Seismogenic Depths Research Articles

Magnitudes of future large earthquakes near Istanbul quantified from 1500 years of historical earthquakes, present-day microseismicity and GPS slip rates

In the past 1500 years, seven sequences of significant earthquakes have occurred on a 160-km-long segment of the North Anatolian Fault zone (NAFZ) centered on Istanbul. The most recent of these sequences will be complete with the future occurrence of one or more Mw ≥ 7.0 earthquakes near the city. From a summation of the seismic moment released by historical earthquakes between 500 and 2000 CE we calculate a mean slip deficit rate of 9.9 mm/year. Our moment summation procedure sums the seismic moment associated with known historical earthquakes in the past 1500 years based on rupture length and assumed seismogenic depth. We observed that in the 1999 Izmit earthquake, strongly coupled fault patches were associated with the highest co-seismic slip. In our calculated map of present-day slip-potential, we recognize several such patches from their low present-day rate of microseismicity, e.g. segments beneath the Kumburgaz and Eastern (Çınarcık) Marmara Basins. In contrast, intervening weakly coupled patches host high present-day background seismicity, e.g. the segment beneath the Western (Tekirdağ) Basin. This lends credibility to an interpretation of future earthquakes occurring in three seismogenic segments of the NAFZ corresponding to three deep basins in the Sea of Marmara. The present-day slip deficits reach up to 1.7 m beneath the Western (Tekirdağ Basin) segment, and 4.0 m and 5.4 m beneath the Central (Central High and Kumburgaz Basin) and Eastern (Çınarcık Basin) segments, respectively. These segments most recently ruptured in August 1766, May 1766 and October 1509 and currently have a potential to generate Mw 7.2, Mw 7.4 and Mw 7.5, earthquakes respectively. Although contiguous ruptures have not occurred historically, ruptures of contiguous segments could occur as a Mw 7.5 earthquake in the west, or a Mw 7.6 earthquake in the east or as a single through-going Mw 7.7 rupture.

Distributed deformation along the subduction plate interface: The role of tectonic mélanges

Recent geophysical monitoring of subduction zones has unraveled a complete spectrum of plate coupling behaviors, from coupled portions rupturing during earthquakes to decoupled portions slipping aseismically. However, the deformation mechanisms and the exhumed rock corresponding to these contrasted behaviors are not yet identified. Tectonic mélange zones are thought to play a major role in the deformation of the plate interface as they represent remnants of the subducted plate scraped off by the overriding plate. In this work we examine several tectonic mélange zones (Hyuga, Okitsu, Mugi) from the Shimanto Belt, an accretionary prism in southwest Japan connecting to the active Nankai subduction zone. These tectonic mélange zones have a block-in-matrix structure, with lenses of sandstones and basalts within a metapelitic matrix, and their deformation is distributed over zones of hundreds of meters in thickness. In addition, the examples of mélange considered here are bounded by sharp faults, some of them bearing pseudotachylyte layers, so that distributed deformation within the mélange and localized deformation on its boundary are juxtaposed. Distributed deformation involves the development of a foliation, as well as of a pervasive network of macroscopic and microscopic shear zones. Along with slip on this network, strain proceeds by fracturing and precipitation of quartz, in long veins parallel to the foliation or in smaller cracks perpendicular to stretching and forming in the neck of competent lenses of sandstones or former quartz veins. The analysis of shear band kinematics shows in all three examples a dominant, top-to-the-trench sense of shear, consistent with deformation along the plate boundary during subduction. Moreover, most shear zones, when foliation is restored back to syn-subduction position, are extensional structures. Finally, the geometry and kinematics of the mélange-bounding faults, as well as radiometric constraints, show that in most cases the faults (=localized structures) were formed during a later stage than mélange internal deformation. These findings bear several consequences on the structure and dynamics of the subduction plate boundary at seismogenic depths. First, there is no support for a model of plate boundary fault zone composed simultaneously of localized slip zones and domains of more distributed deformation. Second, rather than proposed models of underplating, where all deformation is localized into the thrusts bounding the tectonic sheets, we suggest that underplating was to a large extent accommodated by distributed deformation within the mélange sheets. This underplating model accounts for (i) the large amount of strain within the mélange, (ii) the absence of contractional structures during underplating, (iii) thinning of the mélange required by the network of extensional shear bands and stretched boudins. Third, mélanges appear as likely candidates for portions of the plate interface deforming by aseismic slip. The seismic vs. aseismic character of the plate interface might depend on the ability of sediments on top of the subducting plate to undergo distributed strain, which in turn depends on the efficiency of pressure solution to operate.

Relation between crustal stress and thickness of earth's crust

Available models for estimation of crustal stress consider the vertical stress component as weight of above laying rocks, while horizontal stress components are estimated as function of vertical component. Herein, it is assumed that horizontal stress is effect of strain formed at seismogenic depth and transferred to shallower depth of rock mass. Strain rate is different depending on thickness of the crust and is different in different locations. Using available data regarding the measured maximum horizontal stress and depth of Moho discontinuity trend between these is obtained. Trend shows that maximum horizontal stress is decreasing with the thickness of the Earth’s crust and vice versa. Finally, expression that defines maximum horizontal stress from Moho depth and deformation modulus of rock mass is provided.

Significant Effects of Shallow Seismic and Stress Properties on Phase Velocities of Rayleigh Waves Up to 20 s

Seismic data are increasingly used to monitor subsurface velocity changes associated with tectonic and environmental processes that occur in different depth sections. To clarify the differences between effects associated with shallow and deep changes of properties, we conduct numerical experiments using simple layered models that include low velocities, low attenuation coefficients and stress-sensitivity of cracked rocks in the shallow crust. We find significant phase-velocity drops in the period range of 5–20 s when large structural changes occur in the top 1–3 km. The apparent velocity changes (δv/v) measured from the first part of the synthetic Rayleigh waves with a cross-correlation based technique show significant velocity drops in the period bands of 5–10 s and 10–20 s that are consistent with reported values of changes at seismogenic depth. The results highlight the importance of accounting for low velocities, attenuation coefficients and stress-sensitivity of parameters in the top 1–3 km in studies aiming to determine the source region of temporal changes. Analyses using different frequency ranges and calculations of apparent delay times over multiple period bands are essential for resolving the depth range of temporal changes of properties. For temporal changes occurring at seismogenic depths, the measured δv/v values at 5–10 s are significantly larger than those of 10–20 s, which are not observed for shallow changes.

Frictional Heating Processes and Energy Budget During Laboratory Earthquakes

AbstractDuring an earthquake, part of the released elastic strain energy is dissipated within the slip zone by frictional and fracturing processes, the rest being radiated away via elastic waves. While frictional heating plays a key role in the energy budget of earthquakes, it could not be resolved by seismological data up to now. Here we investigate the dynamics of laboratory earthquakes by measuring frictional heat dissipated during the propagation of shear instabilities at stress conditions typical of seismogenic depths. We estimate the complete energy budget of earthquake rupture and demonstrate that the radiation efficiency increases with thermal‐frictional weakening. Using carbon properties and Raman spectroscopy, we map spatial heat heterogeneities on the fault surface. We show that an increase in fault strength corresponds to a transition from a weak fault with multiple strong asperities and little overall radiation, to a highly radiative fault behaving as a single strong asperity.

Tomography of Southern California Via Bayesian Joint Inversion of Rayleigh Wave Ellipticity and Phase Velocity From Ambient Noise Cross‐Correlations

AbstractA self‐consistent regional‐scale seismic velocity model with resolution from seismogenic depth to the surface is crucial for seismic hazard assessment. Though Southern California is the most seismically imaged region in the world, techniques with high near‐surface sensitivity have been applied only in disparate local areas and have not been incorporated into a unified model with deeper resolution. In the present work, we obtain isotropic values for Rayleigh wave phase velocity and ellipticity in Southern California by cross‐correlating daily time series from the year 2015 across 315 regional stations in period ranges 6 to 18 s. Leveraging the complementary sensitivity of the two Rayleigh wave data sets, we combine H/V and phase velocity measurements to determine a new 3‐D shear velocity model in a Bayesian joint inversion framework. The new model has greatly improved shallow resolution compared to the Southern California Earthquake Center CVMS4.26 reference model. Well‐known large‐scale features common to previous studies are resolved, including velocity contrasts across the San Andreas, San Jacinto, Garlock, and Elsinore faults, midcrustal high‐velocity structure beneath the Mojave Desert, and shallow Moho beneath the Salton Trough. Other prominent features that have previously only been imaged in focused local studies include the correct sedimentary thickness of the southern Central Valley, fold structure of the Ventura and Oak Ridge Anticlines, and velocity contrast across the Newport‐Inglewood fault. The new shallow structure will greatly impact simulation‐based studies of seismic hazard, especially in the near‐surface low‐velocity zones beneath densely populated areas like the Los Angeles, San Bernardino, and Ventura Basins.

Continuous Measurement of Stress‐Induced Travel‐Time Variations at SAFOD

Author(s): Yang, C; Niu, F; Daley, TM; Taira, T | Abstract: In situ stress measurement at seismogenic depth is critically important for deciphering fault zone processes. In this study, we conducted a second active-source crosswell field experiment at the Parkfield San Andreas Fault Observatory at Depth (SAFOD) drill site to investigate the detectability of stress-induced seismic velocity changes at the top part of the seismogenic zone. We employed the same configuration of our previous experiments, which deployed a piezoelectric source and a three-component (3C) accelerometer at 1 km deep inside the pilot and main holes, respectively. We also added a hydrophone, which is attached to the source, to monitor the repeatability of the source waveforms. Over a 40-day recording period, we confirmed an ∼0:04% travel-time variation in S wave and coda that roughly follows the fluctuation of barometric pressure. We attributed this correlation to stress sensitivity of seismic velocity and the stress sensitivity is estimated to be 2:0 × 10 −7 Pa −1 , which is approximately two orders of magnitude higher than those measured in laboratory with dry rock samples, but is consistent with our previous results. Our results confirm the hypothesis that substantial cracks and/or pore spaces exist at seismogenic depths and thus may be used to monitor the subsurface stress field with active-source crosswell seismic.

Laboratory studies of slip along faults as a physical basis for a new approach to short-term earthquake prediction

The physical effects that may prove useful for developing a new approach to short-term earthquake prediction have been studied in laboratory conditions. In seismology and earthquake foci mechanics, one of the major challenges is searching for indicators of an upcoming seismic event and attempting to reliably record such indicators by available instruments. In this regard, the best result of the laboratory studies of dynamic slip along faults would be the identification of specific macroscopic parameters controlling the deformation process, which are measurable in field. Dynamic stiffness of a fault zone seems to be an appropriate parameter. The recent laboratory experiments have shown that the value of this parameter predetermines the slip mode along the fault (unstable slip, creep, tremor, etc.), and a radical decrease in shear stiffness takes place as the fault zone reaches the metastable state. The effect discovered in the laboratory conditions gives grounds to suggest that changes in the stress-strain state of the fault zone at the final stage of earthquake preparation are detectable from the parameters of microseismic noise in the low-frequency range. Apparently, the noise records during and after the arrival of surface waves from distant earthquakes can provide the best opportunity for determining the parameters characterizing the study area. The wave oscillations with a period of a few dozen seconds have significant amplitudes and duration, which contributes to the excitation of resonance oscillations of the blocks. There are problems requiring additional laboratory experiments: estimating the size of a fault, which predetermines regularities in decreasing of the own frequency of the block-fault system; determining the ratio of the mechanical parameters of the fault in the nucleation zone and on the periphery of the future rupture, etc. Having analyzed the results of experimental studies carried out by other researchers, we conclude that laboratory experiments under normal conditions and low pressures can successfully address a number of fundamental issues on the way to creating a new approach to short-term earthquake prediction. Increasing pressure and temperature to values characteristic of seismogenic depths does not lead to the occurrence of any fundamentally new features in the behavior of the block-fault system at the stage when dynamic slip is being prepared. During slip, friction reduces due to melting, physical and chemical transformations at the micro- and nanoscales and other processes on the slipping surface, but these effects play no role at the stage when dynamic rock failure and the onset of slip are being prepared.

Geometric Complexity of Earthquake Rupture Surfaces Preserved in Pseudotachylyte Networks

AbstractRecent earthquakes have demonstrated that rupture may propagate through geometrically complex networks of faults. Ancient exhumed faults have the potential to reveal the details of complex rupture at seismogenic depths. We present a new set of field observational criteria for determining which of a population of pseudotachylyte fault veins formed in the same earthquake and apply it to map rupture networks representing single earthquakes. An exceptional exposure of an exhumed ancient strand of the Norumbega Shear Zone preserves evidence of multistranded earthquake rupture in the deep seismogenic zone of a continental transform fault. Individual fault strands slipped at least 2–18 cm, so significant slip is represented by each rupture network. Our data show that synchronously slipped faults intersect at angles of 0 to ∼55°, with the opening angles of fault intersections directed toward the dilational quadrants for dextral slip. Multistranded rupture on a fault network instead of rupture of a single fault may result in greater and/or more variable slip and cause slip rake to vary spatially and temporally. Slip on intersecting faults unequivocally means that there will be motion perpendicular to the average fault plane. Modern earthquakes displaying non‐double‐couple components to focal mechanism solutions and spatially varying rake, slip, and anomalous stress drop may be explained by rupture across fault networks that are too close (spatially and temporally) to be resolved seismically as separate events.

Tectonic History of the Kaapvaal Craton: Constraints From Rheological Modeling

AbstractIn this study, the lithospheric strength of the Kaapvaal Craton is determined using deformation laws describing the failure of rocks at lithospheric depths. Shear wave velocity results from a previous study were used to constrain the rheological profiles associated with each of the 64 broadband seismic stations deployed across the craton. The other constraint, are the geotherms estimated assuming steady state conditions. In order to address the random and complex nature of uncertainties inherent in geotherms at lithospheric depths, a Monte Carlo simulation approach is used to find well‐constrained geotherms for each of the terrains within the craton. The optimized geotherms were subsequently used to determine the rheological profiles associated with each of the 64 localities. The resultant rheological profiles show seismogenic depths of 5–16 km for the craton and further show that the craton can basically be delineated into three strength domains. Most parts of the Kimberley terrain and minor parts elsewhere in the craton show a mechanically weak crust with a strength ratio less than 0.5, a substantial portion of the Witwatersrand terrain shows moderate strength ratios between 0.5 and 1.0, and the rest of the craton shows a mechanically strong crust corresponding to strength ratios greater than 1.0. Strength ratio is an indication of the extent of lower crustal to mantle strength. Low strength ratio regions correlate with parts of the craton that had a significant deformation history while high strength ratio regions correlate with a stable lithosphere that had minor or no deformation history.

Deep deformation of the Longmenshan fault zone related to the 2008 Wenchuan earthquake

This paper reviews studies of the past ten year related to the crustal deformation of the Longmenshan fault zone (LMSFZ) which was struck by the M 8.0 Wenchuan earthquake on May 12, 2008. The tectonics of the LMSFZ located at the eastern margin of the Tibetan Plateau is complex, with complicated fault geometry and a heterogeneous velocity structure. The Wenchuan earthquake rupture extended for about 300 km along the middle and northern segments of the LMSFZ, with a very complicated rupture process involving multiple sub-events. The Wenchuan slip distribution is characterized by two major slip patches near Hongkou/Yingxiu and Beichuan with peak slip of 5.0–15.5 m and extended from near the surface to below 20 km depth. Prior to the Wenchuan earthquake, the LMSFZ had been seismically quiet for several centuries and there were no hints that suggested that such an M 8.0 earthquake might strike the area. The long-term geological investigations and short-term geodetic measurements before the Wenchuan earthquake generally agree that the horizontal slip rate along the LMSFZ is no more than 3 mm per year. The low slip rate observed at the surface around the LMSFZ may not reveal the real state of accumulated strain at depth where the devastating Wenchuan earthquake nucleated. Rates of aseismic slip at depth derived from seismological investigation of repeating microearthquakes were found to be approximately twice as large as the interseismic rates inferred from surface GPS and geological data. Most of the clusters of repeating microearthquakes are located at the edge of locked areas where large coseismic slips were observed during the Wenchuan earthquake, suggesting a close relationship between microearthquakes and impending large earthquakes. A two-dimensional viscoelastic finite-element model produces a depth-related slip rate pattern around the LMSFZ that is consistent with that revealed by the seismological observation of repeating earthquakes. The measured in situ deep slip rates increase with depth and vary from 3.5 to 9.6 mm/a over a depth range of 4–18 km.The seismological observations of deep slip rates and microseismicity in the three decades before the Wenchuan earthquake reveal that the LMSFZ is indeed not as “quiet” as traditionally assumed in comparison with its neighboring fault systems. Considering the deep slip rates (3.5−9.6 mm/a) from the repeating microearthquakes and coseismic peak offsets of 5.0–15.5 m, the recurrence interval of Wenchuan-like events is estimated to be about 500−4500 years. The estimated recurrence interval based on the deep slip rates is much smaller than those estimates using the same coseismic displacements divided by GPS-derived or geological slip rates. Slip rate increases with depth were also recognized in the Parkfield section of the San Andreas fault zone and in the northeastern Japan subduction zone before the ruptures of the 2004 M w6.0 Parkfield earthquake and the 2011 M w9.0 Tohoku-oki earthquake, respectively. Accelerated slip is thought to have preceded a number of recent large subduction zone earthquakes and the 2008 Wenchuan earthquake, and repeating earthquakes may document short-term precursory slip at depth. Alternatively, the rapid slip rates indicated by the repeating microearthquakes may represent transiently accelerated slip preceding the Wenchuan mainshock. We suggest that slip rates at seismogenic depths are of critical importance in seismic hazard analysis. Repeating earthquakes can be regarded as “deep creepmeters” that measure the in - situ deep slip rate on otherwise aseismically slipping faults. For less well defined and widespread faults within the continents, it is essential to reveal a fault’s or region’s seismic history over different time scale. Combining a better understanding of earthquake diversity with modern technology is the key to effective and comprehensive hazard mitigation practices. The potential earthquake hazard of locked faults with unusually high inferred deep slow slip rates should be paid more attention. The 2008 Wenchuan earthquake is the best-studied continental earthquake to date with a large number of scientific publications enabled by the vast collected data sets. The research community efforts have provided first-order information about this unexpected event regarding its coseismic slip distribution and fault geometry. However, there are many remaining questions about the basic nature of this earthquake to be further constrained with multiple data sets, including the relationship of its rupture with prior coupling and interseismic creep, the varying dynamical rupture processes with depth, the frequency dependence of seismic radiation across the fault zone, and the role of multiple postseismic deformation processes.

Surface Rupture of Multiple Crustal Faults in the 2016 Mw 7.8 Kaikōura, New Zealand, Earthquake

Multiple (>20 >20 ) crustal faults ruptured to the ground surface and seafloor in the 14 November 2016 M w Mw 7.8 Kaikōura earthquake, and many have been documented in detail, providing an opportunity to understand the factors controlling multifault ruptures, including the role of the subduction interface. We present a summary of the surface ruptures, as well as previous knowledge including paleoseismic data, and use these data and a 3D geological model to calculate cumulative geological moment magnitudes (M G w MwG ) and seismic moments for comparison with those from geophysical datasets. The earthquake ruptured faults with a wide range of orientations, sense of movement, slip rates, and recurrence intervals, and crossed a tectonic domain boundary, the Hope fault. The maximum net surface displacement was ∼12 m ∼12 m on the Kekerengu and the Papatea faults, and average displacements for the major faults were 0.7–1.5 m south of the Hope fault, and 5.5–6.4 m to the north. M G w MwG using two different methods are M G w MwG 7.7 +0.3 −0.2 7.7−0.2+0.3 and the seismic moment is 33%–67% of geophysical datasets. However, these are minimum values and a best estimate M G w MwG incorporating probable larger slip at depth, a 20 km seismogenic depth, and likely listric geometry is M G w MwG 7.8±0.2 7.8±0.2 , suggests ≤32% ≤32% of the moment may be attributed to slip on the subduction interface and/or a midcrustal detachment. Likely factors contributing to multifault rupture in the Kaikōura earthquake include (1) the presence of the subduction interface, (2) physical linkages between faults, (3) rupture of geologically immature faults in the south, and (4) inherited geological structure. The estimated recurrence interval for the Kaikōura earthquake is ≥5,000–10,000 yrs ≥5,000–10,000 yrs , and so it is a relatively rare event. Nevertheless, these findings support the need for continued advances in seismic hazard modeling to ensure that they incorporate multifault ruptures that cross tectonic domain boundaries.

Estimation and applicability of attenuation characteristics for source parameters and scaling relations in the Garhwal Kumaun Himalaya region, India

Source parameters of the small to moderate earthquakes are significant for understanding the dynamic rupture process, the scaling relations of the earthquakes and for assessment of seismic hazard potential of a region. In this study, the source parameters were determined for 58 small to moderate size earthquakes (3.0 ≤ Mw ≤ 5.0) occurred during 2007–2015 in the Garhwal-Kumaun region. The estimated shear wave quality factor (Qβ(f)) values for each station at different frequencies have been applied to eliminate any bias in the determination of source parameters. The Qβ(f) values have been estimated by using coda wave normalization method in the frequency range 1.5–16 Hz. A frequency-dependent S wave quality factor relation is obtained as Qβ(f) = (152.9 ± 7) f(0.82±0.005) by fitting a power-law frequency dependence model for the estimated values over the whole study region. The spectral (low-frequency spectral level and corner frequency) and source (static stress drop, seismic moment, apparent stress and radiated energy) parameters are obtained assuming ω−2 source model. The displacement spectra are corrected for estimated frequency-dependent attenuation, site effect using spectral decay parameter “Kappa”. The frequency resolution limit was resolved by quantifying the bias in corner frequencies, stress drop and radiated energy estimates due to finite-bandwidth effect. The data of the region shows shallow focused earthquakes with low stress drop. The estimation of Zúñiga parameter (ε) suggests the partial stress drop mechanism in the region. The observed low stress drop and apparent stress can be explained by partial stress drop and low effective stress model. Presence of subsurface fluid at seismogenic depth certainly manipulates the dynamics of the region. However, the limited event selection may strongly bias the scaling relation even after taking as much as possible precaution in considering effects of finite bandwidth, attenuation and site corrections. Although, the scaling can be improved further with the integration of large dataset of microearthquakes and use of a stable and robust approach.

Fluid properties and dynamics along the seismogenic plate interface

Fossil structures, such as exhumed accretionary prisms, are the only direct recorders of the fluids wetting the plate interface near the base of the seismo­ genic zone. By studying exhumed accretionary prisms, it is thus possible to determine the physicochemical properties of fluids and the geometry and dynamics of their circulation. We considered here two transects encompass­ ing the brittle­plastic transition (BPT) zone, in the Franco­Italian Alps and the Shimanto Belt in Japan, and compared our data with a broader set of exam­ ples from the literature. On quartz that grew synkinematically at peak burial conditions, we inferred fluid properties indirectly from quartz trace­element concentrations (using cathodoluminescence [CL] imaging) and directly from fluid­inclusion composition and P­ρ­T properties (using Raman and micro­ thermometry). At ~250 °C, quartz grew principally through fracturing and two types of quartz, a CL­brown and a CL­blue, precipitated alternately. At ~350 °C, where plastic deformation and recrystallization is pervasive, only a single, ho­ mogeneously CL­brown quartz is present. The salinity of the fluid in the inclu­ sions shallower than the BPT is consistently of the order or lower than sea­ water, while salinities are very scattered deeper than the BPT and often exceed seawater salinity. The gas dissolved in the fluid is predominantly CH 4 shal­ lower than the BPT, and either CH 4 or CO 2 deeper than the BPT, depending on the nature of the host rock and in particular on the proportion of carbonates. Cathodoluminescence properties, salinity, and nature of the gas all point to a closed­system behavior in rocks deeper than the BPT. In contrast, shallower than the BPT (i.e., at seismogenic depths), textures revealed by CL­imaging evidence the episodic influx of an external fluid, leading to the crystallization of CL­blue quartz. The scale of the circulation leading to the generation of the CL­blue quartz, or its relationship with the seismic cycle, is still unclear. Be­ sides, the fluid pressure recorded in the abundant water­rich fluid inclusions is systematically much lower than the corresponding lithostatic pressure, ir­ respective of the depth domain considered. For inclusions trapped at large depth, the low fluid pressure recorded in the inclusions reflects post­entrap­ ment reequilibration. For inclusions trapped at shallower conditions, typically at seismogenic depths, the low fluid pressure may as well be the result of large fluid pressure drop after earthquakes.

Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

Strength evolution of simulated carbonate-bearing faults: The role of normal stress and slip velocity

A great number of earthquakes occur within thick carbonate sequences in the shallow crust. At the same time, carbonate fault rocks exhumed from a depth < 6 km (i.e., from seismogenic depths) exhibit the coexistence of structures related to brittle (i.e., cataclasis) and ductile deformation processes (i.e., pressure-solution and granular plasticity). We performed friction experiments on water-saturated simulated carbonate-bearing faults for a wide range of normal stresses (from 5 to 120 MPa) and slip velocities (from 0.3 to 100 μm/s). At high normal stresses (σn > 20 MPa) fault gouges undergo strain-weakening, that is more pronounced at slow slip velocities, and causes a significant reduction of frictional strength, from μ = 0.7 to μ = 0.47. Microstructural analysis show that fault gouge weakening is driven by deformation accommodated by cataclasis and pressure-insensitive deformation processes (pressure solution and granular plasticity) that become more efficient at slow slip velocity. The reduction in frictional strength caused by strain weakening behaviour promoted by the activation of pressure-insensitive deformation might play a significant role in carbonate-bearing faults mechanics.

Effect of Seismogenic Depth and Background Stress on Physical Limits of Earthquake Rupture Across Fault Step Overs

AbstractEarthquakes can rupture geometrically complex fault systems by breaching fault step overs. Quantifying the likelihood of rupture jump across step overs is important to evaluate earthquake hazard and to understand the interactions between dynamic rupture and fault growth processes. Here we investigate the role of seismogenic depth and background stress on physical limits of earthquake rupture across fault step overs. Our computational and theoretical study is focused on the canonical case of two parallel strike‐slip faults with large aspect ratio, uniform prestress and friction properties. We conduct a systematic set of 3‐D dynamic rupture simulations with different seismogenic depth, step over distance, and initial stresses. We find that the maximum step over distance Hc that a rupture can jump depends on seismogenic depth W and strength excess to stress drop ratio S, commonly used to evaluate probable rupture velocity, as , where n = 2 when Hc/W &lt; 0.2 (or S &gt; 1.5) and n = 1 otherwise. The critical nucleation size, largely controlled by frictional properties, has a second‐order effect on Hc. Rupture on the secondary fault is mainly triggered by the stopping phase emanated from the rupture end on the primary fault. Asymptotic analysis of the peak amplitude of stopping phases sheds light on the mechanical origin of the relations between Hc, W, and S, and leads to the scaling regime with n = 1 in far field and n = 2 in near field. The results suggest that strike‐slip earthquakes on faults with large seismogenic depth or operating at high shear stresses can jump wider step overs than observed so far in continental interplate earthquakes.

Analysis of Fan Waves in a Laboratory Model Simulating the Propagation of Shear Ruptures in Rocks

The fan-shaped mechanism of rotational motion transmission in a system of elastically bonded slabs on flat surface, simulating the propagation of shear ruptures in super brittle rocks, is analyzed. Such ruptures appear in the Earth’s crust at seismogenic depths. They propagate due to the nucleation of oblique tensile microcracks, leading to the formation of a fan domino-structure in the rupture head. A laboratory physical model was created which demonstrates the process of fan-structure wave propagation. Equations of the dynamics of rotational motion of slabs as a mechanical system with a finite number of degrees of freedom are obtained. Based on the Merson method of solving the Cauchy problem for systems of ordinary differential equations, the computational algorithm taking into account contact interaction of slabs is developed. Within the framework of a simplified mathematical model of dynamic behavior of a fan-shaped system in the approximation of a continuous medium, the approximate estimates of the length of a fan depending on the velocity of its motion are obtained. It is shown that in the absence of friction a fan can move with any velocity that does not exceed the critical value, which depends on the size, the moment of inertia of slabs, the initial angle and the elasticity coefficient of bonds. In the presence of friction a fan stops. On the basis of discrete and continuous models, the main qualitative features of the behavior of a fan-structure moving under the action of applied tangential forces, whose values in a laboratory physical model are regulated by a change in the inclination angle of the rupture plane, are analyzed. Comparison of computations and laboratory measurements and observations shows good correspondence between the results.

Work Optimization Predicts the Evolution of Extensional Step Overs Within Anisotropic Host Rock: Implications for the San Pablo Bay, CA

AbstractRecent geophysical imaging indicates that the Hayward Fault hard links to the Rodgers Creek Fault at 5 m depth within the San Pablo Bay, CA, suggesting that earthquakes may be able to rupture continuously through the fault network. To investigate fault propagation, interaction, and linkage in segmented fault networks, including those within the San Pablo Bay, we simulate the development of two idealized, underlapping faults within an extensional step over at seismogenic depths using work optimization. We test the sensitivity of fault growth to strength anisotropy, material heterogeneities, and initial fault geometry. The optimal faults propagate toward each other until linking with the other fault at its tip and form a single hard‐linked transverse fault. These faults propagate with relatively high propagation power or rate of efficiency gain. Less efficient faults form wider basins and develop with reduced propagation power. Models with initial fault geometries that more closely match the shallowly imaged Hayward and Rodgers Creek faults suggest that the faults link at seismogenic depths if a mapped segment of the Rodgers Creek that extends into the San Pablo Bay is currently inactive. Predictions of average slip rate, slip per earthquake, and earthquake magnitude from these models closely match paleoseismic estimates. The hard linkage of the Hayward and Rodgers Creek faults imaged in the near‐surface, and predicted by these models, increases local seismic hazard by increasing the upper limit of throughgoing earthquakes to M 7.6.

Partitioning of oblique convergence coupled to the fault locking behavior of fold‐and‐thrust belts: Evidence from the Qilian Shan, northeastern Tibetan Plateau

AbstractOblique plate convergence is common, but it is not clear how the obliquity is achieved by continental fold‐and‐thrust belts. We address this problem in the Qilian Shan, northeastern Tibetan Plateau, using fieldwork observations, geomorphic analysis, and elastic dislocation modeling of published geodetic data. A thrust dips SSW from the northern range front and underlies steeper thrusts in the interior. Cenozoic thrust‐related shortening across the Qilian Shan is ~155–175 km, based on two transects. Elastic dislocation modeling indicates that horizontal strain in the interseismic period is consistent with oblique slip on a single low‐angle detachment thrust below ~26 km depth, dipping SSW at ~17°. We suggest that this detachment is located above North China Block crust, originally underthrust during Paleozoic orogeny. Horizontal shear strain is localized directly above the updip limit of creep on the detachment and is coincident with the left‐lateral Haiyuan Fault. This configuration implies that oblique slip on the detachment below seismogenic depths is partitioned in the shallow crust onto separate strike‐slip and thrust faults. This is consistent with strain partitioning in oceanic subduction zones but has not previously been found by dislocation models of continental interiors. The marginal, strike‐slip, Altyn Tagh Fault influences thrusting within the Qilian Shan for 100–200 km from the fault but does not control the regional structure, where Paleozoic basement faults have been reactivated. The Qilian Shan resembles the main Tibetan Plateau in nascent form: active thrusts are marginal to an interior that is developing plateau characteristics, involving low relief, and low seismicity.