SEISMOTECTONICS OF THE SAN ANDREAS FAULT SYSTEM IN THE SAN GORGONIO PASS REGION

Regional contraction within the San Gorgonio Pass region is associated with the southern Big Bend of the San Andreas fault in Southern California. In this region, the San Andreas exhibits greater geometric complexity than anywhere else along its length, with multiple non-parallel fault strands that branch and join both laterally and at depth. The complexity of past and present faulting in this region has led Jon Matti, Doug Yule and others to call this the “San Gorgonio knot”. Due to its complexity, this part of the San Andreas fault system may serve as both a barrier and an initiation point for devastating earthquakes. Research to understand and unravel the San Gorgonio knot has important implications for seismic hazard in southern California. Much of Doug Yule’s research since the mid-1990s focused on better understanding this enigmatic region. Inspired by Doug Yule’s work in this area, the authors joined him in coordinating a five-year research effort to investigate active faulting within the San Gorgonio Pass, involving many researchers in the Southern California Earthquake Center (SCEC) community with support from their various organizations and institutions. Much of the results of this effort has been compiled in a Geosphere thematic issue, with 11 papers that span earthquake geology, geomorphology, geodesy, potential field geophysics, crustal deformation models, and dynamic rupture models. Here, we highlight findings from these studies, including our own work, and honor the role that Doug played in bringing together diverse earthquake scientists to better understand the rupture potential through the complex faults of the San Gorgonio Pass. The studies confirm the complex evolution of faults in the region and elucidate both the spatially variable long-term slip rates and the capacity for rupture propagation of damaging earthquakes.