Seismic Rupture Propagation Research Articles

SEISMOGENIC ZONE OF CAPE SHARTLAY (LAKE BAIKAL): SPECIFIC FEATURES OF STRUCTURE, DISPLACEMENTS AND RUPTURE GROWTH

Seismogenic deformations of Cape Shartlay represent a very young fault system on the northwestern coast of Lake Baikal. Their study is providing an important opportunity to measure earthquake magnitudes, to identify areas where earthquakes are more likely to occur, and to estimate the probability of earthquake occurrence as applied to seismically active Baikal region. In this connection, the present work was aimed at characterizing in detail the structure, displacements, and reconstruction of the rupture propagation model. The study is based on photogrammetric processing and interpretation of the unmanned aerial survey data, as well as on morphostructural analysis of the displacement profiles and georadiolocation (GPR) data. It has been found that seismogenic ruptures of Cape Shartlay formed under prevailing extension conditions during no less than two earthquakes with magnitudes Mw≥7.0, Ms≥7.2. Seismic rupture propagation was primarily northward. The main rupture with displacement amplitude of more than 2 m contributed 39 to 93 % to the total surface displacement depending on the amount of dislocations on the transverse profile. It is shown that the length of a certain rupture increased almost instantaneously, then displacements along some of the ruptures stopped. A significant elongation of ruptures is primarily due to their merging. The present-day seismogenic zone is highly permeable. According to the tectonophysical model of formation of inner structure of the fault zone, the development of the seismogenic rupture system of Cape Shartlay corresponds to the late disjunctive stage. This means that the rupturing process in this segment of the North Baikal fault may not have stopped yet, and the lack of large earthquakes in the instrumental record implies the accumulation of stress in its southern part. The obtained results provide an opportunity to reconstruct the development of large fault zones by studying the displacement profiles and, therefore, to localize more precisely the places where future earthquakes may occur.

Fracturing, comminution and grain-size-sensitive creep as a record of coseismic loading in the middle-crust: Insights from the Urtiga mylonitic pluton (NE Brazil)

Evidence for mid-crustal seismic slip is scarce due to the limited capacity of downward propagation of rupture damage imposed by the confining pressure. Nevertheless, structures indicative of elevated stresses occur throughout the crustal profile. To further investigate this setting, we have studied the fabrics of the Urtiga pluton, emplaced at the south Patos shear zone (northeast Brazil), a major Neoproterozoic crustal boundary. K-feldspar and plagioclase porphyroclasts are fractured, with fine-grained K-feldspar + plagioclase mixtures filling cracks. The edges of the clasts are rimmed by fine feldspar grains that form a fine-grained matrix. Quartz ribbons are parallel to the mylonitic foliation and show microstructures and a fabric indicating deformation via dislocation creep. Chemical compositions of feldspars are typically similar between porphyroclasts, fractures and matrix, with plagioclase grains locally being more albitic within fractures. Crystallographic preferred orientations (CPO) in grains within fractures are host-controlled by the adjacent porphyroclast, while fine grains rimming the clasts show a weak CPO and are mostly strain-free. These characteristics suggest that grain size reduction in the Urtiga mylonitic pluton occurred through fracturing and subsequent grain-size-sensitive creep under mid-crustal conditions, which were possibly attained via downward propagation of seismic rupture from the overlying seismogenic zone during transient, seismic slip episodes, giving rise to spatially related pseudotachylytes at the boundaries of the southern Patos shear zone.

Geodetic source models of the 2016–2022 Menyuan Earthquake sequence (Northeastern Tibet) inferred from InSAR and optical observations

SUMMARY We study the 2016 January 21 (${{{M}}}_{\rm{w}}$ 5.9) and 2022 January 8 (${{{M}}}_{\rm{w}}$ 6.7) earthquake sequence that struck the Menyuan region in northwest China's Qinghai province. These two earthquakes are destructive events that occurred around/on the Lenglongling fault (LLLF). Here, we derive the line-of-sight displacement fields of the two earthquakes using Interferometric Synthetic Aperture Radar (InSAR) measurements of Sentinel-1 SAR data, and map the range and horizontal offset fields of the 2022 event using Sentinel-1 amplitude images and Planet-Lab optical images. Based on the offset maps, we determine the detailed surface rupture trace of the 2022 event. We perform slip inversions for the two earthquakes on triangle fault patches whose size increases with depth. Results show that the western branch segment of the 2022 event has a ∼0.5-m normal dip-slip motion. This result contradicts previous inferences on dip-slip sense of this branch segment. We identify a left-stepping fault structure with a ∼5-km step width in the transition zone between the Tuolaishan fault (TLSF) and LLLF, which may serve as a kinematic barrier to prevent further propagation of seismic rupture along the TLSF. Stress calculation shows that a stress drop of ∼0.4 bar produced by the 2016 event on a ∼5-km long LLLF segment may act as a negative stress barrier to suppress rupture propagation of the 2022 event toward the southeast of the LLLF.

Late Quaternary Fault Activity of the Southern Segment of the Daliangshan Fault along the Southeastern Margin of the Tibetan Plateau and Its Implications for Fault Rupture Behaviour at Stepovers

Abstract Stepovers have been widely suggested to be important structural boundaries that control earthquake rupture extent and therefore the size of earthquakes. Previous studies suggested that ~4-5 km wide stepovers are likely to arrest fault rupture. However, recent earthquake cases show that even much wider stepovers (e.g., ≥7-8 km wide) sometimes may not effectively impede seismic rupture propagation, which requires us to further explore cascading rupture mechanism of large earthquakes at wider stepovers. Here, we constrained slip rates and paleoseismic earthquakes of two fault sections that bound a constraining stepover with width of approximately 7-8 km along the southern segment of the Daliangshan fault along the southeastern margin of the Tibetan Plateau. Multiple landform offset and radiocarbon dating results constrained that the two fault sections show a moderate slip rate at approximately 5 mm/yr. Moreover, three or four paleoseismic events Z through W, in 1489 AD, 620-515 BC, 4475-3700 BC, and 6265-4510 BC, were revealed on the Jiaojihe fault section. Based on the aforementioned results, we suggest that the most recent seismic event might exhibit a jump over the restraining stepover, whereas ruptures of the older events might be arrested by the stepover. Furthermore, we suggest that moderate slip-rate faults might have similar potential with that of high slip-rate faults to rupture through wider stepovers, which increases us in understanding the generation of cascading ruptures on strike-slip faults and is helpful for evaluating seismic hazards.

New Megathrust Locking Model for the Southern Kurile Subduction Zone Incorporating Viscoelastic Relaxation and Non‐Uniform Compliance of Upper Plate

AbstractDense Global Navigation Satellite System (GNSS) observations enable the development of megathrust interseismic locking models for the southern Kurile subduction zone where many great earthquakes have occurred. Inversion of these data assuming uniform elastic Earth has yielded slip deficit rates that are unreasonably high and/or full locking depth that is unreasonably large. Using the finite element method, here we construct a new Kurile locking model that includes interseismic viscoelastic stress relaxation and non‐uniform compliance of the elastic upper plate. Inverting the same geodetic data using the new subduction zone model alleviates the previously seen unreasonable features in inferred megathrust locking state. In the new model, full locking extends to shallower depths than the downdip limit of some large megathrust earthquakes including the 2003 Mw 8.0 Tokachi‐oki earthquake, supporting the notion of the shrinking of the locked area before the earthquakes and/or propagation of seismic rupture into creeping areas as previously predicted by friction or dynamic rupture models. By modeling the effects of a few recent M 8 earthquakes, we show that postseismic transients of recent earthquakes, although second‐order, should be addressed in deriving megathrust locking models. The locking state near the trench cannot be resolved by the land‐based GNSS data regardless of the improved model rheology and structure, although independent observations, such as slow earthquakes, may be used to speculate on the near‐trench locking state in various part of the margin in the absence of seafloor geodetic observations.

Selective clast survival in an experimentally-produced pseudotachylyte

On-fault processes during earthquakes contribute to seismic rupture propagation and slip. Here we investigate clast fragmentation in an experimental pseudotachylyte (solidified seismic melt) produced with a rotary shear machine. We slid for 0.44 m (corresponding to Mw ≥ 6 earthquakes), at slip rates > 1 m/s, pre-cut samples of quartz + phyllosilicates + plagioclase + sillimanite + garnet -bearing ultramylonite, that hosts pseudotachylytes in nature. The ultramylonite minerals extensively preserved as clasts in the experimental pseudotachylyte are quartz, plagioclase, and sillimanite. Garnet is scarcely preserved, despite having a melting temperature similar to plagioclase, probably due to having low thermal shock resistance. This selective clast survival is identical to the one found in the natural pseudotachylytes. Based on these experimental observations and assuming non-equilibrium melting, the preservation of a mineral, as a clast, in the melt appears to be controlled by its thermal shock properties as well as by its melting temperature. Since the mechanical effects of rupture propagation in these experiments were negligible, we conclude that, for Mw ≥ 6 earthquakes, (i) frictional slip and heating of the slipping zone plus (ii) thermomechanical properties of minerals, rather than fault rupture processes, control mineral comminution and clast survival in frictional melts.

Aseismic transient slip on the Gofar transform fault, East Pacific Rise

Oceanic transform faults display a unique combination of seismic and aseismic slip behavior, including a large globally averaged seismic deficit, and the local occurrence of repeating magnitude (M) [Formula: see text] earthquakes with abundant foreshocks and seismic swarms, as on the Gofar transform of the East Pacific Rise and the Blanco Ridge in the northeast Pacific Ocean. However, the underlying mechanisms that govern the partitioning between seismic and aseismic slip and their interaction remain unclear. Here we present a numerical modeling study of earthquake sequences and aseismic transient slip on oceanic transform faults. In the model, strong dilatancy strengthening, supported by seismic imaging that indicates enhanced fluid-filled porosity and possible hydrothermal circulation down to the brittle-ductile transition, effectively stabilizes along-strike seismic rupture propagation and results in rupture barriers where aseismic transients arise episodically. The modeled slow slip migrates along the barrier zones at speeds ∼10 to 600 m/h, spatiotemporally correlated with the observed migration of seismic swarms on the Gofar transform. Our model thus suggests the possible prevalence of episodic aseismic transients in M [Formula: see text] rupture barrier zones that host active swarms on oceanic transform faults and provides candidates for future seafloor geodesy experiments to verify the relation between aseismic fault slip, earthquake swarms, and fault zone hydromechanical properties.

The Transient and Intermittent Nature of Slow Slip

To first order, faults are locked while stress builds up to a devastating earthquake. However, we know that faults also slip slowly. After decades of geophysical observation, slow slip is now recognized as part of a continuum of transient deformation ranging from the dynamic propagation of seismic rupture to aseismic events over a wide range of durations and sizes. A growing body of evidence suggests that large‐scale slow slip events can be decomposed into a multitude of smaller, temporally clustered events. Slow slip is more frequent and more dynamic than is suggested by conceptual models of rate‐strengthening, stable slip.

From fault creep to slow and fast earthquakes in carbonates

Abstract A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L’Aquila (Mw 6.1) in 2009 and Norcia (Mw 6.5) in 2016 in central Italy. Surprisingly, within this region, fast (≈3 km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite its relevance for seismic hazard studies, the transition from fault creep to slow and fast seismic rupture propagation is still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures were obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3–5 km depth (i.e., P = 100 MPa and T = 100 °C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of seismicity in central Italy. The transition from slow to fast rupture is explained by an increase in the ambient temperature, which enhances the elastic loading stiffness of the fault, i.e., the slip velocities during nucleation, allowing flash weakening and, in turn, the propagation of fast ruptures radiating intense high-frequency seismic waves.

Static Stress Increase in the Outer Forearc Produced by MW 8.2 September 8, 2017 Mexico Earthquake and its Relation to the Gravity Signal

An Mw 8.2 earthquake affected on September 8, 2017 the Cocos plate beneath the Caribbean plate next to the transform contact with the North American plate across the Tehuantepec gap, one of the two gaps that exist along the Mexican subduction zone offshore Chiapas. The epicenter occurred at a depth of 47 km and was associated with a highly steep normal fault parallel to the trench. The origin can be related to slab pull forces, dehydration processes and reactivation of outer rise faults. This earthquake produced positive changes in the Coulomb static stress, which would favor the development of thrust displacements along the interplate contact across the Tehuantepec gap instead of liberating tensions accumulated on it for decades. In the case that stresses were not released steadily in the future, a thrusting-related event with a magnitude of Mw 7.9 or higher could be expected after the September 8, 2017 Chiapas earthquake. However, the differential weight of the forearc offshore (that contains a region with a high positive gravity gradient) could inhibit rupture propagation to the coast precluding higher magnitudes. Combined and satellite only earth gravity field models show a highly heterogeneous density structure along the forearc of the North America-Caribbean plates and in particular at the studied zone. Maximum displacements in the rupture zone correlate to minimum gravity-derived anomalies showing a probable relationship between the differential weight of the forearc structure and the extensional faulting affecting the bending of the Cocos plate at depth. In this sense, variable sediment thicknesses over the forearc, and a high gravity gradient zone parallel to the trench in the outer forearc, could be reflecting physical heterogeneities that influence the propagation of the rupture zone. Additionally, to the north, a highly positive gradient signal related to the subducted Tehuantepec Fz projection beneath the North American plate marks the ending of the main slip patch acting as a barrier to the seismic rupture propagation. This event exemplifies (1) how extensional ruptures in the downgoing slab of a subduction zone can increment accumulated elastic strain across a seismic gap instead of liberating tensions, (2) that rupture propagation for this intraplate event was probably controlled (at shallow depths) by the density structure of the forearc, similarly to recent interplate earthquakes along the Chilean margin; (3) how a low gravity gradient signal and low density structures identified from the inversion model in the region of the outer forearc, where the normal stress increased after the main seismic event, suggest a higher risk of occurrence of a great megathrust earthquake.

Fault reactivation mechanisms and dynamic rupture modelling of depletion-induced seismic events in a Rotliegend gas reservoir

AbstractUnderstanding the mechanisms and key parameters controlling depletion-induced seismicity is key for seismic hazard analyses and the design of mitigation measures. In this paper a methodology is presented to model in 2D the static stress development on faults offsetting depleting reservoir compartments, reactivation of the fault, nucleation of seismic instability, and the subsequent fully dynamic rupture including seismic fault rupture and near-field wave propagation. Slip-dependent reduction of the fault's strength (cohesion and friction) was used to model the development of the instability and seismic rupture. The inclusion of the dynamic calculation allows for a closer comparison to field observables such as borehole seismic data compared to traditional static geomechanical models. We applied this model procedure to a fault and stratigraphy typical for the Groningen field, and compared the results for an offset fault to a fault without offset. A non-zero offset on the fault strongly influenced the stress distribution along the fault due to stress concentrations in the near-fault area close to the top of the hanging wall and the base of the footwall. The heterogeneous stress distribution not only controlled where nucleation occurred within the reservoir interval, but also influenced the subsequent propagation of seismic rupture with low stresses inhibiting the propagation of slip. In a reservoir without offset the stress distribution was relatively uniform throughout the reservoir depth interval. Reactivation occurred at a much larger pressure decrease, but the subsequent seismic event was much larger due to the more uniform state of stress within the reservoir. In both cases the models predicted a unidirectional downward-propagating rupture, with the largest wave amplitudes being radiated downwards into the hanging wall. This study showed how a realistic seismic event could be successfully modelled, including the depletion-induced stressing, nucleation, dynamic propagation, and wave propagation. The influence of fault offset on the depletion-induced stress is significant; the heterogeneous stress development along offset faults not only strongly controls the timing and location of a seismic slip, but also influences the subsequent rupture size of the dynamic event.

An 8 month slow slip event triggers progressive nucleation of the 2014 Chile megathrust

AbstractThe mechanisms leading to large earthquakes are poorly understood and documented. Here we characterize the long‐term precursory phase of the 1 April 2014 Mw8.1 North Chile megathrust. We show that a group of coastal GPS stations accelerated westward 8 months before the main shock, corresponding to a Mw6.5 slow slip event on the subduction interface, 80% of which was aseismic. Concurrent interface foreshocks underwent a diminution of their radiation at high frequency, as shown by the temporal evolution of Fourier spectra and residuals with respect to ground motions predicted by recent subduction models. Such ground motions change suggests that in response to the slow sliding of the subduction interface, seismic ruptures are progressively becoming smoother and/or slower. The gradual propagation of seismic ruptures beyond seismic asperities into surrounding metastable areas could explain these observations and might be the precursory mechanism eventually leading to the main shock.

Static versus dynamic fracturing in shallow carbonate fault zones

Moderate to large earthquakes often nucleate within and propagate through carbonates in the shallow crust. The occurrence of thick belts of low-strain fault-related breccias is relatively common within carbonate damage zones and was generally interpreted in relation to the quasi-static growth of faults. Here we report the occurrence of hundreds of meters thick belts of intensely fragmented dolostones along a major transpressive fault zone in the Italian Southern Alps. These fault rocks have been shattered in-situ with negligible shear strain accumulation. The conditions of in-situ shattering were investigated by deforming the host dolostones in uniaxial compression both under quasi-static (strain rate ∼10−5 s−1) and dynamic (strain rate >50 s−1) loading. Dolostones deformed up to failure under low-strain rate were affected by single to multiple discrete extensional fractures sub-parallel to the loading direction. Dolostones deformed under high-strain rate were shattered above a strain rate threshold of ∼120 s−1 and peak stresses on average larger than the uniaxial compressive strength of the rock, whereas they were split in few fragments or remained macroscopically intact at lower strain rates. Fracture networks were investigated in three dimensions showing that low- and high-strain rate damage patterns (fracture intensity, aperture, orientation) were significantly different, with the latter being similar to that of natural in-situ shattered dolostones (i.e., comparable fragment size distributions). In-situ shattered dolostones were thus interpreted as the result of high energy dynamic fragmentation (dissipated strain energies >1.8 MJ/m3) similarly to pulverized rocks in crystalline lithologies. Given their seismic origin, the presence of in-situ shattered dolostones can be used in earthquake hazard studies as evidence of the propagation of seismic ruptures at shallow depths.

Dislocation Motion and the Microphysics of Flash Heating and Weakening of Faults during Earthquakes

Earthquakes are the result of slip along faults and are due to the decrease of rock frictional strength (dynamic weakening) with increasing slip and slip rate. Friction experiments simulating the abrupt accelerations (>>10 m/s2), slip rates (~1 m/s), and normal stresses (>>10 MPa) expected at the passage of the earthquake rupture along the front of fault patches, measured large fault dynamic weakening for slip rates larger than a critical velocity of 0.01–0.1 m/s. The dynamic weakening corresponds to a decrease of the friction coefficient (defined as the ratio of shear stress vs. normal stress) up to 40%–50% after few millimetres of slip (flash weakening), almost independently of rock type. The microstructural evolution of the sliding interfaces with slip may yield hints on the microphysical processes responsible for flash weakening. At the microscopic scale, the frictional strength results from the interaction of micro- to nano-scale surface irregularities (asperities) which deform during fault sliding. During flash weakening, the visco-plastic and brittle work on the asperities results in abrupt frictional heating (flash heating) and grain size reduction associated with mechano-chemical reactions (e.g., decarbonation in CO2-bearing minerals such as calcite and dolomite; dehydration in water-bearing minerals such as clays, serpentine, etc.) and phase transitions (e.g., flash melting in silicate-bearing rocks). However, flash weakening is also associated with grain size reduction down to the nanoscale. Using focused ion beam scanning and transmission electron microscopy, we studied the micro-physical mechanisms associated with flash heating and nanograin formation in carbonate-bearing fault rocks. Experiments were conducted on pre-cut Carrara marble (99.9% calcite) cylinders using a rotary shear apparatus at conditions relevant to seismic rupture propagation. Flash heating and weakening in calcite-bearing rocks is associated with a shock-like stress release due to the migration of fast-moving dislocations and the conversion of their kinetic energy into heat. From a review of the current natural and experimental observations we speculate that this mechanism tested for calcite-bearing rocks, is a general mechanism operating during flash weakening (e.g., also precursory to flash melting in the case of silicate-bearing rocks) for all fault rock types undergoing fast slip acceleration due to the passage of the seismic rupture front.

Interseismic coupling on the main Himalayan thrust

AbstractWe determine the slip rate and pattern of interseismic coupling on the Main Himalayan Thrust along the entire Himalayan arc based on a compilation of geodetic, interferometric synthetic aperture radar, and microseismicity data. We show that convergence is perpendicular to the arc and increases eastwards from 13.3 ± 1.7 mm/yr to 21.2 ± 2.0 mm/yr. These rates are comparable to geological and geomorphic estimates, indicating an essentially elastic geodetic surface strain. The interseismic uplift rate predicted from the coupling model closely mimics the topography, suggesting that a small percentage of the interseismic strain is permanent. We find that the fault is fully locked along its complete length over about 100 km width. We don't find any resolvable aseismic barrier that could affect the seismic segmentation of the arc and limit the along‐strike propagation of seismic ruptures. The moment deficit builds up at a rate of 15.1 ± 1 × 1019 N m/yr for the entire length of the Himalaya.

From Geodetic Imaging of Seismic and Aseismic Fault Slip to Dynamic Modeling of the Seismic Cycle

Understanding the partitioning of seismic and aseismic fault slip is central to seismotectonics as it ultimately determines the seismic potential of faults. Thanks to advances in tectonic geodesy, it is now possible to develop kinematic models of the spatiotemporal evolution of slip over the seismic cycle and to determine the budget of seismic and aseismic slip. Studies of subduction zones and continental faults have shown that aseismic creep is common and sometimes prevalent within the seismogenic depth range. Interseismic coupling is generally observed to be spatially heterogeneous, defining locked patches of stress accumulation, to be released in future earthquakes or aseismic transients, surrounded by creeping areas. Clay-rich tectonites, high temperature, and elevated pore-fluid pressure seem to be key factors promoting aseismic creep. The generally logarithmic time evolution of afterslip is a distinctive feature of creeping faults that suggests a logarithmic dependency of fault friction on slip rate, as observed in laboratory friction experiments. Most faults can be considered to be paved with interlaced patches where the friction law is either rate-strengthening, inhibiting seismic rupture propagation, or rate-weakening, allowing for earthquake nucleation. The rate-weakening patches act as asperities on which stress builds up in the interseismic period; they might rupture collectively in a variety of ways. The pattern of interseismic coupling can help constrain the return period of the maximum- magnitude earthquake based on the requirement that seismic and aseismic slip sum to match long-term slip. Dynamic models of the seismic cycle based on this conceptual model can be tuned to reproduce geodetic and seismological observations. The promise and pitfalls of using such models to assess seismic hazard are discussed.

Fractal particle size distribution of pulverized fault rocks as a function of distance from the fault core

AbstractThe size distributions of particle in pulverized rocks from the San Andreas fault and the Arima‐Takatsuki Tectonic Line were measured. The rocks are characterized by the development of opening mode fractures with an apparent lack of shear. Fragments in the rocks in both fault zones show a fractal size distribution down to the micron scale. Fractal dimensions, dependent on mineral type, decrease from 2.92 to 1.97 with increasing distance normal to the fault core. The fractal dimensions of the rocks are higher than those of both natural and experimentally created fault gouges measured in previous studies. Moreover, the dimensions are higher than the theoretically estimated upper fractal limit under confined comminution. Dimensions close to 3.0 have been reported in impact loading experiments. The observed characteristics indicate that pulverization is likely to have occurred by a dynamic stress pulse with instantaneous volumetric expansion, possibly during seismic rupture propagation similar to impact loading.

Sampling the uncertainty associated with segmented normal fault interpretation using a stochastic downscaling method

A large-scale normal fault may be composed of several overlapping fault segments separated by relay zones at a finer scale. Fault segmentation may be critical to the understanding and the forecast of physical phenomena at the reservoir or basin scale (e.g. fluid flow, seismic rupture propagation). In this paper, we propose an automatic and stochastic method to sub-divide (downscale) below the data resolution a segmented normal fault into en-echelon segments that may be linked by connecting faults, based on variations in the orientation of the fault. The downscaling algorithm is composed of three main steps. (1) The first step involves detecting the segments using geometrical criteria. (2) Then the overlapping segments are modeled using isolated fault descriptions and statistics. (3) Lastly, the maturity of the simulated relay zones is evaluated based on relay geometry. If a given maturity threshold is reached, the relay ramp is breached. These three downscaling steps depend on seven parameters that can be defined as constant or randomly chosen from probability distributions to sample uncertainties. The method was applied to a large normal fault that laterally limits a hydrocarbon reservoir and is poorly imaged from seismic data. This resulted in one hundred possible 3D fault array models. The analysis of the emerging parameters shows a strong variability of the number and length of segments, and of the overlap and spacing between segments.

Structure of pseudotachylyte vein systems as a key to co-seismic rupture dynamics: the case of Gavilgarh–Tan Shear Zone, central India

The secondary fractures associated with a major pseudotachylyte-bearing fault vein in the sheared aplitic granitoid of the Proterozoic Gavilgarh–Tan Shear Zone in central India are mapped at the outcrop scale. The fracture maps help to identify at least three different types of co-seismic ruptures, e.g., X–X′, T1 and T2, which characterize sinistral-sense shearing of rocks, confined between two sinistral strike-slip faults slipping at seismic rate. From the asymmetric distribution of tensile fractures around the sinistral-sense fault vein, the direction of seismic rupture propagation is predicted to have occurred from west-southwest to east-northeast, during an ancient (Ordovician?) earthquake. Calculations of approximate co-seismic displacement on the faults and seismic moment (M 0) of the earthquake are attempted, following the methods proposed by earlier workers. These estimates broadly agree to the findings from other studied fault zones (e.g., Gole Larghe Fault zone, Italian Alps). This study supports the proposition by some researchers that important seismological information can be extracted from tectonic pseudotachylytes of all ages, provided they are not reworked by subsequent tectonic activity.

Seismic rupture propagation to trench axis, Nankai area

Seismic rupture propagation to trench axis, Nankai area