Geometry of the San Andreas Fault and Sedimentary Basin in the Northern Salton Trough

Abstract:

The Salton Seismic Imaging Project (SSIP) was undertaken, in part, to provide more accurate information on the plate-boundary faults and basin geometry in the Salton Trough region. One of these faults, the southernmost San Andreas Fault (SAF) zone in the northern Salton Trough (Coachella Valley), is considered by many to be likely to produce a large-magnitude earthquake in the near future. We report here on the geometry of the SAF and the adjacent sedimentary basin beneath the Coachella Valley.

We interpret two seismic profiles in the northern Salton Trough that are orthogonal to the axis of the Coachella Valley. Seismic imaging, potential-field studies, and (or) earthquake hypocentral relocations along these profiles indicate that the active strand of the SAF dips NE. On a southern profile, through the Mecca Hills, we obtain a reflection image of the SAF zone in the depth range of 6-12 km that coincides with the microearthquake pattern reported by Hauksson et al. (2012), dipping ~ 60° NE. Steeply dipping reflectors above 6 km emerge at the surface at mapped secondary fault traces in the Mecca Hills, clearly defining a “flower structure” for the upper SAF. On the second profile, from Palm Springs to Landers, two alternative interpretations of SAF structure are possible. By one interpretation, which is based on earthquakes alone, the Banning and Garnet Hill Faults are two closely spaced faults, dipping ~ 60° NNE that pass through two aftershock clusters of the 1986 M 5.9 North Palm Springs earthquake. By the second interpretation, which is based on our reflection imaging on this line, the Banning and Garnet Hills faults converge at 10-km depth; below that depth, a single SAF dips ~ 60° NNE. In the second interpretation, the faults above 10 km resemble the flower structure interpreted beneath the Mecca Hills on our southern profile. The deeper fault in the second interpretation is subparallel to the closely spaced faults of the first interpretation but a few km deeper.

Sedimentary basin depth (Vp < 5.5 km/s, estimated) ranges from 3-4 km in the northern Coachella Valley to a maximum of 5 km in the southern Coachella Valley. Because of the generally flat magnetic field, basement beneath the Coachella Valley southwest of the SAF is interpreted to be similar to that in the Peninsular Ranges.