Seismotectonics of the San Andreas Fault System in the San Gorgonio Pass region: A synthesis

Abstract

In the San Gorgonio Pass region of Southern California, USA, the San Andreas fault exhibits greater geometric complexity than anywhere else along its length, with multiple nonparallel fault strands that branch and join both laterally and at depth (Fig. 1). These structures are associated with regional contraction within the San Gorgonio Pass region due to the nearby Big Bend of the San Andreas fault. 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. The first author of this manuscript, Doug Yule, investigated slip rates and earthquake records in this enigmatic region from the mid-1990s until his death in 2022 (Fig. 2). During the 2010s, many researchers, including co-authors Cooke and Oglesby, were inspired by Doug Yule’s work in this area. Together, the three authors coordinated 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 SCEC, their various organizations, and institutions. Many of the results of this effort are compiled in this 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 and honor the role that Doug played in bringing together earthquake scientists from a variety of disciplines to better understand the rupture potential through the complex faults of the San Gorgonio Pass. Much of this science can be organized around a few key questions.