EVOLUTION OF THE SAN ANDREAS PLATE BOUNDARY FAULTS IN SOUTHERN CALIFORNIA: GEODYNAMIC INSIGHTS

In southern California, the Pacific-North America relative plate motion is accommodated by a complex system of plate boundary faults that include the San Andreas Fault (SAF) proper, the Garlock fault, the Eastern California Shear Zone (ECSZ), and numerous young (< 3 Ma) faults subparallel to the SAF (the San Jacinto, the Elsinore, and the offshore dextral faults). Using a 3D viscoelastoplastic finite element model, we have explored the geodynamics of the inception of the ECSZ and the young faults and their impact on strain rate distribution in southern California. Our results suggest that the formation of the Big Bend of the SAF 5-12 million years ago impeded fault slip on the SAF and localized strain along the ECSZ, causing its inception to provide an alternative path for accommodating the relative plate motion. Similarly, the San Jacinto Fault, the Elsinore Fault, and the offshore dextral faults all initiated along zones of localized strain as the plate boundary faults adjust and evolve to improve the mechanical efficiency of accommodating the relative plate motion. Initiation of each new faults changes the apportionment of slip rates on other faults, which may explain some of the discrepancies between geological and geodetic measurements of fault slip rates. The model results help to explain present-day crustal deformation in southern California, including slip rate variations among the major faults and the distribution of seismicity.