Vertical Strike-slip Fault Research Articles

Three-dimensional fault model and features of chained hazards of the Luding MS 6.8 earthquake, Sichuan Province, China

The MS 6.8 Luding earthquake in 2022 is located on the NNW-trending Moxi segment of the Xianshuihe fault with left-lateral strike-slip behavior. This area is where the Xianshuihe, Anninghe, Daliangshan and Longmenshan faults intersect. China Earthquake Administration has identified that intersection area, among the Moxi segment of the Xianshuihe fault, the Anninghe fault, the Daliangshan fault and the southern part of the Longmenshan fault, as a high-magnitude earthquake hazard area. According to existing data on the Luding earthquake, including the focal parameters, the spatial distribution of re-located aftershocks, dominated azimuth of the earthquake intensities and earthquake-induced ground fissures, we built a 3D earthquake fault model. We found that two discontinuous NNW-trending vertical strike-slip faults with left stepping were the seismogenic faults of the Luding earthquake. Its coseismic left-lateral dislocation triggered transtensional slips and aftershocks on the NW-trending secondary faults at its northernmost tensile area. Meanwhile, local crustal coseismic shortening on the side of Mt. Gongga triggered the aftershocks on the NE- and NW-trending secondary conjugated strike-slip faults, which were confirmed by GNSS observations and InSAR deformation field around the epicenter. This earthquake rupturing pattern also controlled the spatial distribution of the earthquake intensity IX area and earthquake chain hazards. The Coulomb stress calculation shows that the Luding earthquake increases the risk of high-magnitude earthquake occurrence on the southernmost part of the Xianshuihe fault and the Anninghe fault. Finally, we suggested doing good monitoring of the Anninghe fault and the southernmost part of the Xianshuihe fault and avoiding active faults with seismogenic capacity and areas prone to earthquake-chained hazards during the site selection and planning of reconstruction.

The Largest Geodetic Coseismic Assessment of the 2020 Mw = 6.4 Petrinja Earthquake

On 28 December 2020, the area of the city of Petrinja was hit by two strong earthquakes of magnitudes 5.0 and 4.7 on the Richter scale, and the following day, 29 December 2020, the same area was hit by an even stronger earthquake of magnitude 6.2. It was one of the two strongest instrumentally recorded earthquakes that hit the territory of the Republic of Croatia in the last hundred years, and the strongest earthquake in the Banovina area after the great earthquake in 1909. Increased seismic activity in this area is caused by two vertical strike–slip faults, Pokupski and Petrinjski. This article aims to determine the displacements of the Earth’s crust caused by seismic activity in this area using GNSS measurements and InSAR techniques and comparing their results. Our study showed that horizontal coseismic displacements of 20 cm and more were limited to a radius of 20 km from the epicenter, with a maximum displacement of around half a meter. Considering the original plate tectonic movements of the region and the time elapsed since the previous earthquake of similar magnitude, the geodynamic movements of the Dinarides area are in substantial part sudden displacements associated with earthquakes.

Dynamic off-fault failure and tsunamigenesis at strike-slip restraining bends: Fully-coupled models of dynamic rupture, ocean acoustic waves, and tsunami in a shallow bay

Inelastic off-fault deformation can lead to large tsunamigenesis in different tectonic settings. Here a mechanism for tsunami generation by strike-slip earthquakes that involves dynamic off-fault failure at restraining bends is presented. Dynamic rupture on a vertical strike-slip fault is modeled with undrained inelastic off-fault response incorporated by the Drucker-Prager yield criterion. I show that in a local transpressive stress regime dynamic off-fault failure at restraining bends produces significantly larger and more localized surface uplift than is produced by purely elastic dislocation models, resulting in a positive flower and coseismic pop-up structure. The larger uplift is due to frictional sliding with a thrust component on conjugate microfractures, modeled by inelastic deformation. The short-wavelength inelastic uplift, largely controlled by bend size, is shown to generate localized tsunami efficiently in a shallow bay by fully coupling dynamic rupture, ocean acoustic waves, and tsunami. Fully-coupled models also indicate that supershear rupture on a vertical planar strike-slip fault does not generate large tsunami as the large kinetic energy of supershear rupture is carried away by ocean acoustic waves. Dynamic off-fault failure at fault complexities, such as restraining bends and compressional stepovers, may need be urgently incorporated in the current tsunami hazard assessments in strike-slip environments.

Response of inelastic SDOF systems subjected to dynamic rupture simulations involving directivity and fling step

Directivity is a phenomenon perceived during fault ruptures wherein the ground motion and spectral response in the direction of the rupture is more significant than in any other direction. The objective of this study is to evaluate the variability in characteristics of near-fault ground motions as well as the ductility demand on structures due to directivity based on simulations of strike-slip earthquake events by employing SPECFEM3D, a physics-based ground motion simulation code. To understand the finite fault propagation effect, a source effect, the vertical strike-slip fault is considered to be embedded in an elastic half-space, to prevent the influence of path and site effects. Two scenarios are designed based on the positioning of nucleation asperity (NA): (1) in the middle of the fault face to simulate bilateral (BL) rupture and (2) shifted to one end for the case of unilateral (UL) rupture. The ground motions at near-fault stations, located in a racetrack configuration around the surface trace of the fault, are analyzed. In addition to a high spectral content in the forward directivity stations as a result of UL rupture, directivity velocity pulses identified in the fault-normal components are higher than the fling step velocity pulses in the fault-parallel component for the racetrack stations considered. Furthermore, the study examines the correlation between ductility demands computed based on elastoplastic rheology with direct point parameter and ground motion intensity measures for directivity and fling step stations.

Near-Source Ground Motions and Their Variability Derived from Dynamic Rupture Simulations Constrained by NGA-West2 GMPEs

ABSTRACTEmpirical ground-motion prediction equations (GMPEs) lack a sufficient number of measurements at near-source distances. Seismologists strive to supplement the missing data by physics-based strong ground-motion modeling. Here, we build a database of ~3000 dynamic rupture scenarios, assuming a vertical strike-slip fault of 36 × 20 km embedded in a 1D layered elastic medium and linear slip-weakening friction with heterogeneous parameters along the fault. The database is built by a Monte Carlo procedure to follow median and variability of Next Generation Attenuation-West2 Project GMPEs by Boore et al. (2014) at Joyner–Boore distances 10–80 km. The synthetic events span a magnitude range of 5.8–6.8 and have static stress drops between 5 and 40 MPa. These events are used to simulate ground motions at near-source stations within 5 km from the fault. The synthetic ground motions saturate at the near-source distances, and their variability increases at the near stations compared to the distant ones. In the synthetic database, the within-event and between-event variability are extracted for the near and distant stations employing a mixed-effect model. The within-event variability is lower than its empirical value, only weakly dependent on period, and generally larger for the near stations than for the distant ones. The between-event variability is by 1/4 lower than its empirical value at periods >1 s. We show that this can be reconciled by considering epistemic error in Mw when determining GMPEs, which is not present in the synthetic data.

Seismicity of the Northern Volcanic Zone of Iceland

A half century of monitoring of the Northern Volcanic Zone of Iceland, a branch of the North America—Eurasia plate boundary, shows that the seismicity is very unevenly distributed, both in time and space. The four central volcanoes at the boundary, Þeistareykir, Krafla, Fremrinámar, and Askja, show persistent but very low-level seismicity, spatially coinciding with their high-temperature geothermal systems. On their rift structures, on the other hand, seismicity is almost absent, except during rifting episodes. Krafla went through a rifting episode in 1975–1984 with inflation, interrupted by 20 diking events with extensive rifting, eruptive activity, and intense seismicity along an 80 km long section of the rift. During inflation periods, the seismicity was contained within the caldera of the volcano, reflecting the inflation level of the magma chamber. Diking events were marked by seismicity propagating away from the volcano into the fissure swarms to the south or north of the volcano, accompanied by rapid deflation of the caldera magma chamber. These events lasted from 1 day to 3 months, and the dike length varied between 1 and 60 km. The area around the Askja volcano is the only section of the Northern Volcanic Zone that shows persistent moderate seismicity. The largest events are located between fissure swarms of adjacent volcanic systems. Detailed relative locations of hypocenters reveal a system of vertical strike-slip faults, forming a conjugate system consistent with minimum principal stress in the direction of spreading across the plate boundary. A diking event into the lower crust was identified in the adjacent fissure swarm at Upptyppingar in 2007–2008. Four nests of anomalously deep earthquakes (10–34 km) have been identified in the Askja region, apparently associated with the movements of magma well below the brittle-ductile transition. Several processes have been pointed out as possible causes of earthquakes in the deformation zone around the plate boundary. These include inflation and deflation of central volcanoes, intrusion of propagating dikes, both laterally and vertically, strike-slip faulting on conjugate fault systems between overlapping fissure swarms, migration of magma in the lower, ductile crust, and geothermal heat mining.

3D outcrop modelling of large discordant breccia bodies in basinal carbonates of the Apulian margin, Italy

Large discordant breccia bodies (LDBBs) are important record keepers of the tectonic and gravitational evolution in platform-to-basinal settings, and have important implications for fluid-flow migration and compartmentalization of tight carbonate reservoirs. In the Gargano Promontory of southern Italy, LDBBs occur within a Cretaceous slope and basinal carbonate succession. We use field mapping and Unmanned Aerial Vehicle (UAV) -based Structure from Motion (SfM) Photogrammetry to document otherwise inaccessible cliff-side exposures of seismic-to subseismic-scale vertical discordant breccias. LDBBs are up to 50 m in width, more than 80 m in height and display internal chaotic or aligned clast fabrics. The formation which generally contains the LDBBs is characterized by beds of cherty pelagic limestone intercalated with calcarenites, calciturbidites and horizons of mass transport deposits. The mass-transport deposits can be correlated across the chaotic breccia bodies, indicating only slight or no vertical displacement across the adjacent walls. The bases of the breccia bodies are always hidden below current sea level, while the rarely exposed tops are capped by bedded intervals of the host rock formation. Timing and origin of the studied breccias were determined using several lines of evidence, such as stratigraphic provenance of clasts in breccias, mutual relationships of structural and sedimentologic features, and previous studies which establish that the age of dolomitizing cements in the LDBBs formed at different times and by different processes (fault shearing and solution collapse). This work investigates the size, shape and geometry of these breccia bodies whilst also providing cm-scale detail of the textural features in otherwise inaccessible outcrops. We suggest that breccias formed as a result of solution exploiting a pre-existing fracture network characterized by large-scale vertical strike-slip or oblique-slip faults. Initial displacement along these faults created a wide fault damage zone, where fluid migration was later focused to create a zone highly susceptible to solution and subsequent periodic sidewall collapse.

3D Finite-Element Modeling of Dynamic Rupture and Aseismic Slip over Earthquake Cycles on Geometrically Complex Faults

ABSTRACTWe develop a new dynamic earthquake simulator to numerically simulate both spontaneous rupture and aseismic slip over earthquake cycles on geometrically complex fault systems governed by rate- and state-dependent friction. The method is based on the dynamic finite-element method (FEM) EQdyna, which is directly used in the simulator for modeling 3D spontaneous rupture. We apply an adaptive dynamic relaxation technique and a variable time stepping scheme to EQdyna to model the quasi-static processes of an earthquake cycle, including the postseismic, interseismic, and nucleation processes. Therefore, the dynamic and quasi-static processes of an earthquake cycle are modeled in one FEM framework. Tests on a vertical strike-slip fault verify the correctness of the dynamic simulator. We apply the simulator to thrust faults with various dipping angles, which can be considered as the simplest case of geometrically complex faults by breaking symmetry, compared with vertical faults, to examine effects of dipping fault geometry on earthquake cycle behaviors. We find that shallower dipping thrust faults produce larger seismic slip and longer recurrence time over earthquake cycles with the same rupture area. In addition, we find an empirically linear scaling relation between the recurrence interval (and the seismic moment) and the sinusoidal function of the dip angle. The dip-angle dependence is likely due to the free-surface effect, because of broken symmetry. These results suggest dynamic earthquake simulators that can handle nonvertical dipping fault geometry are needed for subduction-zone earthquake studies.

Automated Microseismic Processing and Integrated Interpretation of Induced Seismicity during a Multistage Hydraulic-Fracturing Stimulation, Alberta, Canada

ABSTRACT The development of organic-rich, low-permeability formations for hydrocarbon production requires the use of unconventional techniques such as multiwell pad drilling of horizontal wells and massive multistage hydraulic-fracturing stimulations. However, proliferation of these unconventional development methods has been linked to localized cases of fault reactivation during or shortly after hydraulic fracturing. In the Duvernay formation, located in Alberta, Canada, induced seismicity from hydraulic fracturing has occurred on nearly vertical strike-slip faults that are difficult to detect with conventional seismic exploration methods. In such cases, faults may only be discernible from seismic events with precise and accurate locations, which generally requires dense seismic monitoring arrays deployed near the stimulated wells. In this study, we introduce a new, semiautomated workflow for processing passive seismic data from a dense array and then integrate it with a 3D seismic dataset to characterize seismicity clusters related to hydraulic fracturing and pre-existing faults. The reactivated faults inferred from the distribution of the microseismic events directly overlie a system of incised, middle Devonian channels below the Duvernay formation observed in time slices extracted from the 3D seismic data. The channel system exhibits a set of lateral offsets, interpreted as ancient strike-slip fault displacements, the detection of which is further enhanced by use of a similarity attribute calculated from the 3D seismic data. Taken together, integrated interpretation of induced seismicity and 3D seismic data support a model of a regional left-lateral strike-slip fault system that was active during the middle Devonian and reactivated in a reverse sense (right-lateral strike slip) during hydraulic-fracturing operations.

The Community Code Verification Exercise for Simulating Sequences of Earthquakes and Aseismic Slip (SEAS)

Abstract Numerical simulations of sequences of earthquakes and aseismic slip (SEAS) have made great progress over past decades to address important questions in earthquake physics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between earthquake nucleation, dynamic rupture, and aseismic slip, and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing complexity of SEAS modeling calls for extensive efforts to verify codes and advance these simulations with rigor, reproducibility, and broadened impact. In 2018, we initiated a community code-verification exercise for SEAS simulations, supported by the Southern California Earthquake Center. Here, we report the findings from our first two benchmark problems (BP1 and BP2), designed to verify different computational methods in solving a mathematically well-defined, basic faulting problem. We consider a 2D antiplane problem, with a 1D planar vertical strike-slip fault obeying rate-and-state friction, embedded in a 2D homogeneous, linear elastic half-space. Sequences of quasi-dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of results from 11 groups using different numerical methods show excellent agreements in long-term and coseismic fault behavior. In BP1, we found that truncated domain boundaries influence interseismic stressing, earthquake recurrence, and coseismic rupture, and that model agreement is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of fault behavior depends on how well physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard such as earthquake size distributions, moment release, and recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics.

Curved slickenlines preserve direction of rupture propagation

AbstractSlip-parallel grooves (striations) on fault surfaces are considered a robust indicator of fault slip direction, yet their potential for recording aspects of earthquake rupture dynamics has received little attention. During the 2016 Kaikōura earthquake (South Island, New Zealand), >10 m of dextral strike-slip on the steeply dipping Kekerengu fault exhumed >200 m2 of fresh fault exposure (free faces) where it crossed bedrock canyons. Inscribed upon these surfaces, we observed individual striae up to 6 m long, all of which had formed during the earthquake. These were typically curved. Using simulations of spontaneous dynamic rupture on a vertical strike-slip fault, we reproduce the curved morphology of striae on the Kekerengu fault. Assuming strike-slip pre-stress, our models demonstrate that vertical tractions induced by slip in the so-called cohesive zone result in transient changes in slip direction. We show that slip-path convexity is sensitive to the direction of rupture propagation. To match the convexity of striae formed in 2016 requires the rupture to have propagated in a northeast direction, a prediction that matches the known rupture direction of the Kaikōura earthquake. Our study highlights the potential for fault striae to record aspects of rupture dynamics, including the rupture direction of paleo strike-slip earthquakes.

Geophysical characterization of Udaeta Lake as a possible pull-apart basin associated to quaternary tectonic activity along Magallanes-Fagnano fault system

Pull-apart basins are depressions formed at releasing bends or steps in strike-slip fault systems, between two parallel vertical strike-slip fault sections. In the present study, the geometry of Udaeta Lake, located in the central section of the Magellan-Fagnano Fault System (MFFS) and previously defined by some authors as a pull-apart basin, is analysed. The application of multiple geophysical methods, constrained by surface geological information, enabled us to reveal the subsurface geometry of a section of the MFFS, Udaeta Lake area. Magnetic, gravimetric and detailed topographic profiles, together with vertical electrical soundings were surveyed. The obtained results were compared with fieldwork data. This combined methodology allowed the identification of a transtensional zone with two E-W main sinistral strike-slip faults with a normal component that control the North and South coasts of the lake. This central depression occupied by the lake, with a depth of ∼30 m, is also affected by inferred NE subparallel faults. The geophysical characterization of the MFFS also supports the influence of this fault system on the formation of some water bodies located along the fault zone. Finally, in spite of the lack of detailed geological and structural data, Udaeta Lake we suggest a genesis consistent with a pull-apart basin model. This work constitutes an important contribution to understand earthquake rupture parameters and pattern of surface and in-depth deformation and so, improve knowledge of the seismological behaviour of the MFFS.

The 3D seismic characteristics and significance of the strike-slip faults in the Tazhong area (Tarim Basin, China)

The eastern part of Tazhong area in the Tarim Basin consists of three sets of vertical strike-slip faults oriented in north–northeast (36°azimuth), east–northeast (68° azimuth), and west–northwest (126°azimuth) directions that cut the strata from Cambrian to Carboniferous. The fault belts indicate significant horizon upwarp and downwarp deformations and variations in their stratigraphic thickness on seismic profiles. Through detailed interpretation of the 3D seismic data, we consider that these phenomena reflect the different stress properties and active stages of the faults. The horizon upwarp and downwarp within the fault belts correlated respectively to the decrease and increase in stratigraphic thickness within the fault belts in comparison to the coeval counterpart of the bilateral fault blocks. For the same fault, different stratigraphic intervals express different types of horizon deformation and thickness changes. The horizon downwarp and the contemporaneous stratigraphic thickening inside the fault belts suggest the transtensional actions of the fault. The horizon upwarp and the contemporaneous thinning within the fault belts suggest transpressional actions of the fault. Based on this, we inferred the active periods of the three sets of strike-slip faults. The north–northeast-striking faults were formed in the late Ordovician Sangtamu Formation. This set of faults experienced four stages, i.e., sinistral transpression, sinistral transtension, static, and transtension. The east–northeast and west–northwest-striking faults initiated in the mid-Cambrian period as coupled transtension. Activity ceased in the west–northwest faults after the mid-Cambrian and in the east–northeast faults during the late Ordovician. The three sets of strike-slip faults all affect the formation of the hydrothermal dissolution reservoirs that are distributed in the Ordovician carbonate rocks.

Finite-fault source inversion using adjoint methods in 3-D heterogeneous media

Accounting for lateral heterogeneities in the 3-D velocity structure of the crust is known to improve earthquake source inversion, compared to results based on 1-D velocity models which are routinely assumed to derive finite-fault slip models. The conventional approach to include known 3-D heterogeneity in source inversion involves pre-computing 3-D Green’s functions, which requires a number of 3-D wave propagation simulations proportional to the number of stations or to the number of fault cells. The computational cost of such an approach is prohibitive for the dense data sets that could be provided by future earthquake observation systems. Here, we propose an adjoint-based optimization technique to invert for the spatio-temporal evolution of slip velocity. The approach does not require pre-computed Green’s functions. The adjoint method provides the gradient of the cost function, which is used to improve the model iteratively employing an iterative gradient-based minimization method. The adjoint approach is shown to be computationally more efficient than the conventional approach based on pre-computed Green’s functions in a broad range of situations. We consider data up to 1 Hz from a Haskell source scenario (a steady pulse-like rupture) on a vertical strike-slip fault embedded in an elastic 3-D heterogeneous velocity model. The velocity model comprises a uniform background and a 3-D stochastic perturbation with the von Karman correlation function. Source inversions based on the 3-D velocity model are performed for two different station configurations, a dense and a sparse network with 1 and 20 km station spacing, respectively. These reference inversions show that our inversion scheme adequately retrieves the rise time when the velocity model is exactly known, and illustrates how dense coverage improves the inference of peak-slip velocities. We investigate the effects of uncertainties in the velocity model by performing source inversions based on an incorrect, homogeneous velocity model. We find that, for velocity uncertainties that have standard deviation and correlation length typical of available 3-D crustal models, the inverted sources can be severely contaminated by spurious features even if the station density is high. When data from thousand or more receivers is used in source inversions in 3-D heterogeneous media, the computational cost of the method proposed in this work is at least two orders of magnitude lower than source inversion based on pre-computed Green’s functions.

Porosity distribution in a heterogeneous clay-rich fault core by image processing of 14C-PMMA autoradiographs and Scanning Electron Microscopy

AbstractShale formations are considered by a number of countries as the most suitable media to dispose of high-level radioactive waste. This is mainly due to the impermeable, self-sealing, chemical reducing, and sorption properties that tend to retard radionuclide migration. However, shale formations can also contain highly connected fault zones with permeabilities that can differ of several orders of magnitudes with respect to the undeformed host rock. The objective of this work is to use the 14C-PMMA autoradiography method combined with SEM-EDS measurements to understand the porosity variations in and around fault gouges and to define their relationship to mechano-chemical processes. The studied samples were taken from a low permeability shale in a small-scale vertical strike-slip fault at the Tournemire underground research laboratory. Results display significant variations in porosity and mineralogy along the studied gouge zone due to polyphased tectonics and paleo-fluid circulations.

New images of the crustal structure beneath eastern Tibet from a high-density seismic array

An east–west trending, high-density seismic array was deployed along the eastern margin of the Tibetan Plateau to investigate its eastward expansion. The 160 km long array spans the Ruoergai basin, Minjiang fault, Minshan Mountains, Tazang fault, and West Qinling. The array included 330 short-period seismographs spaced at 500 m intervals, which recorded teleseismic 3-component waveforms over a one month period. P-wave receiver functions calculated from 35 teleseismic events provided an image of crustal structure. The results show a massive thrust nappe structure around the Minshan Mountains and beneath the Minjiang fault. We suggest that this nappe formed after the closure of the Paleo-Tethyan ocean. The resultant Triassic thrusting contributed to partial uplift of the eastern Ruoergai basin and the Minshan Mountains in middle-to-late Miocene time. Receiver function images show that the Tazang fault is a crustal-scale rupture cutting across the Moho. The western Tazang fault appears as a nearly vertical strike-slip fault accommodating left lateral shear at the terminus of the eastern Kunlun fault. After clockwise rotation from an approximate east–west orientation to a nearly north–south orientation, the eastern Tazang fault became a west-dipping thrust fault, which caused crustal thickening beneath the Minshan Mountains and the West Qinling. Our results suggest that late Cenozoic uplift of the eastern margin of the plateau is produced by eastward overthrusting and crustal shortening, processes that absorbed slip along the Tazang and Kunlun faults.

Static elastic deformation in an orthotropic half-space with rigid boundary model due to non-uniform long strike slip fault

The solution of static elastic deformation of a homogeneous, orthotropic elastic uniform half-space with rigid boundary due to a non-uniform slip along a vertical strike-slip fault of infinite length and finite width has been studied. The results obtained here are the generalisation of the results for an isotropic medium having rigid boundary in the sense that medium of the present work is orthotropic with rigid boundary which is more realistic than isotropic and the results for an isotropic case can be derived from our results. The variations of displacement with distance from the fault due to various slip profiles have been studied to examine the effect of anisotropy on the deformation. Numerically it has been found that for parabolic slip profile, the displacement in magnitude for isotropic elastic medium is greater than that for an orthotropic elastic half-space, but, in case of linear slip, the displacements in magnitude for an orthotropic medium is greater than that for the isotropic medium.

A finite difference method for off-fault plasticity throughout the earthquake cycle

We have developed an efficient computational framework for simulating multiple earthquake cycles with off-fault plasticity. The method is developed for the classical antiplane problem of a vertical strike-slip fault governed by rate-and-state friction, with inertial effects captured through the radiation-damping approximation. Both rate-independent plasticity and viscoplasticity are considered, where stresses are constrained by a Drucker–Prager yield condition. The off-fault volume is discretized using finite differences and tectonic loading is imposed by displacing the remote side boundaries at a constant rate. Time-stepping combines an adaptive Runge–Kutta method with an incremental solution process which makes use of an elastoplastic tangent stiffness tensor and the return-mapping algorithm. Solutions are verified by convergence tests and comparison to a finite element solution. We quantify how viscosity, isotropic hardening, and cohesion affect the magnitude and off-fault extent of plastic strain that develops over many ruptures. If hardening is included, plastic strain saturates after the first event and the response during subsequent ruptures is effectively elastic. For viscoplasticity without hardening, however, successive ruptures continue to generate additional plastic strain. In all cases, coseismic slip in the shallow sub-surface is diminished compared to slip accumulated at depth during interseismic loading. The evolution of this slip deficit with each subsequent event, however, is dictated by the plasticity model. Integration of the off-fault plastic strain from the viscoplastic model reveals that a significant amount of tectonic offset is accommodated by inelastic deformation ( ∼ 0.1 m per rupture, or ∼ 10% of the tectonic deformation budget).

THE 2014 MW 6.9 NORTH AEGEAN TROUGH (NAT) EARTHQUAKE: SEISMOLOGICAL AND GEODETIC EVIDENCE

A strong earthquake (Mw 6.9) on 24 May 2014 ruptured the North Aegean Trough (NAT) in Greece, west of the North Anatolian Fault Zone (NAFZ). In order to provide unbiased constrains of the rupture process and fault geometry of the earthquake, seismological and geodetic data were analyzed independently. First, based on teleseismic long-period P- and SH- waveforms a point-source solution yielded dominantly right-lateral strike-slip faulting mechanism. Furthermore, finite fault inversion of broad-band data revealed the slip history of the earthquake. Second, GPS slip vectors derived from 11 permanent GPS stations uniformly distributed around the meizoseismal area of the earthquake indicated significant horizontal coseismic slip. Inversion of GPS-derived displacements on the basis of Okada model and using the new TOPological INVersion (TOPINV) algorithm permitted to model a vertical strike slip fault, consistent with that derived from seismological data. Obtained results are consistent with the NAT structure and constrain well the fault geometry and the dynamics of the 2014 earthquake. The latter seems to fill a gap in seismicity along the NAT in the last 50 years, but seems not to have a direct relationship with the sequence of recent faulting farther east, along the NAFZ.

Within-Event and Between-Events Ground Motion Variability from Earthquake Rupture Scenarios

Measurement of ground motion variability is essential to estimate seismic hazard. Over-estimation of variability can lead to extremely high annual hazard estimates of ground motion exceedance. We explore different parameters that affect the variability of ground motion such as the spatial correlations of kinematic rupture parameters on a finite fault and the corner frequency of the moment-rate spectra. To quantify the variability of ground motion, we simulate kinematic rupture scenarios on several vertical strike-slip faults and compute ground motion using the representation theorem. In particular, for the entire suite of rupture scenarios, we quantify the within-event and the between-events ground motion variability of peak ground acceleration (PGA) and response spectra at several periods, at 40 stations—all approximately at an equal distance of 20 and 50 km from the fault. Both within-event and between-events ground motion variability increase when the slip correlation length on the fault increases. The probability density functions of ground motion tend to truncate at a finite value when the correlation length of slip decreases on the fault, therefore, we do not observe any long-tail distribution of peak ground acceleration when performing several rupture simulations for small correlation lengths. Finally, for a correlation length of 6 km, the within-event and between-events PGA log-normal standard deviations are 0.58 and 0.19, respectively, values slightly smaller than those reported by Boore et al. (Earthq Spectra, 30(3):1057–1085, 2014). The between-events standard deviation is consistently smaller than the within-event for all correlations lengths, a feature that agrees with recent ground motion prediction equations.