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Past and Present Deformation along the San Andreas from Microseismicity, Geodesy, and Seismological Constraints

Debi Kilb, Vera Schulte-Pelkum, Thorsten W. Becker, Whitney M. Behr, & Meghan S. Miller

Accepted 2021, SCEC Contribution #11805

The transform plate boundary between the Pacific and North American plates in southern California is complex, with a large, transpressional restraining bend, several active major strike-slip fault strands, and a broad shear zone that combine to accommodate plate motion. Current transform motion was preceded by long-lived subduction and extensional episodes. Here, we investigate the interplay between this tectonic history and present-day deformation by considering microseismicity from published densified catalogs based on clustering and machine learning, surface and subsurface fault geometries, geodetic constraints, and fabric as imaged by receiver functions and other seismological methods. Receiver functions are sensitive to contrasts in P wave anisotropy, in particular dipping foliation. In places, such imaged structures align with along-dip depth-extensions of fault traces as illuminated by microseismicity, for example along the southern San Andreas Fault. Orthogonal structures are visible in seismicity as well as receiver function strikes in areas such as Ridgecrest. However, overall the region shows mainly fault-parallel but distributed foliation. In the southern Sierra Nevada, deep crustal fabric parallels exhumed ductile foliations with the same strike as the reactivated Kern Canyon Fault. High density measurements near Parkfield and on the San Jacinto fault show fault-parallel and dipping deep crustal fabric on both sides of the surface fault trace. We further compare regional deformation indicators for GNSS inferred crustal strain-rates, focal mechanism-derived stress, and seismic anisotropy from shallow splitting and SKS splitting, Pn, P, and surface wave tomography. Instantaneous strain-rates appear aligned with regional N-S compression as expected from relative plate motion. All crustal and lithospheric mantle depth strain markers deviate from instantaneous strain and show better agreement with inherited tectonic fabric. In the asthenosphere, strain is dominated by large- scale mantle circulation. We conclude that tectonic inheritance plays a major role in continental deformation.

Kilb, D., Schulte-Pelkum, V., Becker, T. W., Behr, W. M., & Miller, M. S. (2021). Past and Present Deformation along the San Andreas from Microseismicity, Geodesy, and Seismological Constraints. Oral Presentation at AGU.