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The slip style of continental midcrustal shear zones plays a crucial role in determining the seismogenic potential of faults, but it remains poorly understood because geological observations that can be directly tied to seismic behavior are scarce. We describe frictional-viscous shear zones in the Red River fault, China, which consists of two segments with distinct seismic behaviors and fluid availabilities. The northern segment hosts moderate to large earthquakes, and midcrustal fault slip is localized into mylonitized pseudotachylyte-bearing layers where dynamically recrystallized quartz records flow stresses exceeding 100 MPa and accelerated viscous creep. The southern segment is dominantly aseismic but active microseismically. Fault slip is accommodated in several mylonitized cataclasite layers, comprising interconnected biotite and intervening fractured clasts, with evidence for pervasive dissolution-precipitation creep. Microstructures, paleopiezometry, and microphysical modeling suggest transient aseismic slip in response to increased strain rates during viscous creep at <50 MPa. We interpret that along-strike variations in fluid environment control fault slip styles and seismic behaviors. The dry and strong northern segment is capable of nucleating large earthquakes, while greater fluid availability in the southern segment activates dissolution-precipitation creep at low driving stresses, which limits interseismic elastic strain accumulation at frictional-viscous transition depths. In this model, compaction-driven fluid pressurization and dilatant hardening are invoked to explain the aseismic slip transients in the southern segment. [ABSTRACT FROM AUTHOR]

The article focuses on synthesizing research on the seismotectonics of the San Andreas Fault System within the San Gorgonio Pass region of Southern California. It explores the complex fault geometry, slip distribution, and seismic hazard implications through multidisciplinary studies, including geology, geophysics, and modeling. It aims to enhance understanding of fault behavior and earthquake risk assessment, serving as a model for studying restraining bends in strike-slip faults globally.

Geomorphic mapping and paleoseismologic data reveal evidence for a late Holocene multifault surface rupture along the Calico-Hidalgo fault system of the southern Eastern California Shear Zone (ECSZ). We have identified ~18 km of continuous surface rupture along the combined Calico and Hidalgo faults in the vicinity of Hidalgo Mountain in the southern Mojave Desert. Based on the freshness of geomorphic fault features and continuity of surface expression, we interpret this feature to reflect a simultaneous paleorupture of both faults. Displacement along the paleorupture is defined by 39 field measurements to be generally pure right-slip with a mean offset of 2.3 m. Scaling relationships for this offset amount imply that the original surface rupture length may have been ~82 km (corresponding to a M7.4 earthquake) and that much of the rupture trace was erased by subsequent erosion of sandy and unconsolidated valley alluvium. Eight luminescence ages from a paleoseismic trench across the paleorupture on the Hidalgo fault bracket the timing of the most recent rupture to 0.9-1.7 ka and a possible penultimate event at 5.5-6.6 ka. This timing is generally consistent with the known earthquake clusters in the southern ECSZ based on previous paleoseismic investigations. The ages of these earthquakes also overlap with the age brackets of the most recent events on the Calico fault 42 km to the north and the Mesquite Lake fault 40 km to the south from earlier work. Based on these age constraints and the expected surface rupture length, we propose that the Calico fault system experienced a major, multifault rupture that spanned the entire length of the fault system between the historical Landers and Hector Mine ruptures but preceded these events by ~1-2 k.y. Coulomb stress change modeling shows that the Calico paleorupture may have delayed the occurrence of the Landers-Hector Mine cluster by placing their respective faults in stress shadows and may have also prevented a triggered event from occurring on the Calico fault following the historic events. This work implies that closely spaced ruptures in complex shear zones may repel each other and thereby stretch out the duration of major earthquake clusters. These results also suggest that complex multifault ruptures in the ECSZ may not follow simple, repeatable patterns. [ABSTRACT FROM AUTHOR]

Earthquake surface-fault rupture location uncertainty is a key factor in fault displacement hazard analysis and informs hazard and risk mitigation strategies. Geologists often predict future rupture locations from fault mapping based on the geomorphology interpreted from remote-sensing data sets. However, surface processes can obscure fault location, fault traces may be mapped in error, and a future rupture may not break every fault trace. We assessed how well geomorphology-based fault mapping predicted surface ruptures for seven earthquakes: 1983 M 6.9 Borah Peak, 2004 M 6.0 Parkfield, 2010 M 7.2 El Mayor-Cucapah, 2011 M 6.7 Fukushima-Hamadori, 2014 M 6.0 South Napa, 2016 M 7.8 Kaikoura, and 2016 M 7 Kumamoto. We trained geoscience students to produce active fault maps using topography and imagery acquired before the earthquakes. A geologic professional completed a "control" map. Mappers used a new "geomorphic indicator ranking" approach to rank fault confidence based on geomorphologic landforms. We determined the accuracy of the mapped faults by comparing the fault maps to published rupture maps. We defined predicted ruptures as ruptures near a fault (50-200 m, depending on the fault confidence) that interacted with the landscape in a similar way to the fault. The mapped faults predicted between 12% to 68% of the principal rupture length for the studied earthquakes. The median separation distances between predicted ruptures and strong, distinct, or weak faults were 15-30 m. Our work highlights that mapping future fault ruptures is an underappreciated challenge of fault displacement hazard analysis--even for experts--with implications for risk management, engineering site assessments, and fault exclusion zones. [ABSTRACT FROM AUTHOR]

Typical rocks at shallow depths of seismogenic faults are fluid-rich gouges. During earthquakes, on-fault frictional heating may trigger thermal pressurization and dynamic fault weakening. We show that frictional melting, rather than thermal pressurization, occurred at shallow depths during the 2008 MW 7.9 Wenchuan earthquake, China. One year after the Wenchuan earthquake, we found an ~2-mm-thick, glass-bearing pseudotachylyte (solidified frictional melt) in the fault gouges retrieved at 732.6 m depth from the first borehole of the Wenchuan Earthquake Fault Scientific Drilling Project. The matrix of pseudotachylyte is enriched in barium and cut by barite-bearing veins, which provide evidence of co- and postseismic fluid percolation. Because pseudotachylyte can be rapidly altered in the presence of percolating fluids, its preservation suggests that gouge melting occurred in a recent large earthquake, possibly the Wenchuan earthquake. Rock friction experiments on fluid-rich fault gouges deformed at conditions expected for seismic slip at borehole depths showed the generation of pseudotachylytes. This result, along with the presence of a second slip zone attributed to the Wenchuan earthquake at 589.2 m depth, implies that during large earthquakes, frictional melting can occur at shallow depths and that seismic slip can be accommodated by multiple faults. This conclusion is consistent with the evidence from surface faulting that multiple ruptures propagated during the Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

The complex 2016 MW 7.8 Kaikōura earthquake ruptured >20 faults and caused highly variable uplift and subsidence of an ~110 km stretch of coastline. The earthquake raised questions about fault interactions in regions of oblique convergence and especially subduction to strike-slip transition zones like the Kaikōura region. We integrate 2016 coastal vertical deformation observations with new mapping and dating of Holocene marine terraces to: (1) compare spatial patterns of 2016 coseismic and longer-term vertical motions, (2) investigate possible past multi-fault ruptures or temporal clusters of earthquakes around Kaikōura, and (3) assess the relative contributions of crustal faults and the Hikurangi subduction interface to late Holocene coastal uplift. We identify possible multi-fault ruptures or loose clusters of earthquakes at ca. 850–550 yr B.P. and ca. 350–100 yr B.P. Most (and possibly all) of the Kaikōura coast has been uplifted over the late Holocene; the 25-km-long Parikawa section of coast subsided coseismically in 2016 but appears to be uplifted through reverse slip on an offshore fault. Late Holocene uplift everywhere along the coastline of interest can be attributed to slip on known upper-plate faults; slip on a shallow-dipping (<20°) subduction interface cannot be ruled out but is not required to explain uplift. [ABSTRACT FROM AUTHOR]

Coseismic landslides are observed in higher concentrations around surface-rupturing faults. This observation has been attributed to a combination of stronger ground motions and increased rock mass damage closer to faults. Past work has shown it is difficult to separate the influences of rock mass damage from strong ground motions on landslide occurrence. We measured coseismic off-fault deformation (OFD) zone widths (treating them as a proxy for areas of more intense rock mass damage) using high-resolution, three-dimensional surface displacements from the 2016 Mw 7.8 Kaikōura earthquake in New Zealand. OFD zones vary in width from ~50 m to 1500 m over the ~180 km length of ruptures analyzed. Using landslide densities from a database of 29,557 Kaikōura landslides, we demonstrate that our OFD zone captures a higher density of coseismic landslide incidence than generic "distance to fault rupture" within ~650 m of surface fault ruptures. This result suggests that the effects of rock mass damage within OFD zones (including ground motions from trapped and amplified seismic waves) may contribute to near-fault coseismic landslide occurrence in addition to the influence of regional ground motions, which attenuate with distance from the fault. The OFD zone represents a new path toward understanding, and planning for, the distribution of coseismic landslides around surface fault ruptures. Inclusion of estimates of fault zone width may improve landslide susceptibility models and decrease landslide risk. [ABSTRACT FROM AUTHOR]

Exhumed fault rocks provide a textural and chemical record of how fault zone composition and architecture control coseismic temperature rise and earthquake mechanics. We integrated field, microstructural, and hematite (U-Th)/He (He) thermochronometry analyses of exhumed minor (square-centimeter-scale surface area) hematite fault mirrors that crosscut the ca. 1400 Ma Sandia granite in two localities along the eastern flank of the central Rio Grande rift, New Mexico. We used these data to characterize fault slip textures; evaluate relationships among fault zone composition, thickness, and inferred magnitude of friction-generated heat; and document the timing of fault slip. Hematite fault mirrors are collocated with and crosscut specular hematite veins and hematite-cemented cataclasite. Observed fault mirror microstructures reflect fault reactivation and strain localization within the comparatively weaker hematite relative to the granite. The fault mirror volume of some slip surfaces exhibits polygonal, sintered hematite nanoparticles likely created during coseismic temperature rise. Individual fault mirror hematite He dates range from ca. 97 to 5 Ma, and ~80% of dates from fault mirror volume aliquots with high-temperature crystal morphologies are ca. 25-10 Ma. These aliquots have grain-size-dependent closure temperatures of ~75-108 °C. A new mean apatite He date of 13.6 ± 2.6 Ma from the Sandia granite is consistent with prior low-temperature thermochronometry data and reflects rapid, Miocene rift flank exhumation. Comparisons of thermal history models and hematite He data patterns, together with field and microstructural observations, indicate that seismicity along the fault mirrors at ~2-4 km depth was coeval with rift flank exhumation. The prevalence and distribution of high-temperature hematite grain morphologies on different slip surfaces correspond with thinner deforming zones and higher proportions of quartz and feldspar derived from the granite that impacted the bulk strength of the deforming zone. Thus, these exhumed fault mirrors illustrate how evolving fault material properties reflect but also govern coseismic temperature rise and associated dynamic weakening mechanisms on minor faults at the upper end of the seismogenic zone. [ABSTRACT FROM AUTHOR]

Normal faults are commonly segmented along strike, with segments that localize strain and influence propagation of slip during earthquakes. Although the geometry of segments can be constrained by fault mapping, it is challenging to determine seismically relevant segments along a fault zone. Because slip histories, geometries, and strengths of linkages between normal fault segments fundamentally control the propagation of rupture during earthquakes, and differences in segment slip rates result in differential uplift of adjacent footwalls, we used along-strike changes in footwall morphology to detect fault segments and the relative strength of the mechanical links between them. We applied a new geomorphic analysis protocol to the Wassuk Range fault, Nevada, within the actively deforming Walker Lane. The protocol examines characteristics of footwall morphology, including range-crest continuity, bedrock-channel long profiles, catchment area variability, and footwall relief, to detect changes in strike-parallel footwall characteristics. Results revealed six domains with significant differences in morphology that we used to identify seismically relevant fault segments and segment boundaries. We integrated our results with previous studies to determine relative strength of links between the six segments, informing seismic hazard assessment. When combined with recent geodetic studies, our results have implications for the future evolution of the Walker Lane, suggesting changes in the accommodation of strain across the region. Our analysis demonstrates the power of this method to efficiently detect along-strike changes in footwall morphology related to fault behavior, permitting future researchers to perform reconnaissance assessment of normal fault segmentation worldwide. [ABSTRACT FROM AUTHOR]

Subduction interfaces are loci of interdependent seismic slip behavior, fluid flow, and mineral redistribution. Mineral redistribution leads to coupling between fluid flow and slip behavior through decreases in porosity/permeability and increases in cohesion during the interseismic period. We investigate this system from the perspective of ancient accretionary complexes with regional zones of mélange that record noncoaxial strain during underthrusting adjacent to the subduction interface. Deformation of weak mudstones is accompanied by low-grade metamorphic reactions, dissolution along scaly microfaults, and the removal of fluid-mobile chemical components, whereas stronger sandstone blocks preserve veins that contain chemical components depleted in mudstones. These observations support local diffusive mass transport from scaly fabrics to veins during interseismic viscous coupling. Underthrusting sediments record a crack porosity that fluctuates due to the interplay of cracking and precipitation. Permanent interseismic deformation involves pressure solution slip, strain hardening, and the development of new shears in undeformed material. In contrast, coseismic slip may be accommodated within observed narrow zones of cataclastic deformation at the top of many mélange terranes. A kinetic model implies interseismic changes in physical properties in less than hundreds of years, and a numerical model that couples an earthquake simulator with a fluid flow system depicts a subduction zone interface governed by feedbacks between fluid production, permeability, hydrofracturing, and aging via mineral precipitation. During an earthquake, interseismic permeability reduction is followed by coseismic rupture of low permeability seals and fluid pressure drop in the seismogenic zone. Updip of the seismogenic zone, there is a post-seismic wave of higher fluid pressure that propagates trenchward. [ABSTRACT FROM AUTHOR]

Strike-slip faults can be nonplanar in both their strike and dip dimensions. While a large body of work has investigated the effects of changes in strike on earthquake rupture and arrest, no previous studies have investigated the role of along-strike variations in dip on strike-slip ruptures. Here, I use the three-dimensional finite-element method to conduct dynamic simulations of ruptures on strike-slip faults with linear surface traces and changes in dip along strike. I experiment with the amount of dip change as well as the abruptness of that change under a variety of initial stress conditions. In all of my initial stress cases, I find that a change in dip along strike can cause rupture to stop, and that larger dip changes over shorter distances are more likely to do so. This is largely due to the change in strike at depth that inherently comes from changing the dip; the majority of these behaviors are a result of the rupture front being forced to change direction mid-rupture. While some dip-slip movement does occur on the nonvertical parts of the model fault, it does not have a significant effect on rupture extent. However, linear-surface-trace, nonvertical-dip faults do produce different surface slip, stress, and ground motion patterns compared to corresponding nonlinear-strike, vertical-dip faults. Together, my results show that changes in dip along strike-slip faults do considerably impact the rupture process, suggesting that this type of geometrical complexity should be considered in rupture forecasts and hazard assessments. [ABSTRACT FROM AUTHOR]

Carbon dioxide emissions from dolomite decarbonation play an essential role in the weakening of carbonate faults by lowering the effective normal stress, which is thermally activated at temperatures above 600-700 °C. However, the mechanochemical effect of lowcrystalline ultrafine fault gouge on the decarbonation and slip behavior of dolomite-bearing faults remains unclear. In this study, we obtained a series of artificial dolomite fault gouges with systematically varying particle sizes and dolomite crystallinities using a high-energy ball mill. The laboratory-scale pulverization of dolomite yielded MgO at temperatures below 50 °C, indicating that mechanical decarbonation without significant heating occurred due to the collapse of the crystalline structure, as revealed by X-ray diffraction and solid-state nuclear magnetic resonance results. Furthermore, the onset temperature of thermal decarbonation decreased to ~400 °C. Numerical modeling reproduced this two-stage decarbonation, where the pore pressure increased due to low-temperature thermal decarbonation, leading to slip weakening on the fault plane even at 400-500 °C; i.e., 200-300 °C lower than previously reported temperatures. Thus, the presence of small amounts of low-crystalline dolomite in a fault plane may lead to a severely reduced shear strength due to thermal decomposition at ~400 °C with a small slip weakening distance. [ABSTRACT FROM AUTHOR]

Earthquake ground surface ruptures provide insights into faulting mechanics and inform seismic hazard analyses. We analyze surface ruptures for 11 historical (1968-2018) moment magnitude (Mw) 4.7-6.6 reverse earthquakes in Australia using statistical techniques and compare their characteristics with magnetic, gravity, and stress trajectory data sets. Of the total combined (summative) length of all surface ruptures (~148 km), 133 km (90%) to 145 km (98%) align with the geophysical structure in the host basement rocks. Surface rupture length (SRL), maximum displacement (MD), and probability of surface rupture at a specified Mw are high compared with equivalent Mw earthquakes globally. This is attributed to (1) a steep cratonic crustal strength gradient at shallow depths, promoting shallow hypocenters (~1-6 km) and limiting downdip rupture widths (~1-8.5 km), and (2) favorably aligned crustal anisotropies (e.g., bedrock foliations, faults, fault intersections) that enhanced lateral rupture propagation and/or surface displacements. Combined (modeled and observed) MDs are in the middle third of the SRL with 68% probability and either the ≤33rd or ≥66th percentiles of SRL with 16% probability. MD occurs proximate to or directly within zones of enhanced fault geometric complexity (as evidenced from surface ruptures) in 8 of 11 earthquakes (73%). MD is approximated by 3.3 ± 1.6 (1σ) x AD (average displacement). S-transform analyses indicates that high-frequency slip maxima also coincide with fault geometric complexities, consistent with stress amplifications and enhanced slip variability due to geometric and kinematic interactions with neighboring faults. Rupture slip taper angles exhibit large variations (-90% to +380% with respect to the mean value) toward rupture termini and are steepest where ruptures terminate at obliquely oriented magnetic lineaments and/or lithology changes. Incremental slip approximates AD between the 10th and 90th percentiles of the SRL. The average static stress drop of the studied earthquakes is 4.8 ± 2.8 MPa. A surface rupture classification scheme for cratonic stable regions is presented to describe the prevailing characteristics of intraplate earthquakes across diverse crustal structural-geophysical settings. New scaling relationships and suggestions for logic tree weights are provided to enhance probabilistic fault displacement hazard analyses for bedrock-dominated intraplate continental regions. [ABSTRACT FROM AUTHOR]

To evaluate ground deformation resulting from large (~10 m) coseismic strike-slip displacements, we focus on deformation of the Kekerengu fault during the November 2016 Mw 7.8 Kaikōura earthquake in New Zealand. Combining post-earthquake field observations with analysis of high-resolution aerial photography and topographic models, we describe the structural geology and geomorphology of the rupture zone. During the earthquake, fissured pressure bulges ("mole tracks") initiated at stepovers between synthetic Riedel (R) faults. As slip accumulated, near-surface "rafts" of cohesive clay-rich sediment, bounded by R faults and capped by grassy turf, rotated about a vertical axis and were internally shortened, thus amplifying the bulges. The bulges are flanked by low-angle contractional faults that emplace the shortened mass of detached sediment outward over less-deformed ground. As slip accrued, turf rafts fragmented into blocks bounded by short secondary fractures striking at a high angle to the main fault trace that we interpret to have originated as antithetic Riedel (R') faults. Eventually these blocks were dispersed into strongly sheared earth and variably rotated. Along the fault, clockwise rotation of these turf rafts within the rupture zone averaged ~20°-30°, accommodating a finite shear strain of 1.0-1.5 and a distributed strike slip of ~3-4 m. On strike-slip parts of the fault, internal shortening of the rafts averaged 1-2 m parallel to the R faults and ~1 m perpendicular to the main fault trace. Driven by distortional rotation, this contraction of the rafts exceeds the magnitude of fault heave. Turf rafts on slightly transtensional segments of the fault were also bulged and shortened-relationships that can be explained by a kinematic model involving "deformable slats." In a paleoseismic trench cut perpendicular the fault, one would observe fissures, low-angle thrusts, and steeply dipping strike-slip faults-some cross-cutting one another-yet all may have formed during a single earthquake featuring a large strike-slip displacement. [ABSTRACT FROM AUTHOR]

Active traces of the southern Fairweather fault were revealed by light detection and ranging (lidar) and show evidence for transpressional deformation between North America and the Yakutat block in southeast Alaska. We map the Holocene geomorphic expression of tectonic deformation along the southern 30 km of the Fairweather fault, which ruptured in the 1958 moment magnitude 7.8 earthquake. Digital maps of surficial geology, geomorphology, and active faults illustrate both strike-slip and dip-slip deformation styles within a 10°-30° double restraining bend where the southern Fairweather fault steps offshore to the Queen Charlotte fault. We measure offset landforms along the fault and calibrate legacy 14C data to reassess the rate of Holocene strike-slip motion (=49 mm/yr), which corroborates published estimates that place most of the plate boundary motion on the Fairweather fault. Our slip-rate estimates allow a component of oblique-reverse motion to be accommodated by contractional structures west of the Fairweather fault consistent with geodetic block models. Stratigraphic and structural relations in hand-dug excavations across two active fault strands provide an incomplete paleoseismic record including evidence for up to six surface ruptures in the past 5600 years, and at least two to four events in the past 810 years. The incomplete record suggests an earthquake recurrence interval of =270 years--much longer than intervals <100 years implied by published slip rates and expected earthquake displacements. Our paleoseismic observations and map of active traces of the southern Fairweather fault illustrate the complexity of transpressional deformation and seismic potential along one of Earth's fastest strike-slip plate boundaries. [ABSTRACT FROM AUTHOR]

We studied a paleoseismic trench excavated in 2017 across the Banning strand of the San Andreas fault and herein provide the first detailed record of ground-breaking earthquakes on this important fault in Southern California. The trench exposed an ~40-m-wide fault zone cutting through alluvial sand, gravel, silt, and clay deposits. We evaluated the paleoseismic record using a new metric that combines event indicator quality and stratigraphic uncertainty. The most recent paleoearthquake occurred between 950 and 730 calibrated years B.P. (cal yr B.P.), potentially contemporaneous with the last rupture of the San Gorgonio Pass fault zone. We interpret five surface-rupturing earthquakes since 3.3-2.5 ka and eight earthquakes since 7.1-5.7 ka. It is possible that additional events have occurred but were not recognized, especially in the deeper (older) section of the stratigraphy, which was not fully exposed across the fault zone. We calculated an average recurrence interval of 380-640 yr based on four complete earthquake cycles between earthquakes 1 and 5. The average recurrence interval is thus slightly less than the elapsed time since the most recent event on the Banning strand. The average recurrence interval on the Banning strand is thus intermediate between longer intervals published for the San Gorgonio Pass fault zone (~1600 yr) and shorter intervals on both the Mission Creek strand of the San Andreas fault (~215 yr) and the Coachella section (~125 yr) of the San Andreas fault. [ABSTRACT FROM AUTHOR]

Sulawesi Island is located at the triple junction between the converging Australian, Sunda, and Philippine plates. The magnitude (Mw) 7.5 Palu earthquake occurred on 28 September 2018 on Sulawesi Island and caused serious casualties. The causative fault of the Palu earthquake was the left-lateral, strike-slip Palu-Koro fault, which has a rapid slip rate. We experienced this earthquake in Palu City and conducted field investigations on coseismic surface ruptures 1 d after the earthquake. Field surveys revealed that the coseismic surface ruptures were characterized by left-lateral offset, en echelon tensional cracks, mole tracks within a narrow zone, and large areas of sand liquefaction that increased the damage and losses. We measured the coseismic displacements along surface ruptures and observed a maximum coseismic offset of ~6.2 m. The rupture traces in the north Palu Basin near Palu City mark the previously unmapped Palu-Koro fault. Based on the field investigations, we determined the exact location of the Palu-Koro fault within the Palu Basin and found that the Palu-Koro fault zone can be divided into three branches: F1, F2, and F3, forming a typical flower structure. [ABSTRACT FROM AUTHOR]

High-resolution three-dimensional terrain models are used to evaluate the Ragged Mountain fault kinematics (Katalla, Alaska, USA). Previous studies have produced contradictory interpretations of the fault's kinematics because surface ruptures along the fault are primarily steeply dipping, uphill-facing normal fault scarps. In this paper, we evaluate the hypothesis that these uphill-facing scarps represent extension above a buried thrust ramp. Detailed geomorphic mapping along the fault, using 20-cm-resolution aerial imagery draped onto a 1-m-resolution lidar (light detection and ranging) elevation model, was used to produce multiple topographic profiles. These profiles illustrate scarp geometries and prominent convex-upward topographic surfaces, indicating significant disturbance by active tectonics. A theoretical model is developed for fault-parallel flow over a thrust ramp that shows the geometric relationships between thrust displacement, upper-plate extension, and ramp dip. An important prediction of the model for this study is that the magnitude of upper-plate extension is comparable to, or greater than, the thrust displacement for ramps with dips greater than ~45°. This model is used to analyze profile shapes and surface displacements in Move software (Midland Valley Ltd.). Analyses of scarp heights allow estimates of hanging-wall extension, which we then use to estimate slip on the underlying thrust via the model. Assuming a low-angle (30°) uniformly dipping thrust and simple longitudinal extension via normal faulting, variations in extension along the fault would require a slip gradient from ~8 m in the north to ~22 m in the south. However, the same north-south variation in extension with a constant slip of 8-10 m may infer an increase in fault dip from ~30° in the north to ~60° in the south. This model prediction has broader implications for active-fault studies. Because the model quantifies relationships between hanging-wall extension, fault slip, and fault dip, it is possible to invert for fault slip in blind thrust ramps where hanging-wall extension is the primary surface manifestation. This study, together with results from the St. Elias Erosion and Tectonics Project (STEEP), clarifies the role of the Ragged Mountain fault as a contractional structure within a broadly sinistral shear system in the western syntaxis of the St. Elias orogeny. [ABSTRACT FROM AUTHOR]

Controls on the formation and distribution of mineralization in continental collisional settings remain unclear. However, our synthesis of diverse geophysical data sets from the eastern margin of Tibet revealed that differential crustal rotation played a key role in the production of a variety of mineralization types. Due to Cenozoic continental collision between India and Eurasia, the elongated continental blocks in the eastern margin of Tibet were extruded and reoriented. Prior to block extrusion in the Eocene, two giant porphyry-skarn ore clusters formed at the boundaries between the central segment and both the northern and southern segments of the Jinshajiang-Ailaoshan suture zone. These crustal segment boundaries displayed counterclockwise rotation, due to clockwise rotation of the central segment relative to both the essentially immobile northern and southern segments, combined with crust-mantle decoupling. This is considered to have induced crustal friction and resultant generation of fertile magmas that formed the porphyry-skarn Cu-Au deposits. During Oligocene-Miocene block extrusion, differential rotation of upper crust occurred on the western and eastern sides of the north-northwest-trending Central Axis fault in the Lanping-Simao basin. Two Oligocene-Miocene Mississippi Valley-type ore clusters occur on fault segments with anomalous differential rotation of 70° to 80°, suggesting that this differential rotation resulted in local extension with consequent ore-fluid influx. [ABSTRACT FROM AUTHOR]

Seismic activity below the standard seismogenic zone is difficult to investigate because the geological records of such earthquakes, pseudotachylytes, are typically reacted and/or deformed. Here, we describe unusually pristine pseudotachylytes in lower-crustal granulites from the Lofoten Archipelago, northern Norway. The pseudotachylytes have essentially the same mineralogical composition as their host (mainly plagioclase, alkali feldspar, orthopyroxene) and contain microstructures indicative of rapid cooling, i.e., feldspar microlites and spherulites and "cauliflower" garnets. Mylonites are absent, both in the wall rocks and among the pseudotachylyte clasts. The absence of features recording precursory ductile deformation rules out several commonly invoked mechanisms for triggering earthquakes in the lower crust, including thermal runaway, plastic instabilities, and downward propagation of seismic slip from the brittle to the ductile part of a fault. The anhydrous mineralogy of host and pseudotachylytes excludes dehydration-induced embrittlement. In the absence of such weakening mechanisms, stress levels in the lower crust must have been transiently high. [ABSTRACT FROM AUTHOR]

The 72-km-long Teton normal fault bounds the eastern base of the Teton Range in northwestern Wyoming, USA. Although geomorphic surfaces along the fault record latest Pleistocene to Holocene fault movement, the postglacial earthquake history of the fault has remained enigmatic. We excavated a paleoseismic trench at the Buffalo Bowl site along the southernmost part of the fault to determine its Holocene rupture history and slip rate. At the site, 6.3 m of displacement postdates an early Holocene (ca. 10.5 ka) alluvial-fan surface. We document evidence of three surface-faulting earthquakes based on packages of scarp-derived colluvium that postdate the alluvial-fan units. Bayesian modeling of radiocarbon and luminescence ages yields earthquake times of ca. 9.9 ka, ca. 7.1 ka, and ca. 4.6 ka, forming the longest, most complete paleoseismic record of the Teton fault. We integrate these data with a displaced deglacial surface 4 km NE at Granite Canyon to calculate a postglacial to mid-Holocene (14.4-4.6 ka) slip rate of 1.1 mm/yr. Our analysis also suggests that the postglacial to early Holocene (14.4-9.9 ka) slip rate exceeds the Holocene (9.9-4.6 ka) rate by a factor of 2 (maximum of 3); however, a uniform rate for the fault is possible considering the 95% slip-rate errors. The 5 k.y. elapsed time since the last rupture of the southernmost Teton fault implies a current slip deficit of 4-5 m, which is possibly explained by spatially/temporally incomplete paleoseismic data, irregular earthquake recurrence, and/or variable per-event displacement. Our study emphasizes the importance of minimizing slip-rate uncertainties by integrating paleoseismic and geomorphic data sets and capturing multiple earthquake cycles. [ABSTRACT FROM AUTHOR]

In this work, we report the results of a multidisciplinary study describing the structural architecture and diagenetic evolution of the Rocca di Neto extensional fault zone developed in poorly lithified sandstones of the Crotone Basin, Southern Italy. The studied fault zone has an estimated displacement of ~90 m and consists of: (1) a low-deformation zone with subsidiary faults and widely spaced deformation bands; (2) an ~10-mwide damage zone, characterized by a dense network of conjugate deformation bands; (3) an ~3-m-wide mixed zone produced by tectonic mixing of sediments with different grain size; (4) an ~1-m-wide fault core with bedding transposed into foliation and ultracomminute black gouge layers. Microstructural investigations indicate that particulate flow was the dominant early-stage deformation mechanism, while cataclasis became predominant after porosity loss, shallow burial, and selective calcite cementation. The combination of tectonic compaction and preferential cementation led to a strainhardening behavior inducing the formation of "inclined conjugate deformation band sets" inside the damage zone, caused by the kinematic stress field associated with fault activity. Conversely, conjugate deformation band sets with a vertical bisector formed outside the damage zone in response to the regional extensional stress field. Stable isotope analysis helped in constraining the diagenetic environment of deformation, which is characterized by mixed marine-meteoric signature for cements hosted inside the damage zone, while it progressively becomes more meteoric moving outside the fault zone. This evidence supports the outward propagation of fault-related deformation structures in the footwall damage zone. [ABSTRACT FROM AUTHOR]

Data from International Ocean Discovery Program (IODP) Expedition 371 reveal vertical movements of 1-3 km in northern Zealandia during early Cenozoic subduction initiation in the western Pacific Ocean. Lord Howe Rise rose from deep (~1 km) water to sea level and subsided back, with peak uplift at 50 Ma in the north and between 41 and 32 Ma in the south. The New Caledonia Trough subsided 2-3 km between 55 and 45 Ma. We suggest these elevation changes resulted from crust delamination and mantle flow that led to slab formation. We propose a "subduction resurrection" model in which (1) a subduction rupture event activated lithospheric-scale faults across a broad region during less than ~5 m.y., and (2) tectonic forces evolved over a further 4-8 m.y. as subducted slabs grew in size and drove plate-motion change. Such a subduction rupture event may have involved nucleation and lateral propagation of slip-weakening rupture along an interconnected set of preexisting weaknesses adjacent to density anomalies. [ABSTRACT FROM AUTHOR]

The Gales Creek fault (GCF) is a 60-km-long, northwest-striking dextral fault system (west of Portland, Oregon) that accommodates northward motion and uplift of the Oregon Coast Range. New geologic mapping and geophysical models confirm inferred offsets from earlier geophysical surveys and document ~12 km of right-lateral offset of a basement high in Eocene Siletz River Volcanics since ca. 35 Ma and ~8.8 km of right-lateral separation of Miocene Columbia River Basalt at Newberg, Oregon, since 15 Ma (~0.62 ± 0.12 mm/yr, average long-term rate). Relative uplift of Eocene Coast Range basalt basement west of the fault zone is at least 5 km based on depth to basement under the Tualatin Basin from a recent inversion of gravity data. West of the city of Forest Grove, the fault consists of two subparallel strands ~7 km apart. The westernmost, Parsons Creek strand, forms a linear valley southward to Henry Hagg Lake, where it continues southward to Newberg as a series of en echelon strands forming both extensional and compressive step-overs. Compressive step-overs in the GCF occur at intersections with ESE-striking sinistral faults crossing the Coast Range, suggesting the GCF is the eastern boundary of an R′ Riedel shear domain that could accommodate up to half of the ~45° of post- 40 Ma clockwise rotation of the Coast Range documented by paleomagnetic studies. Gravity and magnetic anomalies suggest the western strands of the GCF extend southward beneath Newberg into the Northern Willamette Valley, where colinear magnetic anomalies have been correlated with the Mount Angel fault, the proposed source of the 1993 M 5.7 Scotts Mills earthquake. The potential-field data and water-well data also indicate the eastern, Gales Creek strand of the fault may link to the NNW-striking Canby fault through the E-W Beaverton fault to form a 30-km-wide compressive step-over along the south side of the Tualatin Basin. LiDAR data reveal right-lateral stream offsets of as much as 1.5 km, shutter ridges, and other youthful geomorphic features for 60 km along the geophysical and geologic trace of the GCF north of Newberg, Oregon. Paleoseismic trenches document Eocene bedrock thrust over 250 ka surficial deposits along a reverse splay of the fault system near Yamhill, Oregon, and Holocene motion has been recently documented on the GCF along Scoggins Creek and Parsons Creek. The GCF could produce earthquakes in excess of Mw 7, if the entire 60 km segment ruptured in one earthquake. The apparent subsurface links of the GCF to other faults in the Northern Willamette Valley suggest that other faults in the system may also be active. [ABSTRACT FROM AUTHOR]

The 1983 Mw 6.9 Borah Peak earthquake generated -36 km of surface rupture along the Thousand Springs and Warm Springs sections of the Lost River fault zone (LRFZ, Idaho, USA). Although the rupture is a well-studied example of multisegment surface faulting, ambiguity remains regarding the degree to which a bedrock ridge and branch fault at the Willow Creek Hills influenced rupture progress. To explore the 1983 rupture in the context of the structural complexity, we reconstruct the spatial distribution of surface displacements forthe northern 16 km of the 1983 rupture and prehistoric ruptures in the same reach of the LRFZ using 252 vertical-separation measurements made from high-resolution (5-10-cm-pixel) digital surface models. Our results suggest the 1983 Warm Springs rupture had an average vertical displacement of -0.3-0.4 m and released -6% of the seismic moment estimated for the Borah Peak earthquake and <12% of the moment accumulated on the Warm Springs section since its last prehistoric earthquake. The 1983 Warm Springs rupture is best described as the moderate-displacement continuation of primary rupture from the Thousand Springs section into and through a zone of structural complexity. Historical and prehistoric displacements show that the Willow Creek Hills have impeded some, but not all ruptures. We speculate that rupture termination or penetration is controlled by the history of LRFZ moment release, displacement, and rupture direction. Our results inform the interpretation of paleoseismic data from near zones of normal-fault structural complexity and demonstrate that these zones may modulate rather than impede rupture displacement. [ABSTRACT FROM AUTHOR]

The so-called "brittle-ductile transition" is thought to be the strongest part of the lithosphere, and defines the lower limit of the seismogenic zone. It is characterized not only by a transition from localized to distributed (ductile) deformation, but also by a gradual change in microscale deformation mechanism, from microcracking to crystal plasticity. These two transitions can occur separately under different conditions. The threshold conditions bounding the transitions are expected to control how deformation is partitioned between localized fault slip and bulk ductile deformation. Here, we report results from triaxial deformation experiments on pre-faulted cores of Carrara marble over a range of confining pressures, and determine the relative partitioning of the total deformation between bulk strain and on-fault slip. We find that the transition initiates when fault strength (σf) exceeds the yield stress (σy) of the bulk rock, and terminates when it exceeds its ductile flow stress (σflow). In this domain, yield in the bulk rock occurs first, and fault slip is reactivated as a result of bulk strain hardening. The contribution of fault slip to the total deformation is proportional to the ratio (σf - σy)/(σflow - σy). We propose an updated crustal strength profile extending the localized-ductile transition toward shallower regions where the strength of the crust would be limited by fault friction, but significant proportions of tectonic deformation could be accommodated simultaneously by distributed ductile flow. [ABSTRACT FROM AUTHOR]

We present a reconstruction of episodic fluid flow over the past ∼250 k.y. along the Malpais normal fault, which hosts the Beowawe hydrothermal system (Nevada, USA), using a novel combination of the apatite (U-Th)/He (AHe) thermochronometer and a model of the thermal effects of fluid flow. Samples show partial resetting of the AHe thermochronometer in a 40-m-wide zone around the fault. Numerical models using current fluid temperatures and discharge rates indicate that fluid flow events lasting 2 k.y. or more lead to fully reset samples. Episodic fluid pulses lasting 1 k.y. result in partially reset samples, with 30–40 individual fluid pulses required to match the data. Episodic fluid flow is also supported by an overturned geothermal gradient in a borehole that crosses the fault, and by breaks in stable isotope trends in hydrothermal sinter deposits that coincide with two independently dated earthquakes in the past 20 k.y. This suggests a system where fluid flow is triggered by repeated seismic activity, and that seals itself over ∼1 k.y. due to the formation of clays and silicates in the fault damage zone. Hydrothermal activity is younger than the 6–10 Ma age of the fault, which means that deep (∼5 km) fluid flow was initiated only after a large part of the 230 m of fault offset had taken place. [ABSTRACT FROM AUTHOR]

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

New ages and strain analysis of syntectonic calcite from the west-central part of the North Anatolian fault zone (NAFZ; northern Turkey) shed light on the debate over the origin and reactivation behavior of this major active fault system. Contrasting views suggest that the west-central part of the NAFZ was already active during the mid-Eocene, mid-Miocene, or early Pliocene. The deformation style is also controversial: early stages of activity are associated with strike-slip, thrust, or normal faulting. Strike-slip deformation of the NAFZ marks the onset of tectonic escape of the Anatolian block westwards and is therefore a key player in tectonic reconstructions. We sampled syntectonic calcite from polished fault surfaces with horizontal slickenlines at four key sites along a 190-km-long sector of the NAFZ. All sites still show active faulting during the Holocene. Combined U-Pb ages (laser ablation--multicollector--inductively coupled plasma--mass spectrometry) and strain analyses of mechanically twinned calcite indicate that the west-central part of the NAFZ was associated with an extensional normal fault system at 42 Ma and reactivated as a dextral strike-slip fault at least 11 m.y. ago. The results indicate that the NAFZ evolved through the reactivation of major preexisting fault structures rather than by a gradual east (13 Ma) to west (5 Ma) propagation. The onset of westward translation of the Anatolian block occurred at least 11 m.y. ago, possibly as a result of the Miocene collision between the Arabian plate and Anatolian block. [ABSTRACT FROM AUTHOR]

Pseudotachylyte veins are solidified frictional melt generated by localized highstrain- rate deformation, and they are commonly considered to be reliable geological indicators of seismic slip. While originally thought to occur primarily in the upper crust, field studies suggest pseudotachylyte can also form far below the frictional-viscous transition. This implies a more complex inter play between brittle and crystal-plastic deformation in crustal-scale fault systems. We report a new example of deep pseudo tachylyte from the Paleoproterozoic granulite-facies strikeslip Cora Lake shear zone in northern Saskatchewan, Canada. Multiple generations of pseudotachylyte have been variably overprinted by plastic deformation and growth of high-grade metamorphic mineral assemblages (including garnet, ortho pyroxene, and/or clinopyroxene). Undeformed pseudotachylyte generation surfaces locally display sinistral offsets, and deformed pseudotachylyte shows unambiguous shear sense indicators compatible with left-lateral Cora Lake shear zone kinematics. The vein networks are commonly proximal to and inherit kinematically compatible semibrittle shear fractures. Thermobarometry and modeling of mineral stability fields for the overprinted pseudotachylyte suggest vein generation between 0.7 and 0.8 GPa and 700 °C and 800 °C (corresponding to depths from 24 to 28 km). Combined with the kinematic compatibility and the well-constrained regional exhumation history, this suggests that the Cora Lake pseudotachylyte formed contemporaneously with the waning stages of deformation in the shear zone and constitutes one of the deepest examples of fault-related pseudotachylyte yet reported in North America. Evidence exists for several possible formation mechanisms, including in situ generation via ductile shear heating--induced plastic instabilities, as well as from interaction with earthquakes nucleating in the overlying brittle fault system. All of these possible mechanisms ultimately suggest a more rheologically complex environment than might typically be expected for these lower-crustal pressure and temperature conditions. [ABSTRACT FROM AUTHOR]

The Vincent fault in the San Gabriel Mountains, southern California, which forms the contact between the Proterozoic gneisses and Mesozoic magmatic arc in the upper plate and the subduction-associated Pelona Schist in the lower plate, has long been regarded as the Laramide subduction zone megathrust. Our investigation of the structural and thermal evolution of the Vincent fault and the associated mylonite zone requires a reevaluation of its tectonic significance. The base of the upper plate and the uppermost section of the lower plate are involved in a mylonite zone up to 1 km thick. Two mylonitic subzones are recognized within the upper-plate mylonites. The lower mylonite zone records pressuretemperature conditions of 0.46-0.52 GPa and 362-409 °C. Estimated shear stresses for the lower mylonite zone based on the recrystallized quartz grain size lie between 17.0 and 29.5 MPa. The upper mylonite zone records pressure-temperature conditions of 0.19-0.22 GPa and 323-341 °C, and shear stresses between 46.9 and 71.4 MPa. The microstructures show that part of the lower mylonite zone was overprinted by the upper mylonite zone and that the mylonites were formed during retrogressive metamorphism and exhumation. Sense of shear indicators preserved in the mylonitized Pelona Schist and the mylonitic gneisses consistently show that the upper plate moved southeastward relative to the lower plate. Zircon fission-track analysis suggests that the upper plate of the Vincent fault is not cut by any major faults, except for late Miocene to recent strike-slip faults of the San Andreas system. These observations do not support the idea that the Vincent fault is the Laramide subduction zone megathrust; instead, we conclude that it is a normal fault that contributed to the exhumation of the Pelona Schist. [ABSTRACT FROM AUTHOR]

The San Andreas and San Jacinto faults are the primary plate-boundary structures in southern California and present a large earthquake hazard for the region. They approach each other in the Cajon Pass area between the San Gabriel and San Bernardino Mountains, where the northern end of the San Jacinto fault forms a 2-km-wide releasing step with the San Andreas fault. In this study, we used paleoseismic data from sites on the San Jacinto and San Andreas faults near their juncture to evaluate spatial and temporal patterns of surface rupture between these major structures of the North American-Pacific transform plate boundary. We present a new 3700 yr paleoseismic record from the northern San Jacinto fault at Mystic Lake, where trench excavations exposed evidence for at least 16 surface ruptures. A sedimentary gap in our trench exposures separates three ruptures in the oldest part of the record from 13 ruptures during the past 2000 yr. For the past 2000 yr, the mean recurrence times varied from 86 to 312 yr, with a mean recurrence interval of 160 yr. This rate of surface rupture is roughly equal to that of the southern San Andreas fault south of the Cajon Pass juncture, but it is half that of the San Andreas fault north of the juncture, indicating that coseismic strain on the San Andreas fault is split between the southern San Andreas and San Jacinto faults south of Cajon Pass. Comparison of the past 2000 yr of the Mystic Lake record to similar paleoseismic records from nearby sections of the San Andreas fault suggests that: (1) the current open interval on these two faults in the study area is longer than their average recurrence intervals, but that similar intervals of quiescence have occurred in the past 2000 yr; (2) the San Andreas and San Jacinto faults have probably ruptured together multiple times in the past 2000 yr; and (3) a joint rupture of the San Jacinto fault with the Mojave section of the San Andreas fault may be a more likely source of a major earthquake in southern California than a rupture on the San Andreas fault from the Mojave segment to the southern end of the fault. [ABSTRACT FROM AUTHOR]

Bedrock fracture is a key element of rock erosion and subsequent surface processes. Here, we test the hypothesis that rock's susceptibility to subcritical cracking, a specific type of fracturing, significantly drives and limits rock erosion. We measured 10Be-derived erosion rates, compressive strength, and crack characteristics on 20 outcrops of different rock units (quartzite, granite, and two metasandstones) in the northern Blue Ridge Mountains of Virginia (USA). We also measured the subcritical cracking index (n), Charles's law velocity constant (A), and fracture toughness (KIC) of samples from four of the same outcrops, representative of each rock type. Erosion rates range from 1.16 ± 0.67 to 32.3 ± 7.8 m/m.y. We find strong correlations--across the four rock units--between average erosion rates and the three subcritical cracking parameters (R² > 0.85, p < 0.05), but not compressive strength (R² = 0.6; p > 0.1). We also find a correlative relationship between n and outcrop fracture length (R² = 0.91; p < 0.05). The latter correlation is consistent with that of published model predictions, further indicating a mechanistic link between subcritical cracking and rock erosion. We infer that subcritical cracking parameters closely tie to erosion rates, because subcritical cracking is the dominant process of mechanical weathering, leading to positive feedbacks relating subcritical cracking rates, crack length, porosity, and water accessibility. These data are the first that directly test and support the hypothesis that subcritical cracking can set the pace of long-term rock erosion. [ABSTRACT FROM AUTHOR]

Data from an amphibious seismic network in Cascadia (northwest North America) provide unique near-source observations to assess the influence of subducting topography on seismicity. Using subspace detection, we detect and locate 222 events in two separate clusters, near a subducted seamount and a possibly accreted seamount. Seismicity in both clusters is largely shallower than the plate interface and exhibits occasional swarm-like behavior. This implies that the seamount is subducting aseismically via weak coupling with the overriding plate, while earthquakes in the upper plate arise from a high degree of fracturing due to seamount interaction, and the accreted seamount induced similar fracturing before off-scraping. [ABSTRACT FROM AUTHOR]

In extensional geologic systems such as mid-ocean ridges, deformation is typically accommodated by slip on normal faults, where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion. However, at slowly spreading mid-ocean ridges where the tectonic plates move apart at rates <80 km m.y.-1, these normal faults may roll over to form long-lived, low-angled detachments that exhume mantle rocks and form corrugated domes on the seabed. Here we present the results of a local micro-earthquake study over an active detachment at 13°20'N on the Mid-Atlantic Ridge to show that these features can give rise to reverse-faulting earthquakes in response to plate bending. During a 6 month survey period, we observed a remarkably high rate of seismic activity, with >244,000 events detected along 25 km of the ridge axis, to depths of ~10 km below seafloor. Surprisingly, the majority of these were reverse-faulting events. Restricted to depths of 3-7 km below seafloor, these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events. This deformation pattern is consistent with flexural models of plate bending during lithospheric accretion. Our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor. These compressive stresses trigger reverse faulting even though the detachment itself is an extensional system. [ABSTRACT FROM AUTHOR]

Many recent thrust fault earthquakes have involved coseismic surface faulting and surface folding strains. This multifaceted nature of active thrust sheet deformation can make attempts to quantify slip and slip rates from surface strains challenging and uncertain. We present new methods for integrating records of surface deformation, subsurface structure, and geochronology to investigate active deformation over multiple rupture cycles across the Southern Junggar Thrust in the southern Junggar Basin, NW China, from ~225 ka to present. Fluvial terraces preserve records of surface faulting as a prominent fault scarp where the Southern Junggar Thrust is surface-emergent. Terraces also exhibit progressive folding strains across fold scarps that are spatially coincident with subsurface fault-bend folds--constrained by seismic reflection data--along the Southern Junggar Thrust. We quantify the fault slip at depth implied by fold scarp relief along Holocene-aged terraces, and the results are corroborated by independent estimates of slip implied by fault scarp relief for the same terraces. Older terraces exhibit a distinct fanning of dips across fold scarps, suggesting active fault-bend folding kinematics involving a component of limb rotation. We developed quantitative relations for fault-bend folds between fault slip and fold dip using a mechanical, forward modeling approach. Using this novel method, we show how Southern Junggar Thrust slip rate has decelerated markedly, from ~4.1 mm/yr in the middle Quaternary to ~1.2 mm/yr throughout the Holocene. These results provide new insight into natural fault-bend folding kinematics and define innovative methods for elucidating accurate estimates of fault slip and slip rates from terrace folds in active thrust sheets. [ABSTRACT FROM AUTHOR]

Natural fracture networks exert a firstorder control on the exploitation of resources such as aquifers, hydrocarbons, and geothermal reservoirs, and on environmental issues like underground gas storage and waste disposal. Fractures and the mechanical stratigraphy of layered sequences have been intensively studied to unravel the relationships between bed thickness and fracture spacing, but less attention has been paid to intrabed fracturing patterns due to the intrinsic local variability of sedimentary processes and products. Among sedimentary rocks, turbidites show great lateral and vertical variability of textural characteristics and depositional facies, which are expected to strongly influence the location and density of fractures. To better understand the contribution of stratigraphic, sedimentologic, and petrophysical properties on fracture patterns, we performed a high-resolution study on a selected stratigraphic interval of jointed foredeep turbidites in the Miocene Marnoso- Arenacea Formation (Northern Apennines, Italy). Cumulative statistical relations of field and laboratory structural, sedimentologic, and petrophysical data significantly improved when analyzed at the sedimentary facies scale. In particular, for facies recording different cross-flow (i.e., longitudinal to the paleocurrents) depositional conditions within the parent turbidity currents, we observed three-dimensional anisotropies of rock hardness (i.e., uniaxial compression) that were positively correlated with normalized fracture intensities, indicating a primary sedimentary control on fracture distribution. This type of intrabed joint distribution has crucial practical implications for the lateral prediction and evaluation of mesoscale fracture patterns in turbidite sequences. [ABSTRACT FROM AUTHOR]

The very large slip up to the subduction front of the 2011 Tohoku-Oki (Japan) earthquake has challenged our classic view of the megathrust undergoing only aseismic slip at shallow depth. Furthermore, the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks. Recent seismic reflection images from the Sumatra subduction zone show frontal landward-vergent thrusts in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis. Using mechanical analysis, we here show that sudden and successive decreases of the effective friction along the megathrust are required to form landward-vergent frontal thrusts. These decreases are most likely caused by dynamic weakening mechanisms, such as thermal pressurization of the pore fluids related to the propagation of earthquakes to the seafloor. Therefore, landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis. The presence of landward-verging structures in the Cascadia and Sumatra accretionary prisms might indicate future frontal rupture of the shallowest portion of the megathrust, resulting in large tsunamis. [ABSTRACT FROM AUTHOR]

The ongoing collision of India with Asia is partly accommodated by slip on the Main Himalayan Thrust (MHT). The 25 April 2015, Mw 7.8 Gorkha earthquake is the most recent major event to rupture the MHT, which dips gently northward beneath central Nepal. Although the geology of the range has been studied for decades, fundamental aspects of its deep structure remain disputed. Here, we develop a structural cross section and a three-dimensional, geologically informed model of the MHT that are consistent with seismic observations from the Gorkha earthquake. A comparison of our model to a detailed slip inversion data set shows that the slip patch closely matches an ovalshaped, gently dipping fault surface bounded on all sides by steeper ramps. The Gorkha earthquake rupture seems to have been limited by the geometry of that fault segment. This is a significant step forward in understanding the deep geometry of the MHT and its effect on earthquake nucleation and propagation. Published models of fault locking do not correlate with the slip patch or our fault model in the vicinity of the earthquake, further suggesting that fault geometry was the primary control on this event. Our result emphasizes the importance of adequately constraining subsurface fault geometry in megathrusts in order to better assess the sizes and locations of future earthquakes. [ABSTRACT FROM AUTHOR]

At the field scale, nearly all fault surfaces contain grooves generated as one side of the fault slips past the other. Grooves are so common that they are one of the key indicators of principal slip surfaces. Here, we show that at sufficiently small scales, grooves do not exist on fault surfaces. A transition to isotropic roughness occurs at 4-500 mm. Although the scale of the transition can vary even between locales on a single fault, the aspect ratio of the roughness at the transition is well defined for a given fault. We interpret the transition between grooved and ungrooved scales as a transition in deformation mode of asperities on the slip surface. Grooves can form when a hard indenter slides past a softer surface. At small scales, the asperities appear to yield plastically and therefore do not generate grooves as hard indenters. The plastic yielding can be a consequence of the high shear strains required to deform the surfaces at small scales where the aspect ratio (roughness) is high. The transition to plastic yielding is predicted to occur at a specific aspect ratio for each fault, as observed. The new observation both shows a limit to one of the most commonly observed features of faults and suggests a change in the mode of failure of faults as a function of scale. [ABSTRACT FROM AUTHOR]

Various methods have been proposed to measure fracture aperture distributions, including X-ray computed tomography (CT) imaging, which has the advantage that it can be combined with dynamic flow experiments. In this paper, we present a calibration-free missing CT attenuation (CFMA) imaging method for measuring fracture apertures that avoids time-consuming calibration. In addition, this model does not assume a homogeneous matrix and thus provides a good estimate of fracture apertures even when rock properties are heterogeneous. The validity of the CFMA model is established by four approaches: comparing apertures calculated with the conventional calibration-based method; evaluating model predictability at different scanner voxel sizes; comparing with calibration coefficients in the literature from a number of experiments with different rocks and X-ray scanners; and comparing aperture measurements for dry and wet scans. We analyze the systematic error and the random error introduced by rock heterogeneities and CT scanning and show that by averaging 5 replicate scans, we reduce the aperture measurement error to ~22 µm. [ABSTRACT FROM AUTHOR]

Determining the earthquake segmentation of plate-boundary transform faults remains a scientific challenge because paleoseismic data sets rarely resolve the end points of past ruptures. In this study, we test whether lacustrine paleoseismology can be used to assess rupture end points and the earthquake segmentation of the Alpine fault, one of the longest and fastest- slipping plate-boundary transform faults on Earth. Sediment cores from Lake Ellery record eight episodes of high-intensity shaking (modified Mercalli intensity [MM] IX) from Alpine fault earthquakes as event sequences of a turbidite produced by coseismic subaqueous mass wasting, overlain by deposits representing sediment flux from co- and postseismic landsliding in the fluvial catchment. Age-depth modeling constrains the timing of shaking events at a decadal resolution, facilitating correlation with two previously published lake records to reconstruct the spatial distribution of MM IX shaking along ~150 km of the Alpine fault. When resolved with existing on- and near-fault paleoseismic records, the lake data set demonstrates that independent ruptures of the South Westland and Central segments occurred in A.D. 845-775 and A.D. 739-646, and A.D. 646-592 and A.D. 416-370, respectively. Lakes adjacent to the Alpine fault provide paleoseismic records with sufficient spatial and temporal resolution to define along-strike differences in the pattern of rupture capable of distinguishing rupture termination at a geometric segment boundary. This multilake study suggests that locating the end points of ruptures using lacustrine paleoseismology will be most applicable in midlatitude convergent plate-boundary settings where along-strike topography and Quaternary glaciation have resulted in the widespread distribution of suitable lakes. [ABSTRACT FROM AUTHOR]

Principal slip surfaces in faults have measurable roughness generated during slip. The roughness both records previous events and poses the boundary conditions for future rupture. Digital, high-precision roughness data are now available at the field scale (tens of centimeters to tens of meters) for at least 22 faults, and at the laboratory scale (millimeters to tens of centimeters) for a subset of these. We quantify the slip surface roughness by measuring the aspect ratio, which is the average asperity height divided by the profile length. Higher aspect ratios indicate rougher surfaces. From the field studies, two major trends have emerged: (1) fault surface roughness lies in a restricted range with aspect ratios in the slip-parallel direction of 0.07%-0.5% for profiles of 1 m length, and (2) fault surfaces are rougher at small scales than large ones. These features can both be interpreted as fingerprints of scale-dependent strength, which sets a limit to the aspect ratio of the surface. The measurements imply that shear strength scales with the observation scale, L, as L-0.4 A. The new understanding of the physical controls on roughness allows generalization of the extant measurements of a wide array of faults. [ABSTRACT FROM AUTHOR]

Studies of active fault zones have flourished with the availability of high-resolution topographic data, particularly where airborne light detection and ranging (lidar) and structure from motion (SfM) data sets provide a means to remotely analyze submeter- scale fault geomorphology. To determine surface offset at a point along a strike-slip earthquake rupture, geomorphic features (e.g., stream channels) are measured days to centuries after the event. Analysis of these and cumulatively offset features produces offset distributions for successive earthquakes that are used to understand earthquake rupture behavior. As researchers expand studies to more varied terrain types, climates, and vegetation regimes, there is an increasing need to standardize and uniformly validate measurements of tectonically displaced geomorphic features. A recently compiled catalog of nearly 5000 earthquake offsets across a range of measurement and reporting styles provides insight into quality rating and uncertainty trends from which we formulate best-practice and reporting recommendations for remote studies. In addition, a series of public and beginner-level studies validate the remote methodology for a number of tools and emphasize considerations to enhance measurement accuracy and precision for beginners and professionals. Our investigation revealed that (1) standardizing remote measurement methods and reporting quality rating schemes is essential for the utility and repeatability of fault-offset measurements; (2) measurement discrepancies often involve misinterpretation of the offset geomorphic feature and are a function of the investigator's experience; (3) comparison of measurements made by a single investigator in different climatic regions reveals systematic differences in measurement uncertainties attributable to variation in feature preservation; (4) measuring more components of a displaced geomorphic landform produces more consistently repeatable estimates of offset; and (5) inadequate understanding of preevent morphology and post-event modifications represents a greater epistemic limitation than the aleatoric limitations of the measurement process. [ABSTRACT FROM AUTHOR]

Detailed analysis of continuously cored boreholes and cone penetrometer tests (CPTs), high-resolution seismic-reflection data, and luminescence and 14C dates from Holocene strata folded above the tip of the Ventura blind thrust fault constrain the ages and displacements of the two (or more) most recent earthquakes. These two earthquakes, which are identified by a prominent surface fold scarp and a stratigraphic sequence that thickens across an older buried fold scarp, occurred before the 235-yr-long historic era and after 805 ± 75 yr ago (most recent folding event[s]) and between 4065 and 4665 yr ago (previous folding event[s]). Minimum uplift in these two scarp-forming events was ~6 m for the most recent earthquake(s) and ~5.2 m for the previous event(s). Large uplifts such as these typically occur in large-magnitude earthquakes in the range of Mw 7.5-8.0. Any such events along the Ventura fault would likely involve rupture of other Transverse Ranges faults to the east and west and/or rupture downward onto the deep, low-angle décollements that underlie these faults. The proximity of this large reverse-fault system to major population centers, including the greater Los Angeles region, and the potential for tsunami generation during ruptures extending offshore along the western parts of the system highlight the importance of understanding the complex behavior of these faults for probabilistic seismic hazard assessment. [ABSTRACT FROM AUTHOR]

Earthquake prehistory of the southern Puget Lowland, in the north-south compressive regime of the migrating Cascadia forearc, reflects diverse earthquake rupture modes with variable recurrence. Stratigraphy and Bayesian analyses of previously reported and new 14C ages in trenches and cores along backthrust scarps in the Seattle fault zone restrict a large earthquake to 1040-910 cal yr B.P. (2σ), an interval that includes the time of the M 7-7.5 Restoration Point earthquake. A newly identified surface-rupturing earthquake along the Waterman Point backthrust dates to 940-380 cal yr B.P., bringing the number of earthquakes in the Seattle fault zone in the past 3500 yr to 4 or 5. Whether scarps record earthquakes of moderate (M 5.5-6.0) or large (M 6.5-7.0) magnitude, backthrusts of the Seattle fault zone may slip during moderate to large earthquakes every few hundred years for periods of 1000-2000 yr, and then not slip for periods of at least several thousands of years. Four new fault scarp trenches in the Tacoma fault zone show evidence of late Holocene folding and faulting about the time of a large earthquake or earthquakes inferred from widespread coseismic subsidence ca. 1000 cal yr B.P.; 12 ages from 8 sites in the Tacoma fault zone limit the earthquakes to 1050-980 cal yr B.P. Evidence is too sparse to determine whether a large earthquake was closely predated or postdated by other earthquakes in the Tacoma basin, but the scarp of the Tacoma fault was formed by multiple earthquakes. In the northeast-striking Saddle Mountain deformation zone, along the western limit of the Seattle and Tacoma fault zones, analysis of previous ages limits earthquakes to 1200-310 cal yr B.P. The prehistory clarifies earthquake clustering in the central Puget Lowland, but cannot resolve potential structural links among the three Holocene fault zones. [ABSTRACT FROM AUTHOR]

The 4 April 2010 moment magnitude (Mw) 7.2 El Mayor-Cucapah earthquake revealed the existence of a previously unidentified fault system in Mexico that extends ∼120 km from the northern tip of the Gulf of California to the U.S.-Mexico border. The system strikes northwest and is composed of at least seven major faults linked by numerous smaller faults, making this one of the most complex surface ruptures ever documented along the Pacific-North America plate boundary. Rupture propagated bilaterally through three distinct kinematic and geomorphic domains. Southeast of the epicenter, a broad region of distributed fracturing, liquefaction, and discontinuous fault rupture was controlled by a buried, southwest-dipping, dextral-normal fault system that extends ∼53 km across the southern Colorado River delta. Northwest of the epicenter, the sense of vertical slip reverses as rupture propagated through multiple strands of an imbricate stack of eastdipping dextral-normal faults that extend ∼55 km through the Sierra Cucapah. However, some coseismic slip (10-30 cm) was partitioned onto the west-dipping Laguna Salada fault, which extends parallel to the main rupture and defines the western margin of the Sierra Cucapah. In the northernmost domain, rupture terminates on a series of several north-northeast-striking cross-faults with minor offset (<8 cm) that cut uplifted and folded sediments of the northern Colorado River delta in the Yuha Desert. In the Sierra Cucapah, primary rupture occurred on four major faults separated by one fault branch and two accommodation zones. The accommodation zones are distributed in a left-stepping en echelon geometry, such that rupture passed systematically to structurally lower faults. The structurally lowest fault that ruptured in this event is inclined as shallowly as ∼20°. Net surface offsets in the Sierra Cucapah average ∼200 cm, with some reaching 300-400 cm, and rupture kinematics vary greatly along strike. Nonetheless, instantaneous extension directions are consistently oriented ∼085° and the dominant slip direction is ∼310°, which is slightly (∼10°) more westerly than the expected azimuth of relative plate motion, but considerably more oblique to other nearby historical ruptures such as the 1992 Landers earthquake. Complex multifault ruptures are common in the central portion of the Pacific North American plate margin, which is affected by restraining bend tectonics, gravitational potential energy gradients, and the inherently three-dimensional strain of the transtensional and transpressional shear regimes that operate in this region. [ABSTRACT FROM AUTHOR]