Fault Zone Materials Research Articles

Mechanical Energy Dissipation During Seismic Dynamic Weakening in Calcite‐Bearing Faults

AbstractEarthquakes are frictional instabilities caused by the shear stress decrease, that is, dynamic weakening, of faults with slip and slip rate. During dynamic weakening, shear stress depends on slip, slip rate, and temperature, according to constitutive laws governing the earthquake rupture process. In the laboratory, technical limitations in measuring temperature during frictional instabilities inhibit the investigation and interpretation of shear stress evolution. Here we conduct high velocity friction experiments on calcite‐bearing simulated faults, both on bare‐rock and on gouge samples, at 20–30 MPa normal stress, 1–6 m/s slip rate and 1–20 m total slip. Seismic slip pulses are reproduced by imposing boxcar and regularized Yoffe slip rate functions. We measured, together with shear stress, slip, and slip rate, the temperature evolution on the fault by employing an innovative two‐color fiber optic pyrometer. The comparison between modeled and measured temperature reveals that for calcite‐bearing faults the heat sink caused by decarbonation reaction controls the temperature evolution. In bare‐rocks, energy is dissipated as frictional heat, and temperature increase is buffered by the heat sink of the calcite decarbonation reaction. In gouges, energy is dissipated as frictional heat and for plastic deformation processes, balanced by the heat sink caused by the decarbonation reaction enhanced by the mechanochemical effect. Our results suggest that in calcite‐bearing rocks, a common fault zone material for earthquake sources in the continental crust at shallow depth, the type of fault materials (bare‐rocks vs. gouges) controls the energy dissipation during seismic slip.

New Zealand Fault-Rupture Depth Model v.1.0: A Provisional Estimate of the Maximum Depth of Seismic Rupture on New Zealand’s Active Faults

ABSTRACT We summarize estimates of the maximum rupture depth on New Zealand’s active faults (“New Zealand Fault-Rupture Depth Model v.1.0”), as used in the New Zealand Community Fault Model v1.0 and as a constraint for the latest revision of the New Zealand National Seismic Hazard Model (NZ NSHM 2022). Rupture depth estimates are based on a combination of two separate model approaches (using different methods and datasets). The first approach uses regional seismicity distribution from a relocated earthquake catalog to calculate the 90% seismicity cutoff depth (D90), representing the seismogenic depth limit. This is multiplied by an overshoot factor representing the dynamic propagation of rupture into the conditional stability zone, and accounting for the difference between regional seismicity depths and the frictional properties of a mature fault zone to arrive at a seismic estimate of the maximum rupture depth. The second approach uses surface heat flow and rock type to compute depths that correspond to the thermal limits of frictional instabilities on seismogenic faults. To arrive at a thermally-based maximum rupture depth, these thermal limits are also multiplied by an overshoot factor. Both the models have depth cutoffs at the Moho and/or subducting slabs. Results indicate the maximum rupture depths between 8 (Taupō volcanic zone) and >30 km (e.g., southwest North Island), strongly correlated with regional thermal gradients. The depths derived from the two methods show broad agreement for most of the North Island and some differences in the South Island. A combined model using weighting based on relative uncertainties is derived and validated using constraints from hypocenter and slip model depths from recent well-instrumented earthquakes. We discuss modifications to the maximum rupture depths estimated here that were undertaken for application within the NZ NSHM 2022. Our research demonstrates the utility of combining seismicity cutoff and thermal stability estimates to assess the down-dip dimensions of future earthquake ruptures.

The 2015–2017 Pamir earthquake sequence: foreshocks, main shocks and aftershocks, seismotectonics, fault interaction and fluid processes

SUMMARYA sequence of three strong (MW7.2, 6.4, 6.6) earthquakes struck the Pamir of Central Asia in 2015–2017. With a local seismic network, we recorded the succession of the foreshock, main shock and aftershock sequences at local distances with good azimuthal coverage. We located 11 784 seismic events and determined 33 earthquake moment tensors. The seismicity delineates the tectonic structures of the Pamir in unprecedented detail, that is the thrusts that absorb shortening along the Pamir’s thrust front, and the strike-slip and normal faults that dissect the Pamir Plateau into a westward extruding block and a northward advancing block. Ruptures on the kinematically dissimilar faults were activated subsequently from the initial MW 7.2 Sarez event at times and distances that follow a diffusion equation. All main shock areas but the initial one exhibited foreshock activity, which was not modulated by the occurrence of the earlier earthquakes. Modelling of the static Coulomb stress changes indicates that aftershock triggering occurred over distances of ≤90 km on favourably oriented faults. The third event in the sequence, the MW 6.6 Muji earthquake, ruptured despite its repeated stabilization through stress transfer in the order of –10 kPa. To explain the accumulation of MW > 6 earthquakes, we reason that the initial main shock may have increased nearby fault permeability, and facilitated fluid migration into the mature fault zones, eventually triggering the later large earthquakes.

Stochastic quantification of CO2 fault sealing capacity in sand-shale sequences

Structural fault trapping of buoyant fluids, such as hydrocarbons and CO2, relies on fault sealing capacity. The traditional approach to quantifying the sealing capacity of fault zone materials is Shale Gouge Ratio (SGR), which implicitly homogenizes fault properties. However, a large range of fault throw and the presence of fault heterogeneity can lead to different trapping capacities for the same structure. This paper presents a stochastic study to statistically determine the possible range of CO2 column height at a normal fault in sand-shale sequences. We present the procedures of two stochastic models including the continuous shale gouge model (CSGM) and the discrete smear model (DSM). The models are applied in an example case for the High Island field in the Gulf of Mexico (GOM) basin combining borehole geophysical data and measurements from laboratory experiments. Both one-dimensional and two-dimensional cases are discussed. The results show that more than 50% of the realizations predict negligible fault sealing capacity. The CSGM method shows that CO2 column height is overall proportional to SGR and breakthrough pressure. The DSM model suggests that clay smear continuity may significantly affect fault sealing capacity. Large fault throws (>100 m) result in smear breaches and therefore in highly variable maximum CO2 column heights, varying from 0 m to a maximum of ∼95 m. Both the DSM method and the CSGM method proposed in this paper permit explaining geologic uncertainties and the resulting variability of column heights observed in the field. Our results together with the field data from the literature demonstrate the utility of combining both methods to help improve column height prediction. Uncertainty quantification of clay ductility, smear location, and fault heterogeneity is important for determining fault sealing capacity and improving reservoir risk management associated with carbon dioxide geological storage.

Fracturing and pore-fluid distribution in the Marlborough region, New Zealand from body-wave tomography: Implications for regional understanding of the Kaikōura area

Relative to more mature fault zones, immature fault zones that have accumulated smaller total displacement are characterized by less efficient strain localization and more complicated earthquake ruptures. How differences in maturation are reflected in regional-scale upper-crustal fracturing is not well known. Recently, complicated earthquake ruptures associated with immature fault zones, such as the 2016 Kaikōura earthquake in New Zealand, have occurred in areas that are in regional proximity (<100 km away) to more mature faults. Here we examine whether inefficient strain localization in less mature fault zones is associated with a broader distribution and anomalously elevated concentration of fractures over distances of tens of kilometers. We use regional seismic arrival-time tomography in a broad area around the Kaikōura earthquake to investigate lateral variations in Vp and Vp/Vs. Focusing on the extensively faulted but compositionally uniform Torlesse-Pahau terrane in the Marlborough region where the earthquake occurred, we attribute lateral variations in Vp and Vp/Vs to differences in concentration of fluid-filled fractures. Using numerical models relating seismic velocities and fracturing, we solve for the lateral variation in concentration of ∼0.01 aspect ratio fluid-filled fractures. We find that areas near the Kaikōura rupture have >3% elevated fracture porosity compared to the adjacent area to the north. The elevated regional fracturing in the Kaikōura area is interpreted to result from more distributed deformation, and broader distribution of earthquakes, due to inefficient localization of strain from a regionally uniform strain rate field, highlighting the relationship between relative maturity of upper-crustal fault zones and lateral variability of regional upper-crustal properties.Plain language summary:When earthquakes occur in immature fault zones (areas that have accumulated small total displacement), they tend to rupture multiple poorly developed faults of diverse orientations. In contrast, more mature fault zones are associated with more developed, smoother faults and more localized earthquake activity. How these differences in maturation are reflected in the regional distribution of fractures is not well known. Here we use the arrival times of P and S waves from earthquakes to constrain seismic velocities (Vp and Vp/Vs) and use numerical models to relate seismic velocities to fracture concentration. We focus on the Marlborough region of the South Island, New Zealand, where immature fault zones recently ruptured during the 2016 Kaikōura earthquake but which also has more mature fault zones <100 km away. We find elevated fracture concentrations (3% higher fracture porosity), indicative of more distributed deformation along immature fault zones relative to more mature fault zones. In immature settings, fracturing is not as effectively localized along individual fault traces, leading to a broadly distributed fracture network.

Fault shape effect on SH waves using finite element method

In this paper, we analyse how the combination of fault zone shape and material properties affects the propagation of seismic waves in a two-dimensional domain. We focus on SH wave propagation through several faults with different thicknesses and bending radii, but the theory is easily generalized to the three-dimensional case. We show how the density of energy released is mostly a function of the radius and does not depend on the velocity inside a fault zone.

Numerical study of fluid injection-induced deformation and seismicity in a mature fault zone with a low-permeability fault core bounded by a densely fractured damage zone

We apply a fully-coupled hydro-mechanical simulation tool to study fluid injection-induced pressure diffusion, poroelastic response, fracture slip, damage growth, and microseismicity occurrence in a complex fault zone. The fault zone has a low-permeability fault core bounded by a high-permeability damage zone, which is characterised by a fracture network with the fracture density exponentially decaying away from the fault core. We employ the discrete fracture network approach to explicitly represent the distribution and behaviour of these fractures in the fault zone. We use the finite element method to solve the coupled governing equations of fluid flow, solid deformation, and damage evolution in the fractured porous rock. We explore a number of scenarios with different orientations for the in-situ stress field and different locations for a constant rate fluid injection. We report an interactive control of fracture density, fracture orientations, in-situ stress state, and injection point position on the seismo-hydro-mechanical behaviour of the fault zone subject to fluid injection. Insight from our research findings may have important implications for injection-related geoengineering activities such as the development of enhanced geothermal systems and unconventional hydrocarbon reservoirs.

In situ LA-ICPMS U[sbnd]Pb dating and geochemical characterization of fault-zone calcite in the central Tarim Basin, northwest China: Implications for fluid circulation and fault reactivation

Direct evidence for fault reactivation is crucial for understanding long-term fault behavior and reconstruction of fluid circulation and hydrocarbon migration and accumulation during active tectonics. However, absolute dating of fault reactivations and fluid circulation is still challenging due to dissolution and recrystallization of fault zone materials and obliteration of previous events by the latest deformation/hydrothermal event. The lower Ordovician limestone strata at 3.8 km depth in the Central Uplift of the Tarim Basin, northwest China experienced multi-stage regional tectonic activities that developed a series of NW–SE striking reverse faults and NE–SW oriented strike-slip faults. In this study, we performed in situ laser-ablation ICP-MS UPb dating and trace element characterization of calcites collected from borehole cores within one of the main fault zones in the Central Uplift of the Tarim Basin. A new in-house calcite standard AHX-1A (209.8 ± 1.3 Ma), collected from the Tarim Basin, calibrated against well-calibrated WC-1 and ASH-15 calcite standards, is used for mass-bias correction. We obtained in situ UPb ages of 9 calcite samples from two wells within a Paleozoic fault zone in the Tarim Basin. They are 456 ± 11 Ma, 454.7 ± 7.2 Ma, 450.4 ± 6.2 Ma, 435.2 ± 9.7 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma from well TZ2, and 371 ± 18 Ma, 390.6 ± 6.0 Ma and 395 ± 14 Ma from well TZ4 (all reported uncertainties are 2σ). Combined with petrographic evidence, these results define at least six generations of secondary calcite precipitations in the fault zone, that occurred during 456 ± 11 to 450.4 ± 6.2 Ma (weighted mean 452.8 ± 4.3 Ma), 435.2 ± 9.7 Ma, 395 ± 14 to 390.6 ± 6.0 Ma (weighted mean 391.3 ± 5.5 Ma), 371 ± 18 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma. Chondrite-normalized rare earth element (REE) + yttrium (Y) patterns of these calcite samples show prominent positive Gd and Y anomalies, lack of negative Ce anomaly, and enrichment in light REE but depletion in heavy REE, similar to the shallow marine platform host-rock limestone. The results suggest multi-stage fluid circulation under shallow burial, low temperature, and non-oxidizing environments along the fault zone, with the secondary calcite precipitating from a similar fluid source that originated from dissolution of host-rock carbonate or early-stage diagenesis. The detail petrographical and mineralogical analysis of this study, combined with the new interpretation of high-resolution seismic data, constrains two main episodes of faulting and related uplift during 456–435 Ma and 395–371 Ma, as well as a younger faulting episode with weaker intensity during 328–308 Ma. This study demonstrates that the central fault zone of the Tarim Basin experienced ~150 Ma of episodical reactivation during the Paleozoic, which has important implications for hydrocarbon exploration.

Structural investigation of background features and normal faults affecting the Calcari con Selce formation, Southern Apennines, Italy

The Calcari con Selce formation (CSf) is a low-porosity multilayer carbonate, partially dolomitized, formation that extensively crops out in the Lucania Apennine chain. This lithostratigraphic succession represents to depth a key formation concerning hydrocarbon exploration and a potential confined acquifer system. In this work we describe the structural features affecting the CSf related to burial and tectonic deformation with emphasis on normal faults related to post-orogenic extensional deformation. A field investigation led to recognition of background structures such as: i) strata-bounded sub-vertical fracture sets (OBj) and bedding-parallel solution seams (BPss), related to lithostatic load; and: ii) oblique-to-bedding solution seams (BOss) induced by syn-orogenic flexural slip. Furthermore, different sets of dolomitic veins extensively affect dolomitized parts of the CSf showing structural relationships with bedding interfaces. Localized low-angle normal faults (LANFs) post-dating the previous structures formed further anisotropies locally associated with sub- vertical fracture sets (SVfs). These structures, enhancing permeability within carbonate beds, promoted fluid compartmentalization. High- angle normal faults and associate fracture sets, characterized by well-connected features, mainly localized along the mature fault zones, further enhanced the permeability of the CSf allowing the development of preferential fluid-flow pathways moving parallel to the fault-zones, as inferred from the structure of calcite veins. This work provides an enhanced characterization of the fracture network affecting the CSf and represents a useful tool aimed at improving hydrological models for fluid circulation within fractured reservoirs.

Probing Fault Frictional Properties During Afterslip Updip and Downdip of the 2017 Mw 7.3 Sarpol‐e Zahab Earthquake With Space Geodesy

AbstractWe use interferometric synthetic aperture radar (InSAR) data collected by the Sentinel‐1 mission to study the coseismic and postseismic deformation due to the 2017 Mw 7.3 Sarpol‐e Zahab earthquake that occurred near the Iran‐Iraq border in Northwest Zagros. We find that most of the coseismic moment release is between 15‐ and 21‐km depths, well beneath the boundary between the sedimentary cover and underlying basement. Data from four satellite tracks reveal robust postseismic deformation during ~12 months after the mainshock (from November 2017 to December 2018). Kinematic inversions show that the observed postseismic InSAR line‐of‐sight (LOS) displacements are well explained by oblique (thrust + dextral) afterslip both updip and downdip of the coseismic peak slip area. The dip angle of the shallow afterslip fault plane is found to be significantly smaller than that of the coseismic rupture, corresponding to a shallowly dipping detachment located near the base of the sediments or within the basement, depending on the thickness of the sedimentary cover, which is not well constrained over the epicentral area. Aftershocks during the same time period exhibit a similar temporal evolution as the InSAR time series, with most of aftershocks being located within and around the area of maximum surface deformation. The postseismic deformation data are consistent with stress‐driven afterslip models, assuming that the afterslip evolution is governed by rate‐strengthening friction. The inferred frictional properties updip and downdip of the coseismic rupture are significantly different, which likely reflect differences in fault zone material at different depths along the Zagros.

Effects of Low‐Velocity Fault Damage Zones on Long‐Term Earthquake Behaviors on Mature Strike‐Slip Faults

AbstractMature strike‐slip faults are usually surrounded by a narrow zone of damaged rocks characterized by low seismic wave velocities. Observations of earthquakes along such faults indicate that seismicity is highly concentrated within this fault damage zone. However, the long‐term influence of the fault damage zone on complete earthquake cycles, that is, years to centuries, is not well understood. We simulate aseismic slip and dynamic earthquake rupture on a vertical strike‐slip fault surrounded by a fault damage zone for a thousand‐year timescale using fault zone material properties and geometries motivated by observations along major strike‐slip faults. The fault damage zone is approximated asan elastic layer with lower shear wave velocity than the surrounding rock. We find that dynamic wave reflections, whose characteristics are strongly dependent on the width and the rigidity contrast of the fault damage zone, have a prominent effect on the stressing history of the fault. The presence of elastic damage can partially explain the variability in the earthquake sizes and hypocenter locations along a single fault, which vary with fault damage zone depth, width and rigidity contrast from the host rock. The depth extent of the fault damage zone has a pronounced effect on the earthquake hypocenter locations, and shallower fault damage zones favor shallower hypocenters with a bimodal distribution of seismicity along depth. Our findings also suggest significant effects on the hypocenter distribution when the fault damage zone penetrates to the nucleation sites of earthquakes, likely being influenced by both lithological (material) and rheological (frictional) boundaries.

The Age and Origin of Miocene‐Pliocene Fault Reactivations in the Upper Plate of an Incipient Subduction Zone, Puysegur Margin, New Zealand

AbstractStructural observations and 40Ar/39Ar geochronology on pseudotachylyte, mylonite, and other fault zone materials from Fiordland, New Zealand, reveal a multistage history of fault reactivation and uplift above an incipient ocean‐continent subduction zone. The integrated data allow us to distinguish true fault reactivations from cases where different styles of brittle and ductile deformation happen together. Five stages of faulting record the initiation and evolution of subduction at the Puysegur Trench. Stage 1 normal faults (40–25 Ma) formed during continental rifting prior to subduction. These faults were reactivated as dextral strike‐slip shear zones when subduction began at ~25 Ma. The dextral shear zones formed part of a transpressional regime (Stage 2, 25–10 Ma) that included minor reverse motion and modest uplift above the leading edge of the subducting slab as it propagated below Fiordland. At 8–7 Ma (Stage 3), trench‐parallel faults accommodated the first and only episode of pure reverse motion. Reconstructions confirm that these faults formed when the slab reached mantle depths and collided with previously subducted crust. At 5–4 Ma (Stage 4), trench‐parallel faults accommodated oblique‐reverse motion when an oceanic ridge collided obliquely with the Puysegur Trench. Stages 3 and 4 both accelerated rock uplift and topographic growth in short pulses. A return to strike‐slip motion occurred after ~4 Ma (Stage 5) when deformation localized onto the Alpine Fault. These results highlight how the rock record of faulting links displacements occurring at Earth's surface to events occurring both at the trench and deep within the lithosphere during subduction.

Carbonaceous Materials in the Longmenshan Fault Belt Zone: 3. Records of Seismic Slip from the Trench and Implications for Faulting Mechanisms

In recent studies on the recognition of graphitized gouges within the principal slip zone (PSZ) of the Longmenshan fault in China, we proposed that the presence of graphite might be evidence of fault slip. Here, we characterized the clay- and carbonaceous-rich gouges of the active fault zone of the Longmenshan fault belt using samples collected from the trench at Jiulong, which was deformed during the 2008 MW-7.9 Wenchuan earthquake, to determine if graphite is present and study both the processes influencing fault behavior and the associated faulting mechanism. Mineralogical and geochemical analyses of the Jiulong trench sample show the presence of a hydrothermal mineral (i.e., dickite) integrated with dramatic relative chemical enrichment and relative depletion within a yellowish zone, suggesting the presence of vigorous high-temperature fluid–rock interactions, which are likely the fingerprint of thermal pressurization. This is further supported by the absence of carbonaceous materials (CMs) given the spectrometric data obtained. Interestingly, the Raman parameters measured near the carbonaceous-rich gouge fall within the recognized range of graphitization in the mature fault zone, implying the origin of a mature fault, as shown in the companion paper. According to both the sharp boundary within the very recent coseismic rupture zone of the 2008 MW-7.9 Wenchuan earthquake and the presence of kinetically unstable dickite, it is strongly implied that the yellow/altered gouge likely formed from a recent coseismic event as aconsequence of hydrothermal fluid penetration. We further surmise that the CM characteristics varied according to several driving reactions, e.g., transient hydrothermal heating versus long-term geological metamorphism and sedimentation.

Numerical Analysis of Strain Localization in Rocks with Thermo-hydro-mechanical Couplings Using Cosserat Continuum

A numerical model for thermo-hydro-mechanical strong couplings in an elasto-plastic Cosserat continuum is developed to explore the influence of frictional heating and thermal pore fluid pressurization on the strain localization phenomenon. This model allows specifically to study the complete stress–strain response of a rock specimen, as well as the size of the strain localization zone for a rock taking into account its microstructure. The numerical implementation in a finite element code is presented, matching adequately analytical solutions or results from other simulations found in the literature. Two different applications of the numerical model are also presented to highlight its capabilities. The first one is a biaxial test on a saturated weak sandstone, for which the influence on the stress–strain response of the characteristic size of the microstructure and of thermal pressurization is investigated. The second one is the rapid shearing of a mature fault zone in the brittle part of the lithosphere. In this example, the evolution of the thickness of the localized zone and the influence of the permeability change on the stress–strain response are studied.

Heating, weakening and shear localization in earthquake rupture.

Field and borehole observations of active earthquake fault zones show that shear is often localized to principal deforming zones of order 0.1-10 mm width. This paper addresses how frictional heating in rapid slip weakens faults dramatically, relative to their static frictional strength, and promotes such intense localization. Pronounced weakening occurs even on dry rock-on-rock surfaces, due to flash heating effects, at slip rates above approximately 0.1 m s-1 (earthquake slip rates are typically of the order of 1 m s-1). But weakening in rapid shear is also predicted theoretically in thick fault gouge in the presence of fluids (whether native ground fluids or volatiles such as H2O or CO2 released by thermal decomposition reactions), and the predicted localizations are compatible with such narrow shear zones as have been observed. The underlying concepts show how fault zone materials with high static friction coefficients, approximately 0.6-0.8, can undergo strongly localized shear at effective dynamic friction coefficients of the order of 0.1, thus fitting observational constraints, e.g. of earthquakes producing negligible surface heat outflow and, for shallow events, only rarely creating extensive melt. The results to be summarized include those of collaborative research published with Nicolas Brantut (University College London), Eric Dunham (Stanford University), Nadia Lapusta (Caltech), Hiroyuki Noda (JAMSTEC, Japan), John D. Platt (Carnegie Institution for Science, now at *gramLabs), Alan Rempel (Oregon State University) and John W. Rudnicki (Northwestern University).This article is part of the themed issue 'Faulting, friction and weakening: from slow to fast motion'.

Slip of a one-body dynamical spring-slider model in the presence of slip-weakening friction and viscosity

&lt;span style="font-family: 新細明體;"&gt;This study is focused on analytic study at small displacements and numerical simulations of slip of a one-body dynamical slider-slider model in the presence of slip-weakening friction and viscosity. Analytic results with numerical computations show that the displacement of the slider is controlled by the decreasing rate, [gamma], of friction force with slip and viscosity, [eta], of fault-zone material. The natural period of the system with slip-weakening friction and viscosity is longer than that of the system without the two factors. There is a solution regime for &lt;span style="font-family: 新細明體;"&gt;[eta]&lt;/span&gt; and &lt;span style="font-family: 新細明體;"&gt;[gamma]&lt;/span&gt; to make the slider slip steadily without strong attenuation. The viscous effect is stronger than the frictional effect. Meanwhile, a change of &lt;span style="font-family: 新細明體;"&gt;[eta]&lt;/span&gt; results in a larger effect on the slip of the slider than a change of &lt;span style="font-family: 新細明體;"&gt;[gamma]&lt;/span&gt;. Numerical simulations are made for a one-body dynamical slider-slider model in the presence of three slip-weakening friction laws, i.e., the thermal-pressurization (TP) friction law, the softening-hardening (SH) friction law, and a simple slip-weakening (SW) friction law, and viscosity. Results show that slip-weakening friction and viscosity remarkably affect slip of the slider. The TP and SW friction laws cause very similar results. The results caused by the SH friction law are quite different from those by the other two. For the cases in study, the fixed points are not an attractor.&lt;/span&gt;

The Longriqu fault zone, eastern Tibetan Plateau: Segmentation and Holocene behavior

AbstractThe dextral Longriba fault system (LFS), ~300 km long and constituting of two fault zones, has recently been recognized as an important structure of the eastern Tibetan plateau (Sichuan province), as it accommodates a significant amount of the deformation induced by the ongoing Indo‐Asian collision. Although previous paleoseismological investigations highlighted its high seismogenic potential, no systematic quantification of the dextral displacements along the fault system has been undertaken so far. As such information is essential to appraise fault behavior, we propose here a first detailed analysis of the segmentation of the Longriqu fault, the northern fault zone of the LFS, and an offset inventory of morphological features along the fault, using high‐resolution Pleiades satellite images. We identify six major segments forming a mature fault zone. Offsets inventory suggests a characteristic coseismic displacement of ~4 m. Two alluvial fans, with minimum ages of 6.7 and 13.2 ka, respectively displaced by 23 ± 7 m and 40 ± 5 m, give an estimate of the maximal horizontal slip rate on the Longriqu fault of 3.2 ± 1.1 mm yr−1. As a result, a minimum ~1340 year time interval between earthquakes is expected.

Scaling in natural and laboratory earthquakes

AbstractLaboratory experiments reproducing seismic slip conditions show extreme frictional weakening due to the activation of lubrication processes. Due to a substantial variability in the details of the weakening transient, generalization of experimental results and comparison to seismic observations have not been possible so far. Here we show that during the weakening, shear stress τ is generally well matched by a power law of slip u in the form (with 0.35 &lt; α &lt; 0.6). The resulting fracture energy Gf can be approximated by a power law in some aspects in agreement with the seismological estimates . It appears that Gf and are comparable in the range 0.01 &lt; u &lt; 0.3 m. However, surpasses Gf at larger slips: at u≈10 m, and Gf≈106. Possible interpretations of this misfit involve the complexity of damage and weakening mechanisms within mature fault zone structures.

Seismic modelling of fault zones

While faults have long been known as primary pathways for fluid migration in sedimentary basins, recent work highlights the importance of fault zone internal architecture, lateral variation, transmissivity, and impact on migration and trapping. The impacts of fault zone architecture and properties on seismic images are investigated to facilitate accurately mapped fault zones, and to predict subseismic flow properties and sealing potential. A wedge-type fault model with a main fault and a synthetic fault displacing a typical North West Shelf siliciclastic succession is used to replicate the geometrical components of a seismic-scale fault. Elastic properties are derived from rock physics models, which are used in a 2D elastic modelling algorithm to produce realistic marine seismic acquisition geometry. These data were subsequently input into a 2D prestack (one-way wave-equation) migration code to produce an interpretable seismic image. Base-case elastic properties are systematically varied; modelling focuses on gouge properties, fractured fault zone material, the sandstone Vp/Vs relationship, and shale-sand velocity contrast. The workflow from geological model building to elastic property substitution and forward seismic modelling is extremely quick and versatile, allowing testing of a wide range of scenarios. So far this approach has yielded valuable insights into internal fault property prediction and interpretation of the fault zone in traditional post-stack seismic datasets. Implications for processing workflow and attenuation of fault shadows are also expected.

In situ observations of velocity changes in response to tidal deformation from analysis of the high‐frequency ambient wavefield

AbstractWe report systematic seismic velocity variations in response to tidal deformation. Measurements are made on correlation functions of the ambient seismic wavefield at 2–8 Hz recorded by a dense array at the site of the Piñon Flat Observatory, Southern California. The key observation is the dependence of the response on the component of wave motion and coda lapse time τ. Measurements on the vertical correlation component indicate reduced wave speeds during periods of volumetric compression, whereas data from horizontal components show the opposite behavior, compatible with previous observations. These effects are amplified by the directional sensitivities of the different surface wave types constituting the early coda of vertical and horizontal correlation components to the anisotropic behavior of the compliant layer. The decrease of the velocity (volumetric) strain sensitivity Sθ with τ indicates that this response is constrained to shallow depths. The observed velocity dependence on strain implies nonlinear behavior, but conclusions regarding elasticity are more ambiguous. The anisotropic response is possibly associated with inelastic dilatancy of the unconsolidated, low‐velocity material above the granitic basement. However, equal polarity of vertical component velocity changes and deformation in the vertical direction indicate that a nonlinear Poisson effect is similarly compatible with the observed response pattern. Peak relative velocity changes at small τ are 0.03%, which translates into an absolute velocity strain sensitivity of Sθ≈5 × 103 and a stress sensitivity of 0.5 MPa−1. The potentially evolving velocity strain sensitivity of crustal and fault zone materials can be studied with the method introduced here.