geodetic cumulative signal 14% better than a pure elastic model inverting for afterslip. This improvement is mainly produced by the better fit to the geodetic signal at the volcanic and back-arc regions due to the inclusion of non-linear viscoelastic processes, which can explain > 60% of the observed surface displacements in these regions. We also show that poroelastic processes play a significant role locally, specifically near the region where the coseismic slip was largest. Here, poroelastic processes explain most of the cumulative observed GNSS uplift signal and produce surface landward patterns that affect the horizontal GNSS component by up to 15% in the opposite direction. If poroelastic processes are ignored, our results reveal that the resulting afterslip amplitude is both amplified and suppressed by up to 40% in regions of ~50 x 50 km². Our findings have implications for the calculation of the postseismic slip budget, and therefore the seismic hazard assessment of future earthquakes.

How to cite: Peña, C., Metzger, S., Heidbach, O., Bedford, J., Bookhagen, B., Moreno, M., Oncken, O., and Cotton, F.: Relative contribution of afterslip, non-linear viscous, and poroelastic processes to the early postseismic deformation field of the 2010 Maule earthquake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-943, https://doi.org/10.5194/egusphere-egu22-943, 2022.

Geodetic, seismological, gravimetric, and geomorphic proxies have widely been used to understand the behavior of the shallow portion of subduction megathrusts and answer questions related to seismic asperities: Where are they located, and how large are they? How close are they to failure, and how strong are they coupled? Our current knowledge of the kinematics and dynamics of megathrust earthquakes is limited due to their offshore location, and that our observations only cover a fraction of one megathrust earthquake cycle. 

The frictional-elastoplastic interaction between the interface and its overriding wedge causes variable surface strain signals such that the wedge strain pattern may reveal the mechanical state of the interface. We here contribute to this discussion using observations and interpretations of controlled analog megathrust experiments highlighting the variability of deformation signals in subduction zones. To examine the interaction, we investigate seismotectonic scale models representing a seismically heterogenous interface and capture the model’s surface displacements by employing a “laboratory-geodetic” method with high spatio-temporal resolution. Our experiments generate physically self‐consistent, analog megathrust earthquake ruptures over multiple seismic cycles at laboratory scale to study the interplay between short-term elastic and long-term permanent deformation. 

Our results demonstrate that frictional-elastoplastic interaction partitions the upper plate into a trench-parallel and -perpendicular strain domain, experiencing opposite strain (contraction vs. extension) during the co- and interseismic phase of the seismic cycle. Moreover, the pattern differs in the off- and onshore segments of the upper plate. This implies that the seismic potential of the shallow (offshore) portion of the megathrust may be underrepresented if only onshore observations are included in the estimate. However, our models suggest that, in the case of strong frictional contrast (velocity weakening vs. strengthening) on the interface, the short-term, onshore strain pattern (dominated by elastic deformation) may suffice to map the frictional heterogeneity of the shallow interface along strike. Finally, the frictional heterogeneity of the shallow interface is well reflected by the permanent surface strain observed offshore and partially in the strain observed at the coastal region. The observed along-trench segmentation predicted by our models is reasonably compatible with short-term, elastic geodetic observations and permanent geomorphic features in nature.

How to cite: Kosari, E., Rosenau, M., Bedford, J., Deng, Z., Metzger, S., Schurr, B., and Oncken, O.: Linking surface strain signals with frictional heterogeneity of the interface in a laboratory-scale subduction megathrust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3209, https://doi.org/10.5194/egusphere-egu22-3209, 2022.

Tectonic faults display a range of slip behaviours including continuous and episodic slip covering rates of more than 10 orders of magnitude. To gain insight into the slip behaviour of brittle faults, we performed laboratory stick-slip experiments at low pressure using dry “sticky” rice as rock analogue material. Rice has been shown to be a valuable material obeying the rate and state friction laws qualitatively and quantitatively and mimicking the full spectrum of seismogenic fault behaviour (“ricemic cycles”) depending on boundary conditions. The deformation mechanism is granular flow and as such transient hardening and weakening phenomena such as strain localization or stick-slip are accompanied by dilation and compaction, respectively. Such a rheology might be similar in rocks at various scales (grain scale to regional tectonic scale).

We here report on ring shear test experiments on a range of rice varieties, including full-grain and crushed sorts. We imposed boundary conditions (i.e., normal load, shear velocity) scaled down from nature under which our fault analogue shows a variety of slip behaviours ranging from slow and quasi-continuous creep to episodic slow slip to dynamic rupture. The experiments demonstrate that significant interseismic creep (up to far-field loading rate) and earthquakes may not be mutually exclusive phenomena for a given location along a fault. Moreover, creep signals vary systematically with the fault’s seismic potential. Accordingly, the transience of interseismic creep scales with fault strength and seismic coupling as well as with the maturity of the seismic cycle. Loading rate independence of creep signals suggests that the long-term stationary mechanical properties of faults (e.g. seismic coupling) can be inferred from short-term observations (e.g. aftershock sequences). Moreover, we observe the number and size of small episodic slip events to systematically increase towards the end of the seismic cycle providing an observable proxy of the relative shear stress state on seismogenic faults. 

Importantly, very weak faults (with low effective normal loads) in a late stage of their seismic cycle might creep at rates very close to far-field loading for extended periods of the interseismic stage (~decades before failure). Given that we typically observe only a fraction of seismic cycles with high resolution (with geodetic methods) in nature, this might lead to the false belief of the fault being aseismic and not hosting large earthquakes. We thus demonstrate that seismic and aseismic behaviour might not necessarily be mutually exclusive.

How to cite: Rosenau, M., Corbi, F., Cubas, N., Funiciello, F., Kosari, E., Maillot, B., Mastella, G., Oncken, O., Rudolf, M., Souloumiac, P., and Visage, S.: Creep on seismogenic faults: Insights from analogue earthquake experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4533, https://doi.org/10.5194/egusphere-egu22-4533, 2022.

In recent years, machine learning has been used to predict earthquake-like failures in various laboratory experiments. The predictions of these approaches have been framed with both regression and classification. Labquakes prediction in direct shear experiments has been achieved by predicting the time to failure of the sample (regression). Similarly, for laboratory analog subduction models, the time to failure has been successfully predicted. In the classification approach, demonstrated on analog models, a time window of “imminence” is predefined and the model determines if failure occurs within this time window or not. These previous approaches suffer from the problem of thresholding: in time-to-failure regression, there is the need to define a velocity or displacement that signals an event has occurred, in imminence classification the choice is the time window that we consider an event to be imminent. Here we remove this thresholding problem by taking a spatiotemporal regression framing that forecasts future surface velocity fields from past ones. In such a framing, the whole seismic cycle is forecast (i.e., interseismic, coseismic, and postseismic). We test this approach on Foamquake.

Foamquake is a novel 3D elastoplastic seismotectonic analog model mimicking the key features of the subduction megathrust seismic cycle in a scaled manner. Foamquake features a wedge-shaped elastic upper plate made of foam rubber. The analog megathrust includes a velocity weakening, rectangular patch embedded in a velocity neutral matrix. Plate convergence is imposed kinematically with a motor-driven belt (analog of the subducting plate) underthrusting the wedge. Foamquake experiences quasi-periodic cycles of stress accumulation and sudden drops through spontaneous nucleation of frictional instabilities. These labquakes are characterized by coseismic displacement of a few tens of meters when scaled to nature and source parameters (seismic moment-duration and moment-rupture area) scaling as real subduction interplate earthquakes. The 3D nature of Foamquake allows running models with two asperities along strike of the subduction zone divided by a barrier. This configuration generates sequences of full and partial ruptures, superimposed cycles, and nested rupture cascades: complex patterns similar to those inferred at natural megathrusts, representing the perfect testbed for developing new prediction strategies.

In particular, we step toward forecasting seismic cycle full surface velocity fields using deep-learning-based approaches from the Computer Vision field. This framing allows simultaneously to forecast the onset of a labquake and illuminate its space-time evolution at different prediction horizons. A variety of deep-learning algorithms have been tested and compared with Random Forest models (which we consider as a baseline machine learning model). We show that Convolutional Recurrent Neural Networks, with spatiotemporal sequences of surface velocities as input, perform the best in forecasting. Preliminary results suggest that the onset and the spatio-temporal propagation of individual lab-quakes can be predicted with relatively high accuracy at prediction horizons that are in the same order of labquake durations. Surface velocities at further horizons than labquake durations appear unpredictable. This study introduces an innovative framing of the earthquake forecasting problem which can open new perspectives for application to natural observations.

How to cite: Mastella, G., Corbi, F., Bedford, J., Funiciello, F., and Rosenau, M.: Forecasting earthquake rupture characteristics with deep learning: a proof of concept using analog laboratory foamquakes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2996, https://doi.org/10.5194/egusphere-egu22-2996, 2022.

During large strike-slip earthquakes, the surface displacement can be measured from correlation of satellite images acquired before and after the event. These measurements allow quantifying the total surface displacement and knowing how it distributes between on and off-fault deformation, which is important for seismic hazard assessment. These measurements are highly variable, partly due to the sparsity of natural examples. This research focuses on analogue modelling to study parameters affecting the surface displacement in a controlled environment. However, current analogue models do not address the issue of surface deformation associated with strike-slip earthquakes. Instead, models using granular materials, such as sand or clay, rather focus on the surface deformation associated with continuous deformation without earthquakes, while models using rigid materials, such as foam or gelatin, focus on the localization and frequency of seismic events without looking at surface deformation.

To analyze the long-term deformation of seismogenic strike-slip faults from analogue experiments, we used a box composed of two juxtaposed PVC plates simulating a vertical and linear basement fault. A strike-slip fault emerges from this discontinuity. The box is filled with rice and rubber pellets in order to produce both aseismic displacements and earthquakes along the evolving strike-slip fault. Dry rice is a stick-slip granular material already used in subduction experiments. We used a twice broken rice with peak, dynamic, and reactivation friction values of respectively 0.78, 0.67 and 0.68 at a constant shear velocity. Those values decrease when the shear velocity is increased (the parameter “a-b” = -0.012 in the rate friction law of the rice). Since rice is too rigid to produce a measurable elastic strain release, we added a basal layer of fine rubber pellets between the basal PVC plates and the rice layer in order to store elastic strain. Applying a constant displacement velocity and taking photos every 25 micrometers of displacement, we follow the surface displacements through image correlation.

We observe that the average displacement along a profile parallel to the fault (taken at a distance of the basal fault corresponding to half the rice layer thickness) only matches the applied displacement when averaged over the whole experiments. Indeed, during most of the experiment, the observed incremental displacement is lower than the applied one, but from time to time it catches up during discreet events that produce large displacements of up to four times the applied incremental displacement. We interpret these events as seismic events. Hence, the evolution of cumulative displacement with time exhibits some phases of creep, more or less at the same rate as the input rate, during the inter-seismic period, and phases of sudden displacements corresponding to sudden release of elastic strain, i.e., earthquakes. However, we never observe a complete blocking phase (sticking phase). These first results show that it is possible to build an experimental strike-slip fault system in a granular medium with a low normal stress, i.e. a free surface, that produces extended periods of partial stress loading during creep phases, alternating with period of sudden stress release during displacement phases. 

How to cite: Visage, S., Souloumiac, P., Maillot, B., Cubas, N., and Klinger, Y.: Experimental strike-slip earthquakes (“ricequakes”), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9868, https://doi.org/10.5194/egusphere-egu22-9868, 2022.

Mirror-like surfaces (MSs) are easily recognizable in the field since they reflect natural visible light, thanks to their low surface roughness (nm-scale). These ultra-polished surfaces are often found in seismogenic fault zones cutting limestones and dolostones (e.g., Siman-Tov et al., 2013; Fondriest et al., 2013; Ohl et al., 2020). Both natural and experimentally-produced fault-related MSs were described in spatial association with ultrafine matrix (grain size <10µm), nanograins (<100nm in size), amorphous carbon, decomposition products of calcite/dolomite (i.e., portlandite, periclase) and larger in size but “truncated” clasts (Verberne et al., 2019). However, the mechanism of formation of MSs is still a matter of debate. Indeed, experimental evidence shows that MSs can develop both under seismic (slip rate ≈1 m/s; Fondriest et al., 2013; Siman-Tov et al., 2013; Pozzi et al., 2018; Ohl et al., 2020), and aseismic (slip rate ≈0.1-10 µm/s; Verberne et al., 2013; Tesei et al., 2017) deformation conditions, involving various physical-chemical processes operating over a broad range of P-T conditions, strain, and strain rates.

To better constrain the formation mechanism of MSs and their role in the seismic cycle, field, and high-resolution microstructural investigations, combined with thermal maturity analyses, were conducted on MSs cutting Triassic bituminous dolostones from the Italian Central Apennines. This region is one of the most seismically active areas in the Mediterranean (e.g., L’Aquila 2009, Mw 6.3 earthquake), with mainshocks and aftershocks propagating along extensional faults, cutting km-thick sequences of carbonates. The studied faults are hosted in the footwall of the younger-on-older Monte Camicia thrust, related to the Pliocene to Holocene in age Apenninic compressional to extensional tectonics and exhumed from < 4 km depth. The MSs samples were collected from faults with evidence of increasing cumulated slip (from few mm to few meters) and different attitudes (variable resolved stresses) to evaluate i) whether the thermal maturity of organic matter on fault surfaces preserved a trace of frictional heating and ii) to estimate the role of variable mechanical work in their formation.

The microstructures of the MSs and the associated slip zones display a polyphasic deformation history; smeared bitumen along the slip surfaces is spatially associated with (i) discrete ultracataclastic slip zones containing fragments of older bitumen-rich slip zones and calcite-rich vein-precipitated matrix and, (ii) lower strain cataclastic layers with evidence of pressure-solution in the dolostone clasts and viscous shear in the bitumen. Such different deformation styles of bitumen-rich materials might be an evidence of high strain rate coseismic embrittlement and long-term aseismic creep during the seismic cycle.

Micro-Raman analyses on the MSs and their wall rocks have been aimed at quantifying the thermal maturity of the organic matter on slip surfaces that can reveal thermal pulses associated to frictional heating during seismic slip. This multidisciplinary study, though finalized to a deep understanding of their formation mechanism, may lead to recognize microstructural or mineralogical/geochemical features specifically associated to earthquake ruptures in natural faults with a potential impact on seismic hazard studies.

How to cite: Chinello, M., Bersan, E., Fondriest, M., Tesei, T., Corrado, S., and Di Toro, G.: Field, microstructural and phase characterization of mirror-like fault surfaces in bituminous dolostones (central Apennines, Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2415, https://doi.org/10.5194/egusphere-egu22-2415, 2022.

The eastern boundary of the Sichuan-Yunnan tectonic block and Tibetan Plateau is marked by a highly active fault zone featuring four left-lateral strike-slip faults, the Xianshuihe, Anninghe, Zemuhe, and Xiaojiang faults. These collectively show the highest seismicity in southwestern China. Since 1977, a portion of the Anninghe faults (AHF) has experienced seismic quiescence for M_L≥4.0 earthquakes. The spatial extent of this quiescent portion has gradually dwindled with time resulting in the formation of the current 130-km-long “Anninghe seismic gap”. To evaluate the seismic potential and model seismogenesis on this part of the AHF, data are needed on the frictional properties of relevant fault zone materials under mid-crustal hydrothermal conditions.

In this study, we report both saw-cut and rotary shear friction experiments performed on sieved granite gouge collected from the AHF and believed to represent the fault rock composition at seismogenic depth. Experiments were conducted on 1 mm thick gouge layers at 100-600℃, effective normal stress of 100-200MPa, pore water pressures of 30MPa and 100 MPa, and sliding velocities of 0.01-100μm/s . The saw-cut tests reached shear displacements up to 4 mm versus 30 mm in the ring shear experiments. Friction coefficient lays in the range 0.6-0.8 in most samples, except that it drops to 0.4 at higher temperatures and low velocity. In the saw-cut experiments performed at 30MPa pore water pressure, velocity-strengthening behaviour occurred below 200℃ (Regime 1), whereas velocity-weakening occurred at 200-600℃ (Regime 2). By contrast, dry saw-cut experiments showed velocity-strengthening at all temperatures investigated (25-600℃). In the rotary shear experiments performed at 100MPa pore water pressure, three temperature-dependent regimes of behaviour were identified, showing potentially unstable, velocity-weakening behaviour at 100-400℃ (Regime 2) and velocity-strengthening at lower and higher temperatures (Regimes 1 and 3). These regimes moved towards higher temperatures with an increase in sliding velocity. Combining all the data, the importance of Regime 2, i.e. the temperature range characterized by velocity-weakening, potentially seismogenic behaviour, decreased with increasing pore water pressure, shear displacement and effective normal stress. Combined with our microstructural observation and previous studies, we explain our results qualitatively in terms of a microphysical model in which changes in friction coefficient and (a-b) are caused by competition between dilatant granular flow and grain-scale creep processes.

Since the geothermal gradient around AHF is approximately 30 ℃/km, direct application of our results suggests velocity-weakening (Regime 2) on the AHF at depths of 2.5-12.5 km, and velocity-strengthening at shallower and deeper levels. By comparison, the depth range of the AHF seismic gap (locking region) is 0 to 15 km, additionally, the relocated small earthquake distribution in southwestern China shows that the depth of hypocenters are mostly less than 15km, which is consistent with our experimental results. However, our experiments show that the velocity-weakening regime for AHF gouge is controlled by many factors besides temperature, such as effective normal stress, pore fluid pressure, shear displacement and velocity. Further progress towards understanding the seismic gap, and allowing rupture nucleation modelling, for example, therefore requires a more quantitative microphysical modelling approach in future.

How to cite: Lei, H., Niemeijer, A. R., Zhou, Y., and Spiers, C. J.: Seismic potential of the Anninghe Fault zone, southeastern Tibetan Plateau: Constrains from friction experiments on natural granite gouge, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6560, https://doi.org/10.5194/egusphere-egu22-6560, 2022.

Tectonic pseudotachylytes are produced by rapid sliding and melting, and then solidified fast in faults during earthquakes, which are considered as fossil earthquake. Pseudotachylytes record the physical-chemical processes related to earthquake in fault zone, which are essential materials for understanding the history of fault activity.  Here we focus on the pseudotachylytes and cataclastic rocks in the East Yibug Caka fault, SN-trending normal fault in the Qiangtang terrane, in the hinterland of the Tibetan Plateau. Combined optical microscope, scanning electron microscope, powder X-ray diffraction (XRD) with in situ X-ray fluorescence (XRF) analyses, their microstructures, mineral composition and elemental distribution were analyzed in detail. Field investigation shows that the dark gray to brown in color pseudotachylytes, associated with cataclastic rocks, are occurred as fault veins and injection veins with thickness ranging from a few mm to 1 cm. Microstructural observations show that multiple lines of evidence, such as embayed quartz fragments, honeycomb-like vesciles and locally developed microcrystallines and cluster aggregates, indicate that the pseudotachylytes were the products of frictional melting during the seismic slip. In addition, pseudotachylytes present as clasts in cataclastic rocks and fault breccias, and younger cataclastic rocks contain breccia of earlier cataclastic rocks, these characteristcs indicate that large seismic events occurred repeatedly in this fault zone. Considering the initial active time of the normal faults in this area is 13.5 Ma, the formation depth of the pseudotachylytes and associated cataclastic rocks is 10 km, the exhumation rate of the these fault rocks from deep depth is at least ~0.74 mm/yr. Pseudotachylytes along normal faults are seldom reported, this is the first time that we find melt-origined pseudotachylytes in the SN-trending normal faults in the Qiangtang terrane, and where present they have important implications for learning regional seismic activity and fault evolution process.

How to cite: Wang, H., Li, H., Sun, Z., and He, X.: Discovery of the pseudotachylytes in the Qiangtang Rift, Tibet, and their petrological characteristics and tectonic significance, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1052, https://doi.org/10.5194/egusphere-egu22-1052, 2022.

The 2017 Mw 5.5 Pohang earthquake in South Korea has been reported as one of the largest triggered earthquakes at an enhanced geothermal system (EGS) site. A fault that was ruptured in Pohang was not identified by geological investigations or geophysical surveys before the Mw 5.5 Pohang earthquake. “Mud balls” showing a fault gouge structure were reported in the Pohang EGS site only at the depth range of 3,790 – 3,816 m. In this study, we present new observation on the fault rocks retrieved from the Pohang EGS site as drill cuttings. The drill cuttings from 3,256 – 3,911 m interval contained mud balls similar to those observed at the depth of 3,790 – 3,816 m. Mud balls contained fine grains and showed foliated clay matrix with well-rounded clasts of quartz or feldspar, which are a typical fault gouge structure. In addition, mud balls retrieved from the depth of 3,256 and 3,260 m contained black fragments. SEM and TEM observation revealed that these black fragments consist of glassy matrix with sub-micrometer size clasts. Abundant vesicles were observed inside the black fragments, and some of the black fragments preserved foliation defined by compositional layering. TEM observation confirmed that the glassy matrix in the black fragments is amorphous material with a chemical composition similar to illite-smectite. These observations indicate that black fragments are resulted from the frictional melting during the coseismic slip.

How to cite: Jung, S., Kang, J.-H., Kil, Y., and Jung, H.: Evidence of frictional melting observed in the fault rock drill cuttings from Pohang enhanced geothermal system (EGS) site, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3617, https://doi.org/10.5194/egusphere-egu22-3617, 2022.

Theoretical studies predict that during earthquake rupture faults slide at non-constant slip velocity, however it is not clear which source time functions are compatible with the high velocity rheology of earthquake faults. Here we present results from high velocity friction experiments with non-constant velocity history, employing a well-known seismic source solution compatible with earthquake source kinematics. The evolution of friction in experiments shows a strong dependence on the applied slip history, and parameters relevant to the energetics of faulting scale with the impulsiveness of the applied slip function. When comparing constitutive models of strength against our experimental results we demonstrate that the evolution of fault strength is directly controlled by the temperature evolution on and off the fault. Flash heating predicts weakening behaviour at short timescales, but at larger timescales strength is better predicted by a viscous creep rheology. We use a steady-state slip pulse to test the compatibility of our strength measurements at imposed slip rate history with the stress predicted from elastodynamic equilibrium. Whilst some compatibility is observed, the strength evolution indicates that slip acceleration and deceleration might be more rapid than that imposed in our experiments. 

How to cite: Harbord, C., Brantut, N., Spagnuolo, E., and Di Toro, G.: Fault friction during simulated seismic slip pulses, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8340, https://doi.org/10.5194/egusphere-egu22-8340, 2022.

Natural faults are heterogeneous features, with complex geometries and material properties. Understanding how the geometrical complexities of a fault affects the dynamics and preparatory phase of earthquakes is of crucial importance for seismic hazard assessment. In laboratory samples, frictional sliding along prefabricated faults may produce so called stick-slips comparable to dynamic ruptures observed during earthquakes. While the effect of roughness has been shown to influence significantly the frictional behavior of laboratory faults, there are only a few studies investigating more complex types of fault heterogeneities. In this study, we conduct friction experiments on granite with inclined sawcut faults, under a constant confining pressure of 35MPa. Samples are loaded using an axial displacement rate of 0.5 µm/s.  At  similar boundary conditions we compare the slip behavior of (1) a smooth fault, (2) a smooth fault with a single asperity, a 7 mm diameter vertical pin traversing the contact interface, and (3) a rough fault prepared by sandblasting the surface with silicon carbide. A key result of this study is that slip behavior depends on fault roughness and is influenced in a non-trivial way by asperities. The smooth fault displays unstable stick-slip as opposed to the rough fault showing predominantly creep. The smooth fault with the pin exhibits a slip behavior in-between, with very regular stress oscillations that seem to be attenuated by the presence of the pin (asperity). Only after failure of the pin, we observe the stress drop during instabilities to increase regularly with cumulative slip. We also show that in the case of a fault with a single asperity, the slip velocity is less than an order of magnitude lower compared to a similar smooth fault without this asperity.

How to cite: Guerin-Marthe, S., Dresen, G., Kwiatek, G., Wang, L., Bonnelye, A., and Martinez-Garzon, P.: Effects of asperities and roughness on frictional slip of laboratory faults, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9997, https://doi.org/10.5194/egusphere-egu22-9997, 2022.

The transition between seismic slip and aseismic creep of faults in the Earth crust suggests a strong time-dependent mechanism for the underlying physics and corresponding mechanical response of fault slip. Asperities establish the real contact on the slipping interface of a fault and serve as stress concentrators that control the initiation of earthquakes. Investigating the interactions between individual asperities and how the global stability of a fault is controlled by the collective effects of their local behaviors are essential for understanding the intrinsic relationships between earthquake swarms and faulting. Here we design a novel direct-shear experimental setup, which allows a thick PMMA (poly-methyl-methacrylate) plate to slide slowly on a customized surface, on which asperities are modeled by spherical PMMA beads and embedded in a softer polymer base, for analogizing tectonic faults. We perform various experiments by applying multiple normal loads and loading rates, with a high-resolution camera employed to capture the detailed activities of asperities. We demonstrate the global stability of a fault could be described by the synthesized behaviors of local asperities. We also prove, for the same asperity, it can experience different slip modes at different time periods. We generate a catalog of fault slip events defined by the slipping velocity of each asperity derived from the image correlation technology, and then we determine slip episodes based on time and space successively. Furthermore, we investigate the distributions of various parameters of the determined slip episodes, including the number of slipping asperities, as well as the duration, mean slip displacement, and moment of slip episodes. We explore the spatiotemporal variations of b-value within one analog seismic cycle and under different normal loads and loading rates. We link the findings at local scales with the bulk mechanical response of the whole fault. Our results bring new insights into the physics and mechanics of seismic and aseismic faulting.

How to cite: Shu, W., Lengliné, O., and Schmittbuhl, J.: Interactions of asperities controlling on fault stability: An experimental approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5254, https://doi.org/10.5194/egusphere-egu22-5254, 2022.

The frictional strength of discontinuities in the upper earth crust controls the stability and dynamics of slip in diverse catastrophic phenomena such as earthquakes and landslides. Natural rock surfaces are rough at various scales with significant variability that affects their frictional behavior. Seismological and geophysical observations of large thrust faults suggest that fault geometry affects earthquake characteristics, yet the exact effects are currently being debated. In this study we show, using laboratory direct shear experiments, that a specific surface geometry enhances sliding instability and that the transition from stable to unstable sliding is non-linearly controlled by the magnitude of the initial roughness. In order to isolate the effect of roughness, we generate six levels of surface roughness in split prisms of Diabase rocks, with four orders of RMS magnitude difference between the smoothest and the roughest samples. The experiments are performed under an imposed constant normal stress of 5 MPa and load point (shear piston) velocity of 0.01 mm/s. The sliding target is typically set to 10 - 13 mm as monitored from two horizontal LVDT’s that are attached to the shear box very close to the tested interface. We show that the amplitude of the stick-slip events diminishes towards the two roughness extremes. The roughest sample (RMS = 1300 µm) exhibits a gradual increase of shear stress to a peak value of ~13 MPa, followed by brittle fracture expressed by a large stress drop of 3 MPa and then by transition to a relatively stable sliding. For the midrange roughness (RMS = 7 µm), stick-slip oscillations are obtained with different levels of stress drops and sliding dynamics characteristics. The smooth sample (RMS = 0.85 µm) slide in a relatively stable manner while the smoothest surface (RMS = 0.7) exhibits local peak friction of 0.18, followed by stable sliding with moderate slip hardening. We further demonstrate, both experimentally and numerically, that stick-slip oscillations commonly referred to as laboratory earthquakes, are constrained to a very limited range of surfaces roughness within which a specific level, defined here as the critical roughness, triggers the highest amplitude of oscillations. We therefore suggest that the roughness amplitude strongly affects the frictional stability and slip dynamics of natural faults.

How to cite: Morad, D., Sagy, A., Tal, Y., and H. Hatzor, Y.: Critical roughness controls sliding instability of laboratory earthquakes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6478, https://doi.org/10.5194/egusphere-egu22-6478, 2022.

Faults exhibit geometrical heterogeneity at all scales, which induces spatial variations in normal stress and hence strength. Additionally, fault zones comprise multiple fractures which can host seismicity and further modify the stress state on the mainshock fault. Here we study how geometrical complexity affects the precursory phase of large earthquakes. We model seismic cycles on fractal faults with uniform velocity-weakening rate-state friction, loaded by a uniform far-field stressing rate. We also include the effect of surrounding damage, represented by a collection of smaller faults with a power-law decay of density with distance from the main fault.
We find that heterogeneity in normal stress σ induced by roughness controls slip behavior: regions with low σ begin to slip aseismically early in the cycle, loading high σ regions (asperities) which eventually fail seismically generating foreshocks. The precursory phase is characterized by a positive feedback between aseismic slip and foreshocks, with stress changes from each process accelerating the other. In simulations including subparallel secondary faults in the damage zone, this process does not take place on the main fault but instead on smaller, off-fault structures. In both cases, mainshocks nucleate on strong asperities at the edge of the preslip area, which is significantly larger and spatially distinct from mainshock nucleation. These features are consistent with a number of observations at different scales, including laboratory experiments, sub-glacial slip events, and foreshock sequences of megathrust earthquakes.

How to cite: Cattania, C. and Segall, P.: Preseismic slip and foreshocks on rough faults embedded in a damage zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10438, https://doi.org/10.5194/egusphere-egu22-10438, 2022.

It has often been suggested that the frictional instability on small fault patches can lead to slow but accelerated creep and growth, which may explain the nucleation of earthquake rupture. Because of the small size of the asperity at depth, the nucleation process is difficult to verify by observations and its possible role for earthquake generation is still debated.

While most earthquake nucleation models are of complex geometry and assume that the asperity itself is static while its stress is increasing, we suggest a concept where the cohesion zone of a fault patch grows steadily and develops a self-induced high stress behind the crack tip. We show that a slip-weakening and velocity strengthening constitutive relation can generate the high stress cohesion zone. The aseismic growth of the asperity is accelerated, and the point to nucleate into a catastrophic rupture depends on the ambient stress on the fault and the stress drop in the centre of creeping segment. Interestingly the model predicts that earthquakes on faults with subcritical and small ambient stress will start with more energetic ruptures, as their Griffith energy is larger. This is unexpected and may question the common assumption that largest earthquake are triggered if the fault is critically stressed and the last earthquake occurred a long time before the average recurrence period.

Our fracture mechanical, theoretical asperity model is unconventional and questions established ideas on earthquake generation. We discuss the possible consequences and the postulated, testable predictions of the model to motivate laboratory and field experiments.

How to cite: Dahm, T. and Hainzl, S.: Earthquake nucleation – viewpoint of dynamically growing asperities controlled by the fracture cohesion-zone and frictional shear, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3541, https://doi.org/10.5194/egusphere-egu22-3541, 2022.

Most faults at seismogenic depths can be described as fractures or discontinuities in a fluid-saturated porous medium and thus, the theory of poroelasticity offers a practical mechanical description of the natural fault environment. However, poroelasticity is rarely considered in simulations of fault slip. Poroelasticity incorporates the two-way coupling of solid and fluid phases where pore-pressure change,  e.g., due to slip, strains the rock matrix and volumetric strain causes changes in pore pressure. During earthquake nucleation, inelastic dilatancy may also induce pore pressure changes. A complex interplay of pore pressure in the bulk and shear zone emerges when we consider the multiple processes coupled to slip on a fault governed by rate-and-state friction. Here, we present an efficient spectral boundary integral code that allows for 2D quasi-dynamic rate-and-state simulations of slow and fast slip with fully coupled and simultaneous state-dependent dilatancy, fluid injection, and two-way coupled diffusive poroelastic bulk response. The method allows for anisotropic shear-zone permeability, while the bulk is considered to be isotropic and homogenous. We can thus simulate three diffusion time scales at once: along the shear zone, across the shear zone, and due to wavelength-dependent bulk diffusion. We apply the code to understand nucleation and repeated fault ruptures with a realistic pore-pressure injection history from a field experiment. We compare different cases with and without dilatancy, larger or smaller differences in drained and undrained poroelastic properties, and varying bulk diffusivity. By systematically increasing the dilatancy coefficient, we observe a transition from highly unstable seismic slip to a migrating slow slip front to quasi-static slip localized to highly pressurized areas. More surprisingly, we find that differences in drained and undrained poroelastic properties and bulk diffusivity strongly influence fault slip stability. A larger difference between drained and undrained Poisson’s ratio or higher bulk diffusivity results in more stable slip during injection, fewer ruptures, and delayed nucleation. These effects appear to be of comparable importance to dilatancy. We conclude that the poroelastic properties of the bulk, which are typically ignored, play a critical role in the stability and determining if slip is seismic or aseismic.

How to cite: Heimisson, E. R., Liu, S., Lapusta, N., and Rudnicki, J.: The role of poroelasticity and dilatancy in governing the transition from aseismic to seismic slip during fluid injection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2795, https://doi.org/10.5194/egusphere-egu22-2795, 2022.

How to cite: Brantut, N.: Dilatancy Toughening of Shear Cracks and Implications for Slow Rupture Propagation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2865, https://doi.org/10.5194/egusphere-egu22-2865, 2022.