Revised 3D Fault Models for the Southern San Andreas Fault System Extending from San Gorgonio Pass to the Salton Sea

Craig Nicholson, Egill Hauksson, & Andreas Plesch

Published 2010, SCEC Contribution #1739

Understanding the 3D geometry and sense of slip of the San Andreas fault (SAF) is critical to accurately evaluating dynamic rupture and ground motion prediction models. We use alignments of hypocenter and focal mechanism nodal planes within a relocated earthquake catalog to develop improved 3D fault models of principal slip surfaces for the southern SAF system. Through San Gorgonio Pass, earthquakes define multiple fault strands, and beneath ~10 km depth, intersecting sets of strike-slip, oblique slip and thrust faults representing a volume deformation of the lower crust. In the northern Coachella Valley, seismicity indicates that the Garnet Hill and Banning fault strands are most likely sub-parallel (with dips of ~70°NE) to depths of 8–10 km, where they intersect and merge with a stack of moderately dipping to low-angle thrust faults. Gravity and water well data confirm that these faults are sub-parallel and near vertical in the upper 2–3 km, suggesting that these faults tend to decrease dip with depth. The active Mission Creek fault (MCF) appears to steepen along strike SE from ~60°NE to ~80°NE in the Indio Hills. Gravity and velocity modeling also suggest a moderately NE-dipping MCF, although this modeled velocity/density contrast may reflect an older ancestral(?) basin-bounding fault, which tends to act as the lower bound to adjacent seismicity located farther east. SE to North Shore and at Bombay Beach, a few earthquakes do locate directly beneath the SAF surface trace, suggesting that a vertical SAF is permissible here and consistent with observed fault geomorphology. Through the Mecca Hills, much of the seismicity east of the SAF surface trace is clearly related to slip on adjacent secondary faults. Although clusters of hypocenters and nodal planes near North Shore and Salt Creek project to the SAF surface trace with dips ~65°NE, these events appear to be more closely related to slip on the NE-dipping Hidden Springs fault and other intervening secondary fault splays, and not to slip on the SAF itself. Along the southern SAF system, hypocenter and nodal plane alignments thus exhibit a persistent pattern of active multiple fault strands that are often sub-parallel throughout much of the seismogenic zone, and that change dip and dip direction along strike and with depth.

Citation
Nicholson, C., Hauksson, E., & Plesch, A. (2010). Revised 3D Fault Models for the Southern San Andreas Fault System Extending from San Gorgonio Pass to the Salton Sea. Oral Presentation at 106th Annual Meeting American Association of Petroleum Geologists.