Fault Evolution and Strain Partitioning in Southern California: Insights from Geodynamic Modeling

Abstract:

In southern California, the Pacific-North American relative plate motion is accommodated by a complex system of transcurrent, transcompressional, and transtensional faults. Although high-precision GPS measurements in recent years have greatly refined the kinematics of crustal motion and strain partitioning among major faults in Southern California, the causes of such strain partitioning and fault evolution remain uncertain. Using a three-dimensional viscoelasto-plastic finite element model, we have explored how the plate boundary fault system evolves to accommodate the relative plate motion in Southern California. Our results show that, when the plate boundary faults are not optimally orientated to accommodate the relative plate motion, new faults will be initiated. In particular, the Big Bend of the San Andreas Fault (SAF), which is the main plate boundary fault, impedes the relative plate motion, thus forces the development of a system of secondary faults. The evolution of these secondary faults is to minimize the work needed to accommodate the relative plate motion, and this mechanism provides a framework for understanding the present-day strain partitioning and earthquake hazards. In particular, the Big Bend and the offshore parallel faults facilitate strain localization in the Western Transverse Ranges in the form of thrust faults and blind faults, and the bends on the southern SAF enhance the development of the dextral faults across the Mojave Desert, where a number of damaging earthquakes occurred in recent years.