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Slip in strike‐slip earthquakes is spatially variable along a fault, but the degree to which variability over short length scales is inherent to the rupture process or introduced by interpretation and measurement has not been quantified. In this study, we examine the effects of interpretation error on apparent short‐wavelength variability in surface slip distributions by comparing numerical landscape evolution models and recent ruptures on strike‐slip faults measured by hand and with image correlation. Surface slip distributions measured by hand from 63 strike‐slip earthquakes have average spatial variability (CVslip‐spatial = standard deviation/mean) of 0.43–0.52, and a total range of 0.14–1.14. Displacement measurements of offset geomorphic markers from numerical models that simulate constant slip along a fault have average spatial variability of ~0.25–0.40 when measured by hand and no spatial variability when measured by image correlation. Slip distributions from seven recent ruptures measured by image correlation have short‐wavelength variability of 0.09–0.29, which is considered inherent to how rupture propagates to the surface. Our results demonstrate that variability innate to the rupture process and introduced by interpretation both contribute substantially to the observed variability in slip distributions measured by hand. Resolving the extent to which short‐wavelength variability is inherent to rupture propagation through near‐surface material versus an artifact of interpretation furthers understanding of the relationship between surface rupture and fault mechanics and informs interpretation of slip distribution and slip‐per‐event in past earthquakes. [ABSTRACT FROM AUTHOR]

Earthquakes and bedrock river incision are fundamental processes in the evolution of tectonically active landscapes, yet little work has focused on understanding how a bedrock river responds to a single large earthquake. Data from the 1999 Mw = 7.6 Chi-Chi earthquake in central Taiwan show dramatic differences in river response that depend on the proximity to the rupture zone. Near the fault scarp, vertical ground deformation intensifies river incision on a time scale of years to decades, while distal to the fault, landslides induced by the earthquake mantle bedrock on the river bed with sediment, impeding incision for decades to centuries as the material is evacuated. This surprising spatial and temporal variability in bedrock incision caused by earthquakes has implications for the paleoseismic interpretation of bedrock terraces and for the evolution of tectonically active landscapes. [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 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]