Lack of continuity of the San Andreas Fault in southern California: Three‐dimensional fault models and earthquake scenarios

Abstract

The 1200‐km‐long San Andreas Fault loses its apparent continuity in southern California near San Gorgonio Pass [Allen, 1957], which raises significant questions given the dominant role of this fault in active California tectonics. What is the fundamental three‐dimensional (3‐D) geometry and kinematic behavior of the San Andreas fault system in this complex region? Is a throughgoing, if complex, San Andreas rupture from the Mojave Desert to the Coachella Valley possible? We have explored the issue of 3‐D continuity by mapping over 60 faults in this region to depths of 15–20 km from hypocenter locations and focal mechanisms. We were able to constrain the 3‐D geometry of the San Andreas fault zone (SAF) near San Gorgonio Pass from the 3‐D geometry of the fault network surrounding it. The most likely configuration is for the San Andreas Fault to merge into the shallow‐dipping San Gorgonio Pass thrust northwest of Indio. We concluded that there is no direct continuity at present but rather a network of faults, and the only kind of rupture possible for the SAF in this region is a complex rupture, involving both strike‐slip and reverse faulting. GPS measurements also suggest that despite the fact that large motions must have occurred in the past based on offset geologic markers, only minor motion is occurring today in this area. Applying our findings about the fault geometry, we explored several simple earthquake scenarios to determine the most favorable conditions for a throughgoing rupture of the San Andreas fault system from the Mojave Desert to the Coachella Valley.