Using GPS measurements, historical earthquake records, and instrumental earthquake data, we investigated GPS slip rates along the rupture zone of the 1668 Great Anatolian Earthquake (M8.1). We found three complete and one incomplete earthquake cycles since 1254 compiling all available historical and paleo-earthquake records in the literature. These records verified that a ~750-kilometer section of the North Anatolian Fault Zone was ruptured in 1668.  To simultaneously estimate segment-based slip rates and locking depths, we combined all available GPS measurements and modeled them using an arctangent approach. Slip rates are used to estimate preliminary inter-seismic slip storages assuming fault segments are fully locked after a mainshock. Large residuals between preliminary slip estimates and co-seismic slips indicate that the fault segments do not store slip for some time after a major earthquake. The creeping and locked stages vary in time and space, as our investigation revealed. Our results show that the slip rates along the NAFZ systematically increase from east to west suggesting that the Aegean extensional regime is the main driving force for the westward movement of the Anatolian Plate. Additionally, the locking depths show an east-to-west decreasing pattern verifying east-to-west thinning of crustal thickness along the Anatolian Plate. The earthquakes over the past three complete cycles and the current incomplete cycle indicate that the failure of the NAFZ begins in the east and moves westward reflecting a decelerating pattern. The failure is typically completed within a time period of 239±3 years.

How to cite: Yıldırım, S. C., Bulut, F., and Garagon, A.: East to West Acceleration of the Slip Rates Along the North Anatolian Fault and Its Implıcations Regarding Plate Tectonics and Earthquake Cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11854, https://doi.org/10.5194/egusphere-egu23-11854, 2023.

The M7.4 1999 İzmit earthquake apparently advanced the occurrence of possible future event or events along the segments of North Anatolian Fault Zone (NAFZ) beneath the Eastern Marmara Sea due to the positive stress load. This part of the NAFZ did not produce any large earthquake since the May 1766 earthquake, constituting a seismic gap close to the city of Istanbul. In the present study we constructed a Coulomb stress evolution model for the seismic gap that includes the effect of coseismic, time-dependent postseismic viscoelastic relaxation of the substrate beneath the elastic crust and secular stress loadings through the multiple earthquake cycles since 715 AD. The snapshots of stress changes before and after the large and destructive earthquakes of 740, 989, 1343, 1509, May 1766 and 1999 İzmit have been carefully examined. It has been estimated that the total stress changes before 989, 1343, 1509 and May 1766 earthquakes were in the range from 26 to131 bars. Present stress values at the eastern, middle and western sampling points on the faults within the gap are computed as 115, 131 and 85 bars respectively. Considering that the global mean of stress drops for continental strike-slip faults is about 35 bars, it is suggested that the earthquake hazard for the seismic gap critically high.

How to cite: utkucu, M., durmuş, H., uzunca, F., nalbant, S., and kIZILBUĞA, S.: The time-dependent stress changes within the seismic gap of the Eastern Marmara Sea (NW Türkiye) through multiple earthquake cycles since 715 AD., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7909, https://doi.org/10.5194/egusphere-egu23-7909, 2023.

High-resolution geodetic measurements of the accumulated strains along active faults are important for faulting dynamics studies and seismic hazard evaluation. InSAR has been widely applied to measure the interseismic strain along active strike-slip faults. However, phase unwrapping errors, tropospheric delays, along with over-smooth effects in calculating the strain from velocity limit its capability of mapping the highly localized strain along faults. Phase-gradient stacking that sums up the wrapped phase differences of adjacent pixels has been successfully applied to reveal localized deformation across coseismic fractures and slow-moving landslides, yet lacks application to reveal interseismic strain along faults. Here, we conduct phase-gradient stacking on Sentinel-1 SAR interferograms, for the first time, to map the interseismic strain along the North Anatolian Fault with unprecedented resolution. We reveal several segments with extremely high strain rates attributed to shallow creep of the fault. By comparing with historical earthquake ruptures, we find that the creeps are either related to afterslip of recent earthquakes, or related to slip deficits of earthquakes occurred decades ago, challenging the opinion that the NAF has a uniform surface strain rate, particularly along its eastern portion. Our results show that the phase-gradient stacking can not only reduce the computation burden from phase unwrapping and tropospheric correction, but also achieve a much higher spatial resolution strain map than the traditional InSAR method. The proposed method can be applied to other large strikes-slip faults for distinguishing segments with surface creep and strong coupling and therefore better quantify the shallow strain budget and its associated hazards.

How to cite: Liu, Z. and Wang, T.: High-resolution Interseismic Strain Mapping from InSAR Phase-Gradient Stacking: Application to the North Anatolian Fault with Implications to the Non-uniform Strain Distribution Related to Historical Earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3915, https://doi.org/10.5194/egusphere-egu23-3915, 2023.

This study aims to forecast the magnitude of future strong (6.0≤M<7.0) and major (7.0≤M<8.0) earthquakes along the East Anatolian Fault Zone (EAFZ), a major fault zone of Turkey and an active plate boundary that lies between Arabian and Anatolian plates. We first investigated the segmentation of the EAFZ in this context after compiling the earlier research on its structural setting and historical earthquakes. In order to determine the distribution of slip deficit rates, we analyzed GPS slip rates to obtain back-slips. The current slip budgets on each fault segment are calculated using the resulting slip deficit estimates. To elaborate on whether b-values might be used to distinguish between locked and creeping fault segments, we also examined the distribution of b-values along the fault. As a result, we found a reverse correlation between slip deficit rates and b-values. According to our findings, the EAFZ has currently a slip deficit of 1.51 m. While there is a segment such as Hacılar with no slip deficit, there is enough slip deficit accumulation to generate three strong and three major earthquakes on the other fault segments. Presently, these fault segments have the potential to re-generate previous earthquakes, within the magnitude range of 6.8-7.4. The latest strong earthquake on January 24, 2020, the Elazığ earthquake (M 6.8) verified our magnitude forecasts for the Sivrice-Pütürge segment.

How to cite: Uçan, K. A. and Bulut, F.: Forecasting Earthquake Magnitudes along the East Anatolian Fault Zone using Fault Zone Segmentation, Historical Earthquakes, and GPS Slip Rates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11401, https://doi.org/10.5194/egusphere-egu23-11401, 2023.

The propagation of the 2021 Mw7.4 Madoi earthquake rupture from the central Jiangcuo fault (JCF) onto the eastern portion exhibits the most complex geometry with a series of conjugate faults, bends, and stepovers. At the east ~50 km of the 2021 epicenter, the surface rupture along the Jiangcuo eastern branch (JCEB) deviating ~12° anticlockwise from the general strike provides a valuable chance for understanding the particularly complex surface ruptures propagation and the branching behavior of the poorly known JCEB. Using sub-metric orthophotos collected by UAV with a ground resolution of 6 cm, complemented by multiple field investigations, we implemented the surface rupture mapping and coseismic slip distribution of the JCEB in detail associated with this earthquake sequence. Our mapping illuminated the sporadic breaks of the tectonic region in the dune area immediately near the branching point and eastward propagated linear rupture trace. The measurements of the high-resolution coseismic slip along the JCEB show that the slip distribution reveals an approximate dogtail shape to the eastern termination with the maximum left-lateral strike-slip offset of 2.9 m. These data might support the perspective that the rupture propagated with a supershear velocity toward the east. Combined with the accrued displacements along the JCEB, these results indicate that the poorly known divergent branch could accumulate pre-2021 surface breaks as an immature fault and bifurcated in the Madoi quake due to the matched regional stress field. We found linear surface breaks along the NW-strike geologic faults indicating triggered coseismic slip on conjugate faults. In the meantime, the intersections with conjugate faults mark discontinuities in rupture geometry and surface slip on the main fault, suggesting strong fault interaction in the eastern tip zone of the Madoi rupture.

How to cite: Yao, W., Liu-Zeng, J., and Wang, Z.: Rupture Branching and Propagation at the Eastern End of the 2021 Mw 7.4 Madoi Earthquake, North Tibet Plateau, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4233, https://doi.org/10.5194/egusphere-egu23-4233, 2023.

Rupture complexity results in difficulty in quantifying seismic hazards, such as the probability of an earthquake on multiple fault segments and spatial distribution of fault displacement on the surface. Here we tried to propose a dynamic model to fit the rupture behavior of the 2018 Mw7.5 Palu earthquake, which splays along several sub-fault plans on the surface. The Palu event was initiated on an unknown fault and propagated on a curved plane on the Palu-Koro and Matano faults. According to the Interferometric Synthetic Aperture Radar (InSAR) data, both principal (on-fault) and distributed (off-fault) faulting were identified and spatial displacement on the surface could be evaluated. To model the complex geometry of the coseismic rupture plane and corresponding deformation, we proposed a dynamic model through the Discrete Element Method (DEM). Our model demonstrated rupture along a planar fault at depth and several splay faulting with various deformations on the surface, corresponding to the observations. The simulations represented temporal rupture behavior that covers several earthquake cycles and the probability of superficial fault displacement, which shed light on subsequent seismic hazard assessment and probabilistic fault displacement hazard analysis, respectively.

How to cite: Jiao, L., Wang, T., Feng, G., Tapponnier, P., Simanjuntak, A. V. H., and Chan, C.-H.: Impact of rupture complexity on seismic hazard: Case of the 2018 Mw7.5 Palu earthquake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7209, https://doi.org/10.5194/egusphere-egu23-7209, 2023.

Studies of Oceanic Transform Faults (OTFs) usually rely on geophysical data because of the OTF inaccessibility on the seafloor. The Húsavík-Flatey Fault (HFF) in northern Iceland is an OTF connecting the onshore rift in Iceland to an offshore rift segment of the Mid-Atlantic Ridge, accommodating 30% to 50% of the relative plate motion at this latitude between North America and Eurasia. The HFF is unique because its easternmost 25 km-long segment is exposed on land, allowing to study the long-term deformation of the fault. Two historical earthquakes of estimated magnitudes M6.5 - M7 have been reported on the eastern HFF in the last 270 years. However, almost no information exists from prior to the 18th century.

To study the Holocene deformation of the HFF and to build a catalogue of past earthquakes, we excavated 11 paleoseismology trenches at two locations, six on an alluvial fan and five in a pull-apart basin. We also excavated and tracked buried river channels to estimate long-term slip rates and to assess the coseismic displacement of single events. We used radiocarbon dating of birch wood samples together with major element compositions of volcanic ashes (tephras) to constrain the timing of events on the fault.

Trenches at both locations show clear evidence of deformation and surface rupturing events. From offset measurements of glacial morphologies and buried river channels, we calculate a Holocene slip rate of 4 - 6 mm/yr, slightly lower than the estimated present-day geodetic slip rate, suggesting that some of the deformation may be distributed. Based on upward terminations of cracks and faults, we identified eight events in the last ~6000 years, yielding fewer major earthquakes than expected from the 270-year record. We thus suggest that large earthquakes of magnitude ~M7 on the HFF, producing significant surface ruptures, are rare, with a return time of 500 to 600 years. We also propose that the short recurrence times often observed on OTFs may therefore not be representative of the full seismic cycle.

How to cite: Matrau, R., Klinger, Y., Harrington, J., Thórdarson, T., Höskuldsson, Á., Gudmundsdöttir, E., Parisi, L., Fittipaldi, M., Avsar, U., and Jónsson, S.: Holocene deformation on a transform fault: Insights from paleoseismology on the Húsavík-Flatey Fault in North Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6567, https://doi.org/10.5194/egusphere-egu23-6567, 2023.

Long-term fault escarpments are built by the accumulation of individual earthquakes producing incremental surface displacements on the fault releasing crustal tectonic loading. Cumulative escarpment studies have revealed a spatial slip variability along active faults as well as a temporal variability with the alternation of phases of intense seismic activity over a short period of time followed by long periods of quiescence. Understanding this spatial and temporal slip variability on individual faults and over a complex fault system provide a better knowledge of co-seismic rupture extents, essential for estimating past earthquakes magnitude and for seismic hazard assessment.

Up to now, most studies have focused on a timeframe over few seismic cycles, making it difficult to apprehend the rupture barriers persistence and cumulative slip distribution.  Here, we aim at quantifying the slip variability over several timescales ranging from a few months to a few million years on the same fault.

Our study focusses on the ~50 km-long Liri fault, SW of the Fucino basin. The fault is located at the contact between Cretaceous limestone and patches of Quaternary deposits locally convering Mio-Pliocene flysch sediments. Detailed mapping of the fault trace on high-resolution Digital Elevation Model (DEM) from UAV-acquired images, Pleiades images and Lidar together with field observations revealed changes in the morphological expression of the fault north and south of an important wind gap located at Capistrello. To the north, the faut trace is ~16 km-long located on the eastern side of ~2km-wide limestone ridge, reaching ~1300m asl elevation. Two bends in the fault trace, made of ~5km long segments, can be observed with the fault strike varying between N115° and N140°. In this northern section, the fault scarp appears subtle and we did not observe Quaternary deposits on the hanging wall. In the 30 km-long section, south of Capistrello, the cumulative scarp composed of numerous splays is evidenced by a sharp trace, offsetting several morphological surfaces and associated Quaternary sediment packages. Three major bends are observed in this section of the fault, separating 10 to 30 km-long segments striking between N110° and N160°. An alluvial surface offset by ~14 m of cumulative displacement was dated at ~35kyr using ³⁶Cl cosmogenic exposure dating suggesting a minimum slip rate of 0.4 mm/yr.  Other morphological markers that have accumulated displacement between ~10 and 70 m-high have also been sampled for ³⁶Cl cosmogenic exposure dating. Moreover, we excavated two small trenches at the base of the fault scarp within the Quaternary deposits affected by the fault revealing 3 rupture-surfacing earthquakes over the last 2500 yr, the last one occurring after 1226 CE. 

We will present those results and will discuss how the displacement varies along the fault both in time and space.

How to cite: Riesner, M., Benedetti, L., Baize, S., Pucci, S., Ferry, M., Gautier, S., Braucher, R., Fleury, J., Jomard, H., Mazzotti, S., and Villani, F.: Quantifying the slip over various time scales on active normal faults in the Apennines (Italy):  the Liri fault from paleoearthquakes to long-term slip rate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5124, https://doi.org/10.5194/egusphere-egu23-5124, 2023.

A major challenge in seismic hazard research is to quantify the frequency of large earthquakes along active faults, more so when the observational time windows of seismic catalogs are much shorter than the average fault recurrence intervals. In this respect, paleoseismology continues to prove to be an excellent tool to extend the seismic catalogs of faults into prehistorical times. The combination of the ever more advanced trenching surveys and accurate numerical dating techniques allows constraining the timing of paleoearthquakes and, for some datasets, approximating their recurrence models. Despite this, paleoseismic data carries along large uncertainties frequently related to dating technique limitations, poor stratigraphic preservation, and along-strike slip variability that hinder the identification of a complete paleoearthquake record. Subsequently, these issues, among others, challenge the constraint of reliable earthquake chronologies along faults and of the parameters defining their earthquake cycle.

We present an automatic workflow capable to compute and constrain earthquake chronologies along a fault based on the correlation of its available paleoseismic records, including multi-site and poorly constrained datasets. Our inherent premise is that the correlation of paleoseismic data from multiple along-fault locations can help to improve the time constraints and completeness of its paleoearthquake record. Given that paleoseismic records are, by definition, underpopulated, event correlation is not restricted to single occurrences. Instead, an event in a site might be simultaneously correlated with more than one in another if time compatible. Furthermore, to avoid subjectivity biases in event timing estimates and correlation, we exclusively rely on the trench numerical dates limiting each event horizon as the inputs. All earthquake chronologies are modelled probabilistically with a four-step algorithm as we detail. First, all earthquake times in each site are computed as probability density functions (PDFs) using the input numerical dates. The event PDFs from all sites are then integrated to derive a mean curve representing the overall event probabilities for the studied fault in the time span investigated. The probability peaks in this curve, which are assumed as indicative of the event timing at the fault scale, are automatically detected based on peak prominence analysis. A final PDF is then computed for each peak by multiplying all site event PDFs intersecting the peak position. The set of product PDFs constitutes the earthquake chronology of the fault, provided to the user in simple output files that can be externally used to calculate fault parameters for the seismic hazard assessment, and to visualize the modelling.

Preliminary tests on several paleoseismic datasets of the Central Apennines (Italy), the Eastern Betics (Spain), the Dead Sea Fault and the Wasatch Fault (US), have provided good outcomes. The approach significantly reduces the uncertainties in event timing of paleoearthquakes and provides an objective and reliable interpretation of the datasets, especially when these are complex or have wide uncertainties. By extension, the workflow has the potential to reduce the uncertainties in earthquake recurrence estimates and can give insight on the recurrence models that better describe the earthquake cycle in the studied faults.

How to cite: Gómez-Novell, O., Pace, B., and Visini, F.: Automated workflow to compute earthquake chronologies on faults from paleoseismic datasets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3813, https://doi.org/10.5194/egusphere-egu23-3813, 2023.

Earthquakes on normal faults in the continental setting are relatively uncommon. The scarcity of surface-rupturing events underpins an absence of surface displacement measurements. It is a common practice to use surface offset as a proxy to understand the fault structure at depth. Hence, the lack of comprehensive surface data impedes the subsurface reconstruction of seismogenic normal faults and prohibits the thorough assessment of earthquake hazards. To supplement the available surface displacement measurements and to make statistically significant inferences, we apply optical image correlation (OIC) methods to historical images from three large continental normal earthquakes in the western United States (1954 Dixie Valley (M_w 6.8) - Fairview Peak (M_w 7.1) earthquake sequence, the 1959 M_w 7.2 Hebgen Lake earthquake and the 1983 M_w 6.9 Borah Peak earthquake). The results of this study are displacement maps with three components of deformation from which we extract high-resolution 3-d measurements everywhere along the surface rupture. 

The high-resolution 3-d data are used to quantify the magnitude and direction of the earthquake-related offset, the percentage of off-fault damage as well as the width of the fault zone. These parameters represent the fault maturity, geometric complexity and subsurface structure of the fault. Our observations confirm behaviours previously observed along strike-slip faults (e.g. magnitude of off-fault deformation is proportional to the rupture complexity). In addition, a comparative assessment of the results from the three study areas demonstrates that features such as excess slip detected close to the fault scarp are not unique and can be found along multiple dip-slip faults. Consequently, this study documents the variation of the quantifiable parameters along the normal faults. It suggests that while some parameters are a universal reflection of the fault characteristics, others vary according to the geology or topography in the area and should not be accepted without further investigation.

How to cite: Andreuttiova, L., Hollingsworth, J., Vermeesch, P., and Mitchell, T.: Assessing distribution and pattern of the earthquake-related deformation caused by large continental normal earthquakes using optical image correlation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16199, https://doi.org/10.5194/egusphere-egu23-16199, 2023.

The Tianshan range, one of the most active mountain building belts in central Asia, has complex geological structures and frequent strong earthquakes since the Cenozoic. Due to lack of sufficient high-resolution geodetic survey measurments, little is known about detailed fault slip rates and seismic hazards related to main active faults in Tianshan. However, in recent years, the improvements in space-based geodetic technologies (Global Navigation Satellite System (GNSS) and interferometric synthetic aperture radar (InSAR)) with growing coverage and accuracy provide us an opportunity to image more subtle features in this area. In this study, we assesses inter-seismic deformation for the period 2014-2022 over the southwestern Tianshan based on ascending and descending Sentinel-1 SAR data.  Combined with GNSS data, we then constructed the 3D crustal deformation with high precision and high spatial resolution to study the active structures in southwestern Tianshan. The results indicate that: (1) The Tianshan orogenic belt (TSOB) has intense crustal deformation and the shortening rate is approximately 20 mm/yr. The Keping fold-thrust belt (KFB) is the most intensely deformed areas in TSOB, it’s convergence rate accounts for 1/3 of the entire southwestern Tianshan. (2) The South Tianshan fault (STF) and the Piqiang fault (PQF) have obvious left-lateral strike-slip components and the South Tianshan fault also has thrust characteristic. (3) The folds in both western and eastern KFB play an important role in accommodating regional strain, the shortening rate in KFB is accommodated by the thrust-anticlinal zone at the KFB front.

How to cite: Wang, X., Motagh, M., and Xu, C.: Crustal deformation of southwestern Tianshan orogenic belt based on InSAR and GPS observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8011, https://doi.org/10.5194/egusphere-egu23-8011, 2023.

Thanks to space geodesy we know with a millimetric precision how the lithosphere deforms at each stage of the seismic cycle. In particular, during the post-seismic phase, it can deform over thousands of kilometers and for decades. These deformations are partly due to viscoelastic relaxation of the asthenosphere.

In a previous work, we have shown that at the temporal and spatial scale of the seismic cycle, the viscoelastic relaxation can be modeled by a linear creep law [Boulze et al. 2022]. Therefore, because of the linearity of the creep law, the superposition principle applies and the present day deformation is simply the sum of the post-seismic deformations induced by past earthquakes. Based on this result, the objective of our work is to determine what slip history is needed on the Chilean subduction interface to reproduce the current deformation of South America, which is well measured by GNSS.

To investigate this challenging problem, we first develop a 3D spherical finite-element model of the Chilean subduction zone. This model covers the entire South American continent and incorporates a slab with a geometry described by Slab2.0 model [Hayes et al. 2018]. Then, we compare different ways to model the seismic cycle using the backslip theory [Savage 1983]. Finally, by comparing GPS time-series with our seismic cycle model prediction, we discuss many ingredients of the model: e.g. the viscosity of the asthenosphere (Maxwell, Burgers), the impact of a flat slab and low viscosity zones, the magnitude and extent of historical earthquakes.

How to cite: Boulze, H., Fleitout, L., Klein, E., Vigny, C., and Garaud, J.-D.: Modeling surface deformations during the seismic cycle along the Chilean subduction zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12158, https://doi.org/10.5194/egusphere-egu23-12158, 2023.

Foreshocks are commonly observed before the happening of earthquakes in seismic catalogs. They provide critical precursors to reveal the process for the nucleation and rupture of earthquakes. Two mechanisms, pre-slip and cascade triggering, are thought to be the main physical process to explain the foreshock sequences and the mainshock. However, different from the regular micro-magnitude foreshock sequences (e.g. M1.0-3.0), some moderate-size (e.g. ~M6) foreshocks are also found before the mainshock (e.g. the M6.4 foreshock before the 2017 M7.1 Ridgecrest earthquake). How these moderate-size foreshocks affect the happen of mainshocks as well as their possible triggering mechanisms are still ambiguous and less studied.

In this study, fortunately, we obtain geodetic observations of moderate-size (M5.8 and M6.5) foreshocks for the 2020 M6.0 Turkey and 2022 M6.9 Taiwan earthquakes using Sentinel-1 Synthetic Aperture Radar (SAR) images. It is very rare for the geodetic observations of such foreshocks as they are very temporally close to the mainshocks within one day (i.e. ~10 hours and ~17 hours). We then invert for the fault geometries and slip distributions for these two earthquakes together with their moderate foreshocks constrained by these geodetic observations. Coulomb stresses on the fault planes of mainshocks produced by the moderate-size foreshocks are also calculated as well as the static stress drops of the mainshocks. Our study provides a unique opportunity to explore the possible triggering mechanism between moderate-size foreshocks and mainshocks as well as the conditions for the happening of earthquakes.

How to cite: Luo, H. and Wang, T.: Geodetic modeling of moderate-size foreshocks with mainshocks and the implication to earthquake trigger mechanisms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15420, https://doi.org/10.5194/egusphere-egu23-15420, 2023.

We study an experimental model of a fault consisting in a stationary shear band in a compressed granular sample. To obtain those bands, we perform a biaxial compression of a granular sample constituted of glass beads during which we observe the spontaneous formation of shear planes along the Mohr-Coulomb directions in the sample. We study the post-failure regime during which all the deformation occurs along the stationary shear bands. Using an interferometric method of measurement of micro-deformations based on multiple scattering, we obtain full-field measurements of the local incremental deformation in the sample. The deformation measured are typically of $10^{-5}$ with a resolution of about 300 microns (3 bead diameters). Our technics gives access to the strain fluctuations inside the shear band and we show that the macroscopic mean deformation in the bands is the result of the accumulation of local, intermittent, shear events. The size distribution of those shear events follows the Gutenberg-Richter law. We observe clustering of those events following Omori's law and we apply a declustering method to reveal the causal structure underlying our sequences of events (Houdoux et al. 2021). 

In my talk, I will focus on recent experimental results regarding the dependence of the series statistics on the driving velocity. We have studied sequences of aftershocks for different compression velocities and we have shown that surprinsingly the aftershock sequences we observe are deformation-dependent and not time-dependent. We discuss such a deformation memory effect in the framework of an Olami-Feder-Christensen model.

Houdoux et al. Commun Earth Environ 2, 90 (2021)

How to cite: Amon, A., Mathey, A., Marsan, D., Weiss, J., and Crassous, J.: Deformation-dependent aftershocks in laboratory earthquakes sequences, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2262, https://doi.org/10.5194/egusphere-egu23-2262, 2023.

Analysis of geodetic and seismological data helps constrain earthquake dynamics and the physics of lithospheric deformation. Here, we discuss a new modeling approach based on an open-source finite-element framework to invert surface deformation data for constitutive laws and their parameters, such as the Poisson’s ratio or shear modulus in the crust and mantle wedge.

These inversions can be realized by using adjoint-based optimization methods which efficiently reduce the misfit between the calculated and observed displacements. To quantify the associated model uncertainties, we extend the inverse approach to a Bayesian inference problem. Since the data are usually informative only in a few directions in parameter space, we use a low-rank Laplace approximation of the posterior distribution to make the inverse problem computationally tractable. The mean and the posterior covariance are approximated by the solution of the inverse problem (MAP point) and the inverse of the Hessian of the negative log posterior evaluated at the MAP point, respectively. We show how smoothly varying parameter fields can be reconstructed satisfactorily from noisy data.

To improve the spatial resolution of the inverse solution we solve a Bayesian optimal experimental design problem to find the best station configuration by maximizing the expected information gain, defined as the Kullback-Leibler divergence between posterior and prior distributions. We show how and why the optimal network improves the material property inference more than evenly spaced stations. Based on our previous work on inverting for fault slip without Green’s function computations, we combine the two inversion schemes to jointly infer both model parameters, the coseismic slip, and material properties distribution. Lastly, we test this numerical forward/inverse framework with an application, the 2011 Tohoku-oki M9 earthquake. Both continuous land-based and six offshore acoustic GNSS stations located around the earthquake epicenter are inverted to jointly estimate the shear modulus and the fault slip during the megathrust event.

Our results demonstrates the potential of our computational framework and the general approach for inferring constitutive laws to evaluate sensitivity to parameters, and define strategies to improve our understanding of relevant parameters for earthquake dynamics. 

How to cite: Becker, T., Puel, S., Villa, U., Ghattas, O., and Liu, D.: An Adjoint-based Method for Inverting for Heterogeneous Material Properties and Fault Slip From Earthquake Surface Deformation Data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9213, https://doi.org/10.5194/egusphere-egu23-9213, 2023.