Geodetic Slip Research Articles

The Induced Mw 5.0 March 2020 West Texas Seismic Sequence

AbstractOn March 26, 2020, a M 5.0 earthquake occurred in the Delaware Basin, Texas, near the border between Reeves and Culberson Counties. This was the third largest earthquake recorded in Texas and the largest earthquake in the Central and Eastern United States since the three M 5.0–5.8 induced events in Oklahoma during 2016. Using multistation waveform template matching, we detect 3,940 earthquakes in the sequence with the first event in the area occurring in May 2018. The M 5.0 earthquake sequence occurred on a ENE (∼082°) normal fault dipping ∼37° toward the south. The earthquake caused 6 mm of oblique surface deformation, and geodetic slip inversion suggests slip was isolated above 6 km depth. We find that the sequence was most likely induced by nearby wastewater disposal operations, and seismicity rates in the region surrounding the M 5.0 will likely continue to increase in the future if disposal operations continue unaltered.

Three-dimensional seismic tomographic imaging beneath the Sea of Marmara: evidence for locked and creeping sections of the Main Marmara Fault

SUMMARYWe present 3-D Vp and Vp/Vs tomographic images for depths down to 22 km along with precise earthquake locations beneath the Sea of Marmara using the seismological data sets collected during the Ocean Bottom Seismometer (OBS) experiments conducted in 2001 and 2006. The unique data set collected by 52 OBSs and augmented by 14 land stations include 3852 P and 2643 S arrival times from 434 earthquakes and 4744 P arrival times from 557 air-gun shot records. From the tomographic depth sections and relocated seismicity, we identify two shear zones which are marked by narrow-width (∼10 km), low Vp (4.3–5.3 km s–1) and high Vp/Vs ratio (∼2.0) anomalies coinciding with the surface trace of the Main Marmara Fault (MMF) and extending near vertically from 8 km depth below the basins into the deeper part of the crust. We consider these shear zones to be the strike-slip deformation signature of the MMF at depth. The western shear zone imaged lies between the Western High and the eastern end of the Central Basin including the 60-km-long segment of the MMF and contains higher seismicity and is visible in the 8–22 km depth range. The eastern shear zone, coinciding with the NW–SE trending surface trace of the MMF, is imaged beneath the Çınarcık Basin with lower seismicity and tomographic resolution. Furthermore, these shear zones may contain fractures filled with deep crustal fluids allowing partial aseismic creep. Within these shear zones, we also identify small-scale, high Vp (6 km s–1) anomalies with low seismicity indicative of strong patches of the fault at depth that can be considered as asperities. Between these shear zones, our tomographic images clearly show a central zone that has homogenously high Vp (∼6.0–6.5 km s–1) and low Vp/Vs ratio (∼1.7) over a large area including the 50-km-long central segment of the MMF lying between the eastern end of the Central Basin and western end of the Çınarcık Basin. Thus, we infer that the central MMF is in a locked state based on its high shear strength, low seismicity and a lack of detectable geodetic slip. Furthermore, a large earthquake might not nucleate on the central MMF since it may be strongly locked at the current level of stress loading, and thus it may behave as a seismic barrier or anti-asperity. On the other hand, the central MMF is a good candidate for a supershear rupture because it has nearly uniform elastic properties and low background seismicity indicating the homogeneity of friction and pre-stress across the fault, which are presented as the main characteristics of supershear fault ruptures. From the seismic hazard perspective, we speculate that a large earthquake may be expected to nucleate within either of the imaged shear zones with the possibility of the central MMF acting as a seismic barrier to stop or to join the rupture in a supershear mode. The imaged features have important implications for various aspects of seismic hazard and crustal dynamics in the Sea of Marmara region.

On the Relationship Between Offshore Geodetic Coverage and Slip Model Uncertainty: Analog Megathrust Earthquake Case Studies

AbstractWe apply a geodetic slip inversion technique to analog subduction megathrust earthquakes to demonstrate how limited offshore geodetic coverage affects coseismic slip models. We analyzed two archetypical megathrust earthquakes: trench‐breaking and non‐trench‐breaking earthquakes. Slip inversion models of analog earthquakes show quantitative and qualitative changes as a function of offshore coverage. Shallow slip cannot be resolved if the observation coverage of the offshore segment is <50%. Moreover, the slip pattern of shallow events flips from landward to trenchward skewed as offshore coverage reduces to <40%. The estimated slip for both event types converges to a similar unimodal pattern when there is no offshore coverage. We infer 5–20% slip overestimation when the observations are above the high slipping zone during trench‐breaking events versus 5–10% underestimation during non‐trench‐breaking events if observations are land limited. The moment magnitude derived for trench‐breaking ruptures might be significantly affected (ΔMw ~ 0.5).

Quaternary slip rates on the White Mountains fault zone, eastern California: Implications for comparing geologic to geodetic slip rates across the Walker Lane

AbstractThe White Mountains fault zone in eastern California is a major fault system that accommodates right-lateral shear across the southern Walker Lane. We combined field geomorphic mapping and interpretation of high-resolution airborne light detection and ranging (LiDAR) digital elevation models with 10Be cosmogenic nuclide exposure ages to calculate new late Pleistocene and Holocene right-lateral slip rates on the White Mountains fault zone. Alluvial fans were found to have ages of 46.6 + 11.0/–10.0 ka and 7.3 + 4.2/–4.5 ka, with right-lateral displacements of 65 ± 13 m and 14 ± 5 m, respectively, yielding a minimum average slip rate of 1.4 ± 0.3 mm/yr. These new slip rates help to resolve the kinematics of fault slip across this part of the complex Pacific–North American plate boundary. Our results suggest that late Pleistocene slip rates on the White Mountains fault zone were significantly faster than previously reported. These results also help to reconcile a portion of the observed discrepancy between modern geodetic strain rates and known late Pleistocene slip rates in the southern Walker Lane. The total middle to late Pleistocene slip rate from the southern Walker Lane near 37.5°N was 7.9 + 1.3/–0.6 mm/yr, ∼75% of the observed modern geodetic rate.

Geodetic slip rate and locking depth of east Semangko Fault derived from GPS measurement

65 km Semangko Fault is part of southern segments of Sumatran Fault Zone (SFZ) which is complex corresponds to the transition from the strike-slip regime of the SFZ to the normal faulting tectonics of the Sunda Strait. The recent publication showed branches of Semangko Fault: West Semangko Fault (WSF) and East Semangko Fault (ESF). This study estimated geodetic slip rate and locking depth of ESF using Global Positioning System (GPS) time series data from 2007 to 2019 from all available GPS sites. GPS velocities refer to Sundaland Plate were used to estimate the fault parameters of the WSF and ESF simultaneously. Non-uniformity of velocity direction shows the complexities of Semangko Fault possibly caused by the contribution of all faults around ESF. An ESF geodetic slip rate, which is 12.5 ± 2 mm/yr was lower than WSF, which is 16.5 ± 2 mm/yr. It is consistent with the rigid block nature of the SFZ system as northern segment slip rates have similar value. Small slip rates are possibly leading to lower generated seismic moment than the major segment of SFZ.

Introduction of covariance components in slip inversion of geodetic data following a non-uniform spatial distribution and application to slip deficit rate estimation in the Nankai Trough subduction zone

SUMMARYInappropriate mathematical treatment of prediction errors associated with inaccurate forward modelling in an inversion scheme may result in significant unnatural short-wavelength components in the estimated slip distribution, which is a typical consequence of overfitting data. When geodetic data in observation stations following a non-uniform spatial distribution are used in a geodetic slip inversion, the spatial non-uniformity of the observation can possibly influence the distribution pattern of the short-wavelength components significantly, which may be confused with slip patterns that are geophysically meaningful. Such situations often occur when land and seafloor geodetic data are used in combination in slip inversions. To avoid overfitting, this study proposes a method that incorporates covariance components in the covariance matrix of the misfit vector, which originate from prediction errors. Because the proposed method retains the linearity of the inversion problem, widely known approaches that introduce prior constraints to a linear inversion problem are easily combined with the proposed method. This study demonstrates a combination of the newly introduced covariance components with a prior constraint on the smoothness of slip distribution, constructing a Bayesian model with unknown hyperparameters, which are objectively determined by minimizing Akaike’s Bayesian information criterion. In the synthetic tests, the proposed method estimated slip deficit rate (SDR) distributions that are closer to the true one, avoiding overfitting the geodetic data with spatial non-uniformity. By contrast, a conventional approach, which does not introduce covariance components, estimates unnaturally rough SDR distributions using the same synthetic data. The proposed method was applied to the estimation of SDR in the Nankai Trough subduction zone, using geodetic data of displacement rates provided by land GNSS stations and seafloor GNSS-Acoustic stations. This method estimates a reasonably smooth distribution of SDR, avoiding overfitting. The spatial distribution of residuals of the displacement rates suggests that the proposed method avoids overfitting some portions of the observed displacement rates that the forward model set for the analyses could not fundamentally explain.

The role of the Polochic Fault as part of the North American and Caribbean Plate boundary: Insights from the infill of the Lake Izabal Basin

AbstractThe Lake Izabal Basin in Guatemala is a major pull‐apart basin along the sinistral Polochic Fault, which is part of the North American and Caribbean plate boundary. The basin infill contains information about the tectonic and sedimentological processes that have imparted a significant control on its sedimentary section. The inception of the basin has been linked to the relative importance of the Polochic Fault in the tectonic history of the plate boundary; yet, its sedimentological record and its inception age have been poorly documented. This study integrates diverse datasets, including industry reports, well logs and reports, well cuttings, vintage seismic data, outcrop observations and geochronological data to constrain the initial infill and age of inception of the basin. The integrated data show that during the Oligocene–Miocene, a marine carbonate platform was established in the region which was later uplifted and eroded in the early Miocene. The fluvial–lacustrine deposits above this carbonate platform are part of the initial infill of the basin and are constrained with zircon weighted‐mean 206Pb/238U ages of 12.060 ± 0.008 from a volcanic tuff ~30 m above the unconformity. Sandstone, mudstone and coal dominate the interval from 12 to 4 Ma, with an increase in conglomerate correlating to the uplift of the Mico Mountains and San Gil Hill at 4 Ma. Fault switch activity between the Polochic and Motagua faults has been hypothesized to explain total offset along the Polochic Fault and the geologic and geodetic slip rates along the two faults. The 12 Ma age determined for the initial infill of the basin confirms this hypothesis. Consequently, our study confirms that at ~12 Ma the Polochic Fault served as the main fault of the plate boundary with inferred slip rates ranging from 13 to 21 mm/yr with a strong possibility that the Polochic Fault was, at some point between 15 Ma and 7 Ma, the only active fault of the plate boundary. The results of this study show that tectonic records preserved in sediments of strike‐slip basins improve the understanding of the relative significance of individual faults and the implications with respect to strain partitioning throughout its tectonic history.

Earthquake‐Scaling Relationships from Geodetically Derived Slip Distributions

AbstractEmpirical earthquake scaling relationships describe expected relationships between moment magnitude and various spatial descriptors of the earthquake rupture (along‐strike length, down‐dip width, rupture area, and peak and mean slip). These scaling relationships play important roles in many seismological, geological, and hazards‐assessment applications. Historically, scaling relationships were defined from various seismological criteria, such as teleseismic finite‐fault models or aftershock distributions. The proliferation of earthquake slip distributions from geodetic observations presents an opportunity to reassess earthquake scaling relationships using observations that more directly sample the spatial characteristics of an earthquake than seismological observations. Here, we present a database of 111 earthquake slip distributions from 73 different earthquakes that were derived from geodetic observations. The earthquakes range in magnitude from Mw 5.3 to 9.1. We extract common spatial descriptors from these slip distributions in four different ways to account for biases introduced by inversion regularization, and we regress these spatial descriptors with moment magnitude to derive new empirical scaling relationships. We additionally assess the shape characteristics of the slip distributions and report the average earthquake shape. We find that our scaling relationships differ in important ways from previous studies, and we show that these differences originate from our use of a geodetic slip‐distribution database rather than from methods for extracting spatial descriptors. Notably, we find that geodetic slip distributions systematically predict smaller fault areas than seismically derived scaling relationships. Because geodetic source inversions are likely contaminated to some degree by aseismic afterslip, this relationship suggests that seismologically determined scaling relationships systematically overpredict earthquake dimensions. We find that fault length, fault width, peak slip, and mean slip differ from previous studies in ways that are more complex and magnitude dependent. Given the high‐model resolution afforded by geodetic observations, our earthquake scaling relationships derived from geodetic slip distributions provide improved constraints on empirical scaling relationships.

A new geological slip rate estimate for the Calico Fault, eastern California: implications for geodetic versus geologic rate estimates in the Eastern California Shear Zone

ABSTRACTAccurate estimation of fault slip rate is fundamental to seismic hazard assessment. Previous work suggested a discrepancy between short-term geodetic and long-term geologic slip rates in the Mojave Desert section of the Eastern California Shear Zone (ECSZ). Understanding the origin of this discrepancy can improve understanding of earthquake hazard and fault evolution. We measured offsets in alluvial fans along the Calico Fault near Newberry Springs, California, and used several techniques to date the offset landforms and determine a slip rate. Our preferred slip rate estimate is 3.2 ± 0.4 mm/yr, representing an average over the last few hundred thousand years, faster than previous estimates. Seismic hazard associated with this fault may therefore be higher than previously assumed. We discuss possible biases in the various slip rate estimates and discuss possible reasons for the rate discrepancy. We suggest that the ECSZ discrepancy is an artefact of limited data, and represents a combination of faster slip on the Calico Fault, off-fault deformation, unmapped fault strands, and uncertainties in the geologic rates that have been underestimated. Assuming our new rate estimate is correct and a fair amount (40%) of off-fault deformation occurs on major ECSZ faults, the summed geologic rate estimate across the Mojave section of the ECSZ is 10.5 ± 3.1 mm/yr, which is equivalent within uncertainties to the geodetic rate estimate.

Structural setting along the Western North Anatolian Fault and its influence on the 2014 North Aegean Earthquake (Mw 6.9)

We investigated microseismicity, geodetic slip and structural setting along the western North Anatolian Fault (NAF) to characterize their influence on pre-, co- and post-seismic stages of the 2014 North Aegean Earthquake (M 6.9). We identified that the NAF in North Aegean Sea (NAS) operates beneath three basins and two transpressional ridges rather than a single through-going basin. Refined hypocenters indicate that NAF is a narrow shear-zone in the east, and systematically expands towards the west. Microseismicity has a wide spread epicentral pattern at pre-seismic stage of the 2014 earthquake, but later tightens during post-seismic stage. This suggests that pre-seismic strain accumulation was completed on the main fault and transferred to surrounding secondary structures, and the slip returned back to the main fault following the mainshock. Overall microseismicity pattern shows that seismogenic zone becomes deeper to the west and shallower to the east. Three fault segments merged with two step-overs have failed during the 2014 North Aegean Earthquake rupturing a ~90 km section of the NAF. The co-seismic slips reach ~90 cm beneath western step-over and remains below ~60 cm beneath eastern step-over. Along-fault pre- and co-seismic geodetic slips show a complementary pattern verifying that the 2014 mainshock generated the highest slip at pre-seismically locked patches, located beneath transpressive ridges hosting two step-overs. The existence of high pre-shock concentrations underneath suggests fracturing at seismogenic basement overcoming frictional strength at these two fault step-overs.

GPS constraints on deformation in northern Central America from 1999 to 2017, Part 1 – Time-dependent modelling of large regional earthquakes and their post-seismic effects

We use continuous and campaign measurements from 215 GPS sites in northern Central America and southern Mexico to estimate coseismic and afterslip solutions for the 2009 M w = 7.3 Swan Islands fault strike-slip earthquake and the 2012 M w = 7.3 El Salvador and M w = 7.4 Guatemala thrust-faulting earthquakes on the Middle America trench. Our simultaneous , time-dependent inversion of more than 350 000 daily GPS site positions gives the first jointly consistent estimates of the coseismic slips for all three earthquakes, their combined time-dependent post-seismic effects and secular station velocities corrected for both the co-seismic and post-seismic deformation. Our geodetic slip solutions for all three earthquakes agree with previous estimates that were derived via static coseismic-offset modelling. Our time-dependent model, which attributes all transient post-seismic deformation to earthquake afterslip, fits nearly all of the continuous GPS site position time-series within their several-millimetre position noise. Afterslip moments for the three earthquakes range from 35 to 140 per cent of the geodetic coseismic moments, with the largest afterslip estimated for the 2012 El Salvador earthquake along the weakly coupled El Salvador trench segment. Forward modelling of viscoelastic deformation triggered by all three earthquakes for a range of assumed mantle and lower crustal viscosities suggests that it accounts for under 20 per cent of the observed post-seismic deformation and possibly under 10 per cent. Our results thus point to afterslip as the primary and perhaps dominant mode of post-seismic deformation for these C The Author(s)

Geologic Slip‐Rate Determinations on the Talas‐Fergana Fault: Mismatch With Geodetic Slip Rate

AbstractDetailed new paleoseismic field investigations at two sites on the Talas‐Fergana fault, a poorly known strike‐slip structure that transects the Tien Shan mountain range, document late Holocene slip rates of 11–16 mm a−1. This prominent structure is distinctive in striking obliquely NW‐SE across the Tien Shan, which is otherwise dominated by contractional structures striking generally E‐W. Moreover, a satellite‐based Global Positioning System network spanning the Tien Shan orogen records active N‐S contraction rates of ~20 mm a−1, but limits slip on the Talas‐Fergana fault to <2 mm a−1. This profound mismatch between long‐term geologic and short‐term geodetic slip rates, which may suggest temporal variability in slip, highlights the importance of field‐based investigations as a complement to remotely sensed data, particularly in evaluating models of lithosphere behavior and earthquake probabilities on presently locked faults such as the Talas‐Fergana.

Revisiting the 1991 Uttarkashi and the 1999 Chamoli, India, earthquakes: Implications of rupture mechanisms in the central Himalaya

The central Himalaya has attracted much attention due to its potential to host a great earthquake – an inference based on the history of earthquakes and the geodetic slip estimates. The largest and earliest historically documented earthquake in the central Himalaya occurred in 1803 (M ∼ 7.7) near Uttarkashi on its western part, much before the great event of 1934 (Mw 8.2, located farther to the east) - both of which were felt severely in the Gangetic Plain south of the mountain front. The seismotectonic model by Seeber and Armbruster (1981) predicts that large/great earthquakes originate on the Main Himalayan Thrust (MHT) and their ruptures propagate southward to the Main Frontal Thrust (MFT). The rupture of the April 25, 2015 (Mw 7.8) Gorkha (Nepal) earthquake, however, was southeast-directed with a southwest-directed slip, and its Mw 7.3 aftershock on May 12 also showed a similar slip direction. Post-2015, we revisited the 1991 Uttarkashi (Mw 6.7) and the 1999 Chamoli (Mw 6.5) earthquakes in India, both of which had originated on the MHT flat, but show SSW-directed slips, as reported in this study. Ruptures of both these earthquakes fell short of reaching the MFT, just as the historically documented 1803 earthquake, but their shaking effects were felt in the southern plains. These observations compel us to conclude that the varying rupture modes exhibited by the Himalayan earthquakes are controlled primarily by the geometry of the flat-ramp-flat system.

Dynamic Rupture Modeling of the M7.2 2010 El Mayor‐Cucapah Earthquake: Comparison With a Geodetic Model

AbstractThe 2010 Mw 7.2 El Mayor‐Cucapah earthquake is the largest event recorded in the broader Southern California‐Baja California region in the last 18 years. Here we try to analyze primary features of this type of event by using dynamic rupture simulations based on a multifault interface and later compare our results with space geodetic models. Our results show that starting from homogeneous prestress conditions, slip heterogeneity can be achieved as a result of variable dip angle along strike and the modulation imposed by step over segments. We also considered effects from a topographic free surface and find that although this does not produce significant first‐order effects for this earthquake, even a low topographic dome such as the Cucapah range can affect the rupture front pattern and fault slip rate. Finally, we inverted available interferometric synthetic aperture radar data, using the same geometry as the dynamic rupture model, and retrieved the space geodetic slip distribution that serves to constrain the dynamic rupture models. The one to one comparison of the final fault slip pattern generated with dynamic rupture models and the space geodetic inversion show good agreement. Our results lead us to the following conclusion: in a possible multifault rupture scenario, and if we have first‐order geometry constraints, dynamic rupture models can be very efficient in predicting large‐scale slip heterogeneities that are important for the correct assessment of seismic hazard and the magnitude of future events. Our work contributes to understanding the complex nature of multifault systems.

Crustal Deformation in the India‐Eurasia Collision Zone From 25 Years of GPS Measurements

AbstractThe India‐Eurasia collision zone is the largest deforming region on the planet; direct measurements of present‐day deformation from Global Positioning System (GPS) have the potential to discriminate between competing models of continental tectonics. But the increasing spatial resolution and accuracy of observations have only led to increasingly complex realizations of competing models. Here we present the most complete, accurate, and up‐to‐date velocity field for India‐Eurasia available, comprising 2576 velocities measured during 1991–2015. The core of our velocity field is from the Crustal Movement Observation Network of China‐I/II: 27 continuous stations observed since 1999; 56 campaign stations observed annually during 1998–2007; 1000 campaign stations observed in 1999, 2001, 2004, and 2007; 260 continuous stations operating since late 2010; and 2000 campaign stations observed in 2009, 2011, 2013, and 2015. We process these data and combine the solutions in a consistent reference frame with stations from the Global Strain Rate Model compilation, then invert for continuous velocity and strain rate fields. We update geodetic slip rates for the major faults (some vary along strike), and find that those along the major Tibetan strike‐slip faults are in good agreement with recent geological estimates. The velocity field shows several large undeforming areas, strain focused around some major faults, areas of diffuse strain, and dilation of the high plateau. We suggest that a new generation of dynamic models incorporating strength variations and strain‐weakening mechanisms is required to explain the key observations. Seismic hazard in much of the region is elevated, not just near the major faults.

A spectral expansion approach for geodetic slip inversion: implications for the downdip rupture limits of oceanic and continental megathrust earthquakes

A spectral expansion approach for geodetic slip inversion: implications for the downdip rupture limits of oceanic and continental megathrust earthquakes

Geodetic Slip Model of the 3 September 2016Mw 5.8 Pawnee, Oklahoma, Earthquake: Evidence for Fault‐Zone Collapse

The 3 September 2016 M w 5.8 Pawnee earthquake in northern Oklahoma is the largest earthquake ever recorded in Oklahoma. The coseismic deformation was measured with both Interferometric Synthetic Aperture Radar and Global Positioning System (GPS), with measureable signals of order 1 cm and 1 mm, respectively. We derive a coseismic slip model from Sentinel‐1A and Radarsat 2 interferograms and GPS static offsets, dominated by distributed left‐lateral strike slip on a primary west‐northwest–east‐southeast‐trending subvertical plane, whereas strike slip is concentrated near the hypocenter (5.6 km depth), with maximum slip of ∼1 m located slightly east and down‐dip of the hypocenter. Based on systematic misfits of observed interferogram line‐of‐sight (LoS) displacements, with LoS based on shear‐dislocation models, a few decimeters of fault‐zone collapse are inferred in the hypocentral region where coseismic slip was the largest. This may represent the postseismic migration of large volumes of fluid away from the high‐slip areas, made possible by the creation of a temporary high‐permeability damage zone around the fault.

Quantitative Analysis of Seismicity in Iran

We use historical and recent major earthquakes and GPS geodetic data to compute seismic strain rate, geodetic slip deficit, static stress drop, the parameters of the magnitude–frequency distribution and geodetic strain rate in the Iranian Plateau to identify seismically mature fault segments and regions. Our analysis suggests that 11 fault segments are in the mature stage of the earthquake cycle, with the possibility of generating major earthquakes. These faults primarily are located in the north and the east of Iran. Four seismically mature regions in southern Iran with the potential for damaging strong earthquakes are also identified. We also delineate four additional fault segments in Iran that can generate major earthquakes without robust clues to their maturity.The most important fault segment in this study is the strike-slip system near the capital city of Tehran, with the potential to cause more than one million fatalities.

Spatial Variations of Aftershock Parameters and their Relation to Geodetic Slip Models for the 2010 Mw8.8 Maule and the 2011 Mw9.0 Tohoku-oki Earthquakes

Recent development in analysis tools and deployments of the geodetic and seismic instruments give an opportunity to investigate aftershock sequences at local scales, which is important for the seismic hazard assessment. In particular, we study the dependencies between aftershock sequences properties and deformational/geological data on a scale of the rupture extension of megathrust earthquakes. For this goal we use, on one hand, published models of inter-, co- and postseismic slip and geological information and, on the other hand, aftershock parameters, obtained by fitting a modified Epidemic Type Aftershock Sequence (ETAS) model. The altered ETAS model takes into account the mainshock rupture extension and it distinguishes between primary and the secondary aftershock triggering involved in the total seismicity rate. We estimate the Spearman correlation coefficients between the spatially distributed aftershock parameters estimated by the modified ETAS model and crustal physical properties for the Maule 2010 Mw8.8 and the Tohoku-oki 2011 Mw9.0 aftershock sequences. We find that: (1) modified ETAS model outperforms the classical one, when the mainshock rupture extension cannot be neglected and represented as a point source; (2) anomalous aftershock parameters occur in the areas of the reactivated fault systems; (3) aftershocks, regardless of their generation, tend to occur in the areas of high coseismic slip gradient, afterslip and interseismic coupling; (4) aftershock seismic moment releases preferentially in regions of large coseismic slip, coseismic slip gradient and interseismically locked areas; (5) b value tends to be smaller in interseismically locked regions.

Long‐lasting seismic repeaters in the Central Basin of the Main Marmara Fault

AbstractThe Main Marmara Fault which crosses the whole Marmara Sea is a significant seismic gap along the North Anatolian Fault. Here we show that nine long‐lasting strike‐slip seismic repeaters exist below the Central Basin within the seismogenic zone, in a 10 km region where deep creep was previously suggested from the analysis of the local seismicity. The typical recurrence time was 8 months during the 2008–2015 period. The cumulative slip of the repeating sequence appears to be compatible with the regional geodetic slip rate if they are assumed to be part of a large single asperity (10 km). The repeaters also exhibit short‐term crises and are possibly related to bursts of creep.