Effects of preexisting and evolving weaknesses on the kinematic evolution of strike-slip restraining bends in the Eastern California shear zone

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This project is advancing understanding of how barriers between faults influence the spatial and temporal occurrence of major earthquakes in the United States via a focused geologic study in southern California. The fault boundary between the Pacific and North American tectonic plates cuts directly through California and poses extreme hazard to its dense population and economic infrastructure via earthquake shaking, ground rupture, and triggered landslides. This boundary does not consist of a single fault, however. In southern California, roughly half of the fault activity occurs along the San Andreas fault, whereas the remainder is distributed across a broader fault system. About 25% of the activity occurs along the 125-kilometer-wide Eastern California Shear Zone, which has produced recent damaging (magnitude7) earthquakes and appears to be in an intensified phase of activity. This shear zone is enigmatic, in that earthquakes are produced via simultaneous or sequential motion on multiple faults that cut across gaps which would have otherwise been expected to be barriers. The zone also exhibits discrepancy in measured rates of motion, as measurements spanning decades using space-based techniques are 2 to 3 times faster than long-term measurements based on geologic offsets. This discrepancy may result from measurement errors, undocumented deformation, or changes in motion rate through time. As a result of these problems, the future earthquake risk of the region, which includes major national defense installations (e.g. Marine Corps Air Ground Combat Center - MCAGCC), as well as how future ruptures may interact with or even trigger events along the remainder of the plate boundary (e.g. near Los Angeles), are not well understood. This project is advancing understanding by documenting the evolution of complex fault connections and quantifying the magnitude of internal tectonic deformation that occurs in-between the major faults of the southern Mojave Desert. This effort involves cutting-edge mapping using high-resolution topography, dating of measured offsets using various geochronometers, and testing of how preexisting weaknesses in the crust may be influencing enigmatic patterns of faulting. Ultimately the project will reconstruct of how faults have evolved and interacted, leading to improved forecasting of what to expect from major earthquakes in southern California as well as an improved framework for understanding how fault barriers control earthquake size that can be applied to the entire United States. This project will also benefit society by improving public scientific literacy via internet-based video outreach and strengthening the STEM (science, technology, engineering and mathematics) workforce.This project is testing conceptual treatments of crustal strength in the context of the origin and evolution of nascent strike-slip faults of the Eastern California Shear Zone. One conceptual view considers strength as an evolving quantity, which may decrease with accumulating displacement as faults smoothen or increase autogenically as geometric complexity grows due to distributed deformation. Another perspective considers how crustal anisotropy (i.e. preexisting structural weaknesses) influences the initial geometry and kinematics of fault arrays. Several characteristics of the Eastern California Shear Zone may relate to initial or evolving strength, such as variability in fault trend, a prevalence of prominent, self-similar restraining bends between fault segments, and complex deformation partitioning, none of which can be clearly related to total fault slip, age, orientation, or spatial position. This project is testing the role of strength in fault development by documenting the evolution of faulting and folding (i.e. integrating or complexifying) through twelve key transpressive zones (e.g. the remarkable but previously unstudied Calico-Hidalgo fault stepover and associated borderlands) and comparing active deformation to patterns of crustal rheological variations via bedrock mapping. Kinematic interpretation of fault bends is based on high-resolution topography and neotectonic mapping, structural analysis, geophysical subsurface imaging, documentation of penetrative strain between fault strands, and chronologies of deformation using comprehensive Quaternary dating. This project is directly contributing to understanding of transpression, the influence of mechanical anisotropy on continental deformation, segmentation and integration of strike-slip faults (including "earthquake gates," which may present barriers to rupture and influence seismic hazards), the cause of the discrepancy between geodetic and geologically determined slip rates, local earthquake hazards, and California tectonics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.