Fast Slip Rate Research Articles

Barrier-Induced Rupture Front Disturbances during the 2023 Morocco Earthquake

Abstract Seismic waveforms, including teleseismic body waves, contain information about the irregular behavior of rupture propagation, which is essential for understanding the evolution process of large earthquakes. Here, a high-degree-of-freedom source inversion is applied to the teleseismic P waves of the 2023 moment magnitude 6.8 Morocco earthquake to reveal the irregular rupture behavior during earthquake growth. The resulting total moment tensor solution is an oblique focal mechanism that exhibits reverse faulting with a strike-slip component. There are two distinct peaks at 2 and 4 s in the moment rate function. The reverse fault component dominates at the beginning of the rupture, but then the strike-slip component increases to the second peak and then decreases. The main rupture propagates first in an east-northeast direction, then both up- and down-dip. The down-dip propagating rupture diminishes shortly, whereas the up-dip propagating rupture becomes dominant. The main rupture propagating in the up-dip direction is temporarily suppressed around a point located at 19 km depth and 10 km east-northeast of the hypocenter (region B). After the rupture propagates surrounding region B, the rupture propagates into region B, where a relatively fast slip rate is observed. It is confirmed that the irregular rupture propagation associated with region B is reproduced even when the model settings and the data sampling interval are slightly changed. The irregular rupture propagation obtained in this study suggests that a barrier with high apparent strength (e.g., high fracture surface energy) can cause the rupture to be initially suppressed within the barrier region, followed by delayed rupture propagation through the apparent barrier. The high-frequency seismic motions caused by such an irregular rupture propagation may have contributed to the increase in earthquake-related damage.

Characteristic versus Gutenberg–Richter Nucleation-Based Magnitude–Frequency Distributions in the New Zealand National Seismic Hazard Model 2022

Abstract Probabilistic seismic hazard analysis requires a seismicity rate model, or in other words, a forecast of earthquake rates. In the New Zealand National Seismic Hazard Model 2022, the seismicity rate model is constructed through independent forecasts of earthquakes on mapped faults and earthquakes distributed over cells in a spatial grid. Here, we explore the seismicity rate model for upper plate (hypocenter ≥ 40 km) events, to investigate the shape of magnitude–frequency distributions (MFDs) considering events nucleating (or for which the hypocenters are located) within individual fault zone. We find that more than 80% of the fault zones have MFDs that are better described by a Gutenberg–Richter (GR) distribution, instead of a characteristic distribution (i.e., rates of larger magnitudes much higher than the GR trend). Furthermore, the MFD classifications are neither influenced by time-dependent (and time-independent) considerations nor directly affected by the size (or area) of the fault zones. Fault zones with faster slip rates (>20 mm/yr) exhibit characteristic MFDs, whereas those with slower slip rates may or may not. Although multifault ruptures are prevalent in the characteristic distributions, large maximum magnitude (Mw >8.0) plays a pivotal role producing a characteristic MFD. On the other hand, physically unconnected multifault ruptures (i.e., involving rupture jumps ≥ 10 km) are mostly observed with GR distributions.

An investigation into the effect of slip rate on the traction coefficient behaviour with a laboratory replication of a locomotive wheel rolling/sliding along a railway track

This paper investigates the effect of slip rate on the traction coefficient of the wheel/rail interface using a twin-disc machine under the replicated tribological and locomotive operating conditions. The traction coefficient decreases with increasing slip rate. The interface materials of the tested samples get less time to accumulate strain hardening, resulting in a lower increase in surface hardness under a faster slip rate. The running-in period is shorter under a higher slip rate, indicating that the surface micro-geometry of the interface materials reaches a steady state faster. The wear and traction coefficient outcomes of this research could be helpful in improving locomotive traction control systems, and in increasing the safety and maintenance efficiency of railway operations.

Foreshock properties illuminate nucleation processes of slow and fast laboratory earthquakes

Understanding the connection between seismic activity and the earthquake nucleation process is a fundamental goal in earthquake seismology with important implications for earthquake early warning systems and forecasting. We use high-resolution acoustic emission (AE) waveform measurements from laboratory stick-slip experiments that span a spectrum of slow to fast slip rates to probe spatiotemporal properties of laboratory foreshocks and nucleation processes. We measure waveform similarity and pairwise differential travel-times (DTT) between AEs throughout the seismic cycle. AEs broadcasted prior to slow labquakes have small DTT and high waveform similarity relative to fast labquakes. We show that during slow stick-slip, the fault never fully locks, and waveform similarity and pairwise differential travel times do not evolve throughout the seismic cycle. In contrast, fast laboratory earthquakes are preceded by a rapid increase in waveform similarity late in the seismic cycle and a reduction in differential travel times, indicating that AEs begin to coalesce as the fault slip velocity increases leading up to failure. These observations point to key differences in the nucleation process of slow and fast labquakes and suggest that the spatiotemporal evolution of laboratory foreshocks is linked to fault slip velocity.

A High‐Order Accurate Summation‐By‐Parts Finite Difference Method for Fully‐Dynamic Earthquake Sequence Simulations Within Sedimentary Basins

AbstractWe present an efficient numerical method for earthquake sequences in 2D antiplane shear that incorporates wave propagation. A vertical strike‐slip fault governed by rate‐and‐state friction is embedded in a heterogeneous elastic half‐space discretized using a high‐order accurate Summation‐by‐Parts finite difference method. Adaptive time‐stepping is applied during the interseismic periods; during coseismic rupture we apply a non‐stiff method, enabling a variety of explicit time stepping methods. We consider a shallow sedimentary basin and explore sensitivity to spatial resolution and the switching criteria used to transition between solvers. For sufficient grid resolution and switching thresholds, simulations results remain robust over long time scales. We explore the effects of full dynamics and basin depth and stiffness, making comparisons with quasi‐dynamic counterparts. Fully‐dynamic ruptures generate higher stresses, faster slip rates and rupture speeds, producing seismic scattering in the bulk. Because single‐event dynamic simulations penetrate further into sediments compared to the quasi‐dynamic simulations, we hypothesize that the incorporation of inertial effects would produce sequences of only surface‐rupturing events. However, we find that subbasin ruptures can still emerge with elastodynamics, for sufficiently compliant basins. We also find that full dynamics can increase the frequency of surface‐rupturing events, depending on basin depth and stiffness. These results suggest that an earthquake's potential to penetrate into shallow sediments should be viewed through the lens of the earthquake sequence, as it depends on basin properties and wave‐mediated effects, but also on self‐consistent initial conditions obtained from seismogenic cycling.

Frictional constitutive behavior of chlorite at low shearing rates and hydrothermal conditions

Chlorite is a common, but understudied phyllosilicate mineral along continental and subduction zone faults. To understand the potential role of chlorite in different modes of fault slip, we measure the constitutive frictional properties of chlorite at low rates of deformation and shallow hydrothermal conditions by shearing powdered samples of water-saturated chlorite in the triaxial saw-cut configuration. Experiments were conducted at 25 to 130 °C, 130 MPa confining pressure, pore pressures from 10 MPa to 120 MPa, slip rates from 0.001 to 10 μm/s, and shear displacements up to 7.5 mm. The frictional strength of chlorite increases with increasing temperature from 25 to 130 °C (μ = 0.4 to 0.46), accompanied by an increased abundance of Riedel shears. Chlorite also transitions from rate-strengthening behavior at fast slip rates (0.1 to 10 μm/s) to rate-weakening or rate-neutral behavior at lower slip rates (0.001 to 0.01 μm/s), consistent with one hypothesis for the cause of slow slip. This transition in frictional behavior reflects different microphysical processes controlling the direct and evolution effects of friction. At low deformation rates, the magnitude and temperature sensitivity of the direct effect (a) are consistent with subcritical fracture. In contrast, the evolution effect (b) is insensitive to temperature and increases systematically with decreasing strain rate. We propose the increase in b and resulting rate-weakening behavior of chlorite with decreasing slip rate is controlled by time-dependent properties of adsorbed water to mineral surfaces.

Large‐Scale Crustal Deformation, Slip‐Rate Variation, and Strain Distribution Along the Kunlun Fault (Tibet) From Sentinel‐1 InSAR Observations (2015–2020)

AbstractLarge‐scale geodetic measurements of crustal deformation in the north‐central Tibetan Plateau are crucial for improved understanding of earthquake‐cycle processes and long‐term seismic hazard assessment. We use GPS velocities and Interferometric Synthetic Aperture Radar (InSAR) observations (2015–2020) on eight descending and eight ascending Sentinel‐1 tracks to map surface motions and their gradients for an area of over ∼2,000 km × 350 km around the Kunlun‐Manyi fault system. The derived line‐of‐sight (LOS) and 3D velocity fields demonstrate ongoing postseismic transients along the ruptured segments of the 1997 Manyi and 2001 Kokoxili earthquakes, interseismic deformation along locked fault segments, and strain accumulation on a large subsidiary fault (i.e., the Kunlun Pass fault). We use elastic dislocation and analytical postseismic deformation models constrained by our dense InSAR measurements to quantify the interseismic and postseismic contributions, which reveal the along‐fault distribution of fault slip rate and locking depth along the entire length of the fault. The results indicate that the slip rate of the Kunlun fault systematically decreases toward the west to the west of the Taiyang Lake fault. We also find that the interseismic strain accumulation rate along the main trace of the Kunlun fault is to first‐order spatially constant (0.1–0.15 microstrain/yr), except around its restraining bend. As the historically unruptured segments, especially the eastern segments (from 95°E to 101°E) of the Kunlun fault, have relatively fast slip rates and high locking depths (>15 km), they have the potential to generate large and damaging earthquakes in this region.

Recognition of broad thermal anomaly around the median tectonic line in central Kii peninsula, southwest Japan: Possible heat sources

AbstractApplication of Raman spectroscopy of carbonaceous material thermometry to samples of metasedimentary rock from the low‐grade Sanbagawa belt in the central Kii peninsula reveals a progressive decrease in temperature from ~390 °C close to the northern boundary, a major continental shear zone—the median tectonic line (MTL)—to ~270 °C and ~7 km to the south and roughly constant temperature distribution thereafter. Within the Sanbagawa belt, the thermal structure is not significantly modified by slip‐on fault boundaries between different geological units or folding. Meso‐ and microstructural observations combined with strain analysis using detrital grains in meta‐mudstone indicate a similar deformation history throughout the area and no correlation between ductile strain and temperature gradients. These observations suggest the observed thermal structure was developed after the main stages of ductile deformation of the Sanbagawa belt were complete and are not due to localized preferential exhumation along with the MTL. The observations also require a heat source along with the MTL. Order of magnitude estimates suggest the influx of warm fluid along the MTL are viable causes of the observed thermal anomaly. Although shear heating would be another possible explanation, thermal calculations require anomalous fast slip rates along the MTL and much greater frictional strength than generally considered reasonable. For these reasons, fluid infiltration is our preferred model.

Contemporary background stress field in the eastern Tibetan Plateau: Insights from 3D geomechanical modeling

Knowledge of the crustal stress state is important for understanding tectonic processes and seismic hazard assessment as well as for various practical applications. In this study, we present a model describing the contemporary 3D crustal background stress field of the eastern Tibetan Plateau. The model incorporates a complex fault system and an appropriate initial stress state that matches both model-independent stress observations and kinematic data well. Our results show that the modeled maximum horizontal stress SH is generally oriented in NW-SE direction. The Bayan Har Block and the Chuan-Dian Block are dominated by a strike-slip faulting stress regime. However, in some local areas, the crust is governed by a normal faulting regime. We infer that the local normal faulting stress regime in the Bamei-Kangding area is caused by the clockwise rotation of the strike of the Xianshuihe fault at Kangding. This normal faulting stress regime would promote the ascent of intrusions and provide the space necessary for the emplacement of the Zheduoshan granite. The stress state on the Longmen Shan fault zone is rather inhomogeneous. For the Yingxiu-Beichuan fault, the northeastern segment has a lower normal stress but a higher shear stress than the southwestern segment. This inhomogeneous stress state on the Yingxiu-Beichuan fault could provide an explanation for the northeastward propagation of the ultra-long unilateral rupture in the Wenchuan Ms8.0 earthquake. According to the stress state on the Xianshuihe fault, we conclude that the Kangding-Shimian segment has a high potential to produce a strong earthquake due to its high normal stress and a fast slip rate and should be carefully monitored. On the Bamei-Kangding segment, which is governed by a normal faulting stress regime and may be lubricated by fluids, slip seems to be facilitated favoring more frequent small to moderate magnitude earthquakes to occur on this segment.

Block Tectonics Across Western Tibet and Multi‐Millennial Recurrence of Great Earthquakes on the Karakax Fault

AbstractFault slip rates are critical to quantify continental deformation. Those along the Karakax fault (northwestern Altyn Tagh Fault: ATF) have been debated, even though it is one of Tibet's most outstanding active faults. At Taersa, using LiDAR measurements of terrace and fan riser offsets (∼6 to ∼500 m) and10Be/26Al dating of alluvial surfaces (<210 ka), we obtain a late Quaternary slip rate of ∼2.5 ± 0.5 mm/yr. This doubles the ∼2.6 ± 0.5 mm/yr rate time span found to the east and west. We interpret the ∼150 km‐long, free‐faced rupture along the fault to be that of theM∼ 7.6 event felt in Hotan in 1882. Characteristic slip (∼6 m) during four large earthquakes since ∼10 ka implies a ∼2500 ± 500 years return time. A ∼3 mm/yr rate is consistent with the ∼80 km offset of the Karakax river since uplift of the West Kunlun range and sediment deposition in the Tarim foreland accelerated, ∼24 Ma ago. The faster slip rate (∼10.5 mm/yr) on the central ATF matches the sum of those along the reactivated West Tibetan terrane boundaries (Karakax and Longmu‐Gozha Co faults) at the Uzatagh triple junction (∼36°N, 83°E). The abrupt termination and altitude drop of the Karakorum range where the Longmu Co and Karakorum faults meet (Angmong junction), also reflect triple junction kinematics. Such localized changes account for the rise of the Karakorum and West Kunlun ranges and support lithospheric block tectonics rather than diffusely distributed deformation.

Fast Holocene slip and localized strain along the Liqui\xf1e-Ofqui strike-slip fault system, Chile

In active tectonic settings dominated by strike-slip kinematics, slip partitioning across subparallel faults is a common feature; therefore, assessing the degree of partitioning and strain localization is paramount for seismic hazard assessments. Here, we estimate a slip rate of 18.8 ± 2.0 mm/year over the past 9.0 ± 0.1 ka for a single strand of the Liquiñe-Ofqui Fault System, which straddles the Main Cordillera in Southern Chile. This Holocene rate accounts for ~ 82% of the trench-parallel component of oblique plate convergence and is similar to million-year estimates integrated over the entire fault system. Our results imply that strain localizes on a single fault at millennial time scale but over longer time scales strain localization is not sustained. The fast millennial slip rate in the absence of historical Mw > 6.5 earthquakes along the Liquiñe-Ofqui Fault System implies either a component of aseismic slip or Mw ~ 7 earthquakes involving multi-trace ruptures and > 150-year repeat times. Our results have implications for the understanding of strike-slip fault system dynamics within volcanic arcs and seismic hazard assessments.

GEOPHYSICAL INVESTIGATIONS OF A POTENTIAL LANDSLIDE AREA IN MAYOON, HUNZA DISTRICT, GILGIT-BALTISTAN, PAKISTAN

The Mayoon landslide in the Hunza District is a slowly developed, non-catastrophic landslide that has gained its importance in the last few years after its rapid activation and fast slip rate. The area is characterized by high earthquake hazards (zone 3 with a peak ground acceleration value of 2.4–3.2 m/s2) by the Building Code of Pakistan due to frequent earth quakes. The past high earthquake activity in the area has displaced the foliated rocks towards the south and is responsible for opening the bedrock joints. The head and body of the landslide are covered by unconsolidated material and have fractures of varying lengths and widths. The non-invasive geophysical techniques, including Ground Penetrating Radar (GPR) and Electrical Resistivity Soundings (ERS), are deployed to evaluate the Mayoon landslide subsurface. The subsurface is interpreted into a two-layer model. Bright reflectors and highly variable resistivity characterize the top layer (Layer-1). This layer is associated with a loose, highly heterogeneous, fragmented material deposited under glacial settings over the existing bedrock. Hyperbolic reflections and intermediate resistivity characterize the bottom layer (Layer-2). This layer is associated with foliated metamorphic bedrock. The hyperbolic reflections show faults/fractures within the bedrock. The extension of these fractures/faults with depth is uncertain due to decay in the GPR signal with depth. The intermediate resistivity shows the bedrock is weathered and foliated. Reflections within Layer-1 have disrupted directly above the fractures/faults suggesting a possible movement. A bright reflection between the two layers highlights the presence of the debonded surface. Loose material within Layer-1 coupled with debonding possesses a significant hazard to generate a landslide under unfavourable conditions, such as an intense rainstorm or earthquake activity.

Seismicity, Fault Slip Rates, and Fault Interactions in a Fault System

AbstractSeismicity and the partitioning of slip rates in interacting and mechanically coupled fault systems are poorly understood. We developed and used three‐dimensional viscoelastoplastic finite element models to simulate earthquakes, earthquake cycles, and synthetic seismicity in single and bifurcated fault systems and investigate seismicity, fault slip rates, and interactions and mechanical coupling between faults in fault systems. The modeled interseismic velocities and coseismic displacements are consistent with those from analytical and quasi‐analytical solutions. We found that the modeled frequency‐magnitude relation satisfies the Gutenberg‐Richter relation and the b value is inversely related to differential stress. We observed that the seismicity on two parallel faults is often interdependent and complementary: high seismic moment release rates on a fault are usually accompanied by low ones on the other fault and vice versa; the two faults collectively accommodate far‐field tectonic loading and crustal relative motion. Model results showed that when two parallel faults have the same or similar strength, they have similar seismicity and long‐term slip rates, but when one of the two faults has lower strength, it has more active seismicity and a faster slip rate than the other; the sum of their slip rates equals the far‐field crustal relative motion in both cases. The results in this study provide scientific insights into the spatiotemporal migration of seismicity and partitioning of fault slip rates in an interacting and mechanically coupled fault system and help to qualify b values and to evaluate the earthquake hazard and risk on each of the faults in such a system.

Water Availability and Deformation Processes in Smectite‐Rich Gouges During Seismic Slip

AbstractSmectite clays occur in subduction zone fault cores at shallow depth (approximately 1 km; e.g., Japan Trench) and landslide décollements (e.g., Vajont, Italy, 1963). The availability of pore fluids affects the likelihood that seismic slip propagates from deeper to shallow fault depths or that a landslide accelerates to its final collapse. To investigate the deformation processes active during seismic faulting we performed friction experiments with a rotary machine on 2‐mm‐thick smectite‐rich gouge layers (70/30 wt % Ca‐montmorillonite/opal) sheared at 5‐MPa normal stress, at slip rates of 0.001, 0.01, 0.1, and 1.3 m/s, and total displacement of 3 m. Experiments were performed on predried gouges under vacuum, under room humidity and under partly saturated conditions. The fault shear strength measured in the experiments was included in a one‐dimensional numerical model incorporating frictional heating, thermal, and thermochemical pressurization. Quantitative X‐ray powder diffraction and scanning electron microscopy investigations were performed on pristine and deformed smectite‐rich gouges. Under dry conditions, cataclasis and amorphization dominated at slip rates of 0.001–0.1 m/s, whereas grain size sensitive flow and, under vacuum, frictional melting occurred at fast slip rates (1.3 m/s). Under partly saturated conditions, frictional slip in a smectite foliation occurred in combination with pressurization of water by shear‐enhanced compaction and, for V = 0.01–1.3 m/s, with thermal pressurization. Pseudotachylytes, the only reliable microstructural markers for seismic slip, formed only with large frictional power (>2 MW/m2), which could be achieved at shallow depth with high slip rates, or, at depth, with high shear stress in dehydrated smectites.

Subseismic to Seismic Slip in Smectite Clay Nanofoliation.

Smectite clays are the main constituent of slipping zones found in subduction zone faults at shallow depth (e.g., <1‐km depth in the Japan Trench) and in the decollements of large landslides (e.g., 1963 landslide, Vajont, Italy). Therefore, deformation processes in smectite clays may control the mechanical behavior from slow creep to fast accelerations and slip during earthquakes and landslides. Here, we use (1) laboratory experiments to investigate the mechanical behavior of partly water‐saturated smectite‐rich gouges sheared from subseismic to seismic slip rates V and (2) nanoscale microscopy to study the gouge fabric. At all slip rates, deformation localizes in volumes of the gouge layer that contain a “nanofoliation” consisting of anastomosing smectite crystals. “Seismic” nanofoliations produced at V = 0.01, 0.1, and 1.3 m/s are similar to “subseismic” nanofoliations obtained at V = 10−5 m/s. This similarity suggests that frictional slip along water‐lubricated smectite grain boundaries and basal planes may occur from subseismic to seismic slip rates in natural smectite‐rich faults. Thus, if water is available along smectite grain boundaries and basal planes, nanofoliations can develop from slow to fast slip rates. Still, when nanofoliations are found highly localized in a volume, they can be diagnostic of slip that occurred at rates equal or larger than 0.01 m/s. In such a case, they could be markers of past seismic events when found in natural fault rocks.

Frictional Behavior of Input Sediments to the Hikurangi Trench, New Zealand

AbstractThe Hikurangi subduction zone hosts shallow slow‐slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow‐slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity‐weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow‐slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10‐km depth without requiring elevated pore fluid pressures. The transition to velocity‐strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow‐sliding velocities, rather than accommodating deformation through ordinary earthquakes.

Paleoseismology and slip rate of the western Tianjingshan fault of NE Tibet, China

We present results from a detailed investigation of the horizontal displacement distribution, timing of paleoearthquakes and left-lateral slip rate on the western Tianjingshan fault. Measurements of 240 offset streams and ridges confirm that the fault is left-lateral and record evidence of repeated ∼3–4m coseismic offsets along the 60–km–long fault. This suggests that ∼6 earthquakes may have occurred along the entire western Tianjingshan fault with repeated occurrence of earthquakes of Mw 7.2–7.5. Structural and stratigraphic relationships exposed in our five trenches in combination with previously reported studies further indicate that the fault has ruptured in as many as six paleoearthquakes since the late Quaternary. Paleoseismic data show that the average recurrence interval for Holocene earthquakes is approximately 5,000yr. The most recent earthquake along the western Tianjingshan fault occurred ∼1.2±0.1kyrBP, indicating that this fault segment did not rupture in the M 7.5 historical earthquake of 1709 that ruptured the central Tianjingshan fault. We estimate that the Holocene slip rate of the western Tianjingshan fault is ∼1.1–1.2mm/yr based on measurements of the age and offset of stream channels. Compared with the relatively fast slip rate of the Haiyuan fault (∼4–6mm/yr), we suggest that the Tianjingshan fault acts as an essential active fault accommodating the sinistral displacement and crustal shortening deformation in NE Tibet.

A Comparative Study of Source Complexity of Two Moderate-Sized Earthquakes in Southern Taiwan: 2016 Mw6.4 and 2010 Mw6.2 Earthquakes

We study the complex rupture history of the 2016 Mw6.4 Meinong earthquake and the 2010 Mw6.2 Jiaxian earthquake in southern Taiwan by simultaneously inverting near-field strong-motion records, local and teleseismic broadband waveforms along with long-period surface waves. The focal mechanism results reveal that both earthquakes may rupture two different blind faults. The rupture of the 2016 event is dominated by the strike-slip motion with minor thrust faulting, while the 2010 event is a relatively high-angle thrust earthquake. Our preferred finite-fault source models suggest that the coseismic rupture history of the 2016 event should be relatively more complex than the 2010 event. During the 2016 event, two main patches of slip characterize the coseismic slip history. The cumulative seismic moment within 12 s of rupture is about 5.36 × 1018 N m, which is 1.9 times of the moment of the 2010 event. The slip of the 2016 event is associated with relatively larger perk slip of 0.7 m at shallower depth (~18–19 km), higher average slip of 0.3 m and faster slip rate with the maximum value of 3.4 m/s. Our kinematic models can shed some light on the cause of the difference in seismic hazard between these two earthquakes, and will be further applied to analyze the deep blind fault structure in southern Taiwan.

Southern San Andreas Fault Evaluation Field Activity: Approaches to Measuring Small Geomorphic Offsets--Challenges and Recommendations for Active Fault Studies

In southern California, where fast slip rates and sparse vegetation contribute to crisp expression of faults and microtopography, field and high‐resolution topographic data (<1 m/pixel) increasingly are used to investigate the mark left by large earthquakes on the landscape (e.g., Zielke et al. , 2010; Zielke et al. , 2012; Salisbury, Rockwell, et al. , 2012, Madden et al. , 2013). These studies measure offset streams or other geomorphic features along a stretch of a fault, analyze the offset values for concentrations or trends along strike, and infer that the common magnitudes reflect successive surface‐rupturing earthquakes along that fault section. Wallace (1968) introduced the use of such offsets, and the challenges in interpreting their “unique complex history” with offsets on the Carrizo section of the San Andreas fault; these were more fully mapped by Sieh (1978) and followed by similar field studies along other faults (e.g., Lindvall et al. , 1989; McGill and Sieh, 1991). Results from such compilations spurred the development of classic fault behavior models, notably the characteristic earthquake and slip‐patch models, and thus constitute an important component of the long‐standing contrast between magnitude–frequency models (Schwartz and Coppersmith, 1984; Sieh, 1996; Hecker et al. , 2013). The proliferation of offset datasets has led earthquake geologists to examine the methods and approaches for measuring these offsets, uncertainties associated with measurement of such features, and quality ranking schemes (Arrowsmith and Rockwell, 2012; Salisbury, Arrowsmith, et al. , 2012; Gold et al. , 2013; Madden et al. , 2013). In light of this, the Southern San Andreas Fault Evaluation (SoSAFE) project at the Southern California Earthquake Center (SCEC) organized a combined field activity and workshop (the “Fieldshop”) to measure offsets, compare techniques, and explore differences in interpretation. A thorough analysis of the measurements from the field activity will be provided separately; this …

Ductile strain rate measurements document long-term strain localization in the continental crust

Quantifi cation of strain localization in the continental lithosphere is hindered by the lack of reliable deformation rate measurements in the deep crust. Quartz-strain-rate-metry (QSR) is a convenient tool for performing such measurements once calibrated. We achieve this calibra- tion by identifying the best piezometer-rheological law pairs that yield a strain rate in agree- ment with that measured on the same outcrop by a more direct method taken as a reference. When applied to two major continental strike-slip shear zones, the Ailao Shan-Red River (ASRR; southwest China) and the Karakorum (northwest India), the calibrated QSR high- lights across-strike strain rate variations, from 1 ◊ 10 −13 s -1 in zones where it is localized. Strain rates integrated across the shear zones imply fast fault slip rates on the order of 1.1 cm yr -1 (Karakorum) and 4 cm yr -1 (ASRR), proving strong strain localization in these strike-slip continental shear zones.