Dynamic coupling between the San Jacinto Fault and the San Andres Fault in Southern California

The southern San Andreas Fault (SAF), which has been seismically quiescent since 1680, is now waiting for the “Big One”, the earthquake that could rupture the entire southernmost segment of the SAF. However, in southern California a significant portion of the Pacific-North American plate motion is accommodated by the subparallel San Jacinto Fault (SJF). While the SAF has been seismically quiescent, the SJF has been active, with nine moderate-size earthquakes (M ~6-7) since 1899. Would these earthquakes on the SJF affect the stress state and loading rate on the SAF and hence delay the “Big One”? How do the neighboring and subparallel SJF and SAF interact each other? Here we explore the dynamic coupling between these two faults in a three-dimensional viscoelastoplastic finite element model. We found that, in addition to the static Coulomb stress changes associated with each earthquake, the SJF and the SAF interact with each other over longer timescales by collectively accommodating the relative plate motion. During the periods when the SJF experiences clusters of moderate earthquakes, such as those since 1899, the loading rate on the SAF can be significantly lowered.