USING GROUND PENETRATING RADAR TO UNDERSTAND SLIP PARTITIONING AND FAULT GEOMETRY, SAN ANDREAS FAULT, CALIFORNIA

Understanding how faults slip and distribute deformation near Earth’s surface is key to improving the resilience of communities and infrastructure exposed to seismic hazards. For example, whether slip focuses onto one or more fault strand(s) directly impacts fault displacement hazard analyses, our understanding of a fault system’s slip history, and appropriate estimates of fault slip rates that inform probabilistic seismic hazard analyses. The San Andreas Fault (SAF) is one of the world’s best-known transform fault systems; yet slip partitioning in the near-surface (10s-of-meters depth) remains poorly understood. The creeping segment of the SAF, while unusual among continental strike slip faults, provides a unique opportunity to observe shallow deformation without requiring a ground-rupturing earthquake. A section of the SAF near Mee Ranch (36.18068N, 120.75855W) is known to be actively creeping at 25 ± 1 mm/yr, as documented by an alignment array and a creepmeter at the site, and active creep is concentrated within a narrow (<5m) shear zone between two alignment array benchmarks.

In 2021, a series of ground penetrating radar (GPR) transects were collected across the SAF at Mee Ranch with lines orientated orthogonal to the observed fault structures. The GPR data clearly show disturbance of stratigraphy just below the surface on several unidentified fault stands as well as various antithetic and synthetic faults associated with the primary and other strands of the fault. The location of the GPR-identified faults extends beyond the narrow active shear zone, indicating that only a small subset of near-surface fault splays is actively deforming. Additional GPR data collection will refine interpretations and attempt to identify offset features in the subsurface through fault-parallel grids to reconstruct slip. In addition to GPR, the team will employ earth resistivity tomography to extend the observations downward to better understand the coalescence of the strands at depth.