FAULT INTERACTION AND EARTHQUAKES: INSIGHTS FROM NUMERICAL MODELING

In broad plate boundary zones such as the San Andreas Fault, the tectonic loading from relative plate motion is accommodated by a system of mechanically coupled faults. Similar fault coupling in mid-continents is suggested by the migrating earthquakes and complementary moment release between widespread fault systems in North China and other regions. To investigate the mechanical coupling between faults over thousands of years with multiple seismic cycles, we developed a three-dimensional viscoelastic-plastic finite element model that simulates stress evolution in the crust with a system of faults. Synthetic earthquakes on these faults, occurring when stress reaches predefined plastic yield strength, are simulated by certain amount of plastic strain or stress drop on the ruptured fault planes. We have applied this model to explore fault interaction in southern California and eastern Tibetan Plateau. In southern California, the Pacific-North America relative plate motion is mainly accommodated by the subparallel San Andreas Fault and the San Jacinto Fault. Our results show that the cluster of nine magnitude 6-7 earthquakes on the San Jacinto fault in the past 300 years may have significantly reduced the loading rate on the southern San Andreas Fault, which has been seismically quiescent during this period and is anticipated to produce a big earthquake. Similarly in eastern Tibetan Plateau, we find that the variations of seismicity on the fast-slipping Xianshuhe fault may influence the loading rate on the adjacent Longmanshan fault, which produced the 2008 M7.9 Wenchuan earthquake.