Fault zone evolution along complex plate boundaries: A case study from the Eastern California shear zone

Spatiotemporal patterns of deformation within plate boundary systems reflect complex interactions across a wide range of processes and scales. For example, stress transfer during earthquakes, interseismic strain accumulation, large-scale variations in lithospheric structure and strength, and climate-solid Earth interactions all influence plate boundary and fault network evolution. At the scale of individual fault segments, the role of variations in fault strength, width, and geometry on long-term slip rates and slip transfer remains to be fully elucidated. The southern Pacific-North America (PNA) plate boundary system provides an ideal natural laboratory to test hypotheses on these topics given the wealth of geologic and geophysical data sets constraining fault network structure across short (seconds) and long-term (Myrs) timescales.

We use a combination of existing data sets (e.g., fault locations, geometry, width, and slip rates) and state-of-the-art forward modeling techniques to test (1) the role of fault strength and width on long-term slip rates within the Eastern California shear zone (ECSZ) and (2) interactions with other key structures of the PNA plate boundary, such as the Garlock and San Andreas fault systems. Using the Geodynamic World Builder (GWB), we integrated the SCEC Community Fault Model (CFM) into the open-source mantle convection and lithospheric code ASPECT to calculate instantaneous long-term slip rates on individual fault strands, which are mechanically defined by reductions in the internal angle of friction and cohesion. To produce fault widths within the range of geologic observations (e.g., 10s of meters), we utilize ASPECT's adaptive-mesh refinement capabilities to achieve significantly higher resolution within faults and between fault strands. Our model results reveal that fault geometry, width and strength exert a first-order control on modeled slip rates. While our focus here is on the ECSZ, our modeling workflow allows straightforward adjustment of the forward simulations to incorporate any portion of the plate boundary system. Our work serves as a bridge to system-wide explorations of fault evolution and comparison with geodetic and geologic activity rates, with the future goal of more accurate modeling of the first-order controls on plate boundary evolution.