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Profile of Shallow Crustal Structure across the San Andreas Fault Zone, Coachella Valley based on Controlled-Source Data from the Salton Seismic Imaging Project (SSIP)

Amanda Hernandez, Patricia Persaud, Klaus Bauer, Joann M. Stock, Gary S. Fuis, John A. Hole, & Mark R. Goldman

Published September 2015, SCEC Contribution #7636

The Coachella Valley is predicted to be at the center of the worst ground shaking in the event of the “Big One,” a M7.8 earthquake on the southern San Andreas Fault (SAF). Structural constraints on existing models of the shallow subsurface are essential in this region of rapidly growing population, since they give an improved understanding of the seismic hazard. In order to validate the structural interpretation of our 200 m grid-spacing 2D tomographic results, we analyzed and interpreted first arrivals from nine explosive shots recorded along Line 5 of the SSIP. This structure-based approach confirms the major features in the 2D tomography, and gives details on basement depths, fault zones and basin geometry. Line 5 comprises 281 receivers and is oriented SW-NE across the Coachella Valley, where it extends 37 km from the Santa Rosa Mountains in the Peninsular Ranges to the Little San Bernardino Mountains crossing the strands of the SAF. Using forward modeling ray tracing, we fit traveltime curves to our first-arrival picks and determined the best fitting 1D P-wave velocity models for receivers east and west of each shot, and a 2D velocity model for the profile. In general, 1D models for receivers east of the shots have deeper basement contacts and lower apparent velocities, ~5 km/s at 4 km depth, while models for receivers to the west of shots have shallower basement contacts and velocities up to 6 km/s at ~2 km depth. Based on mismatches in basement contact depths (assuming 5-6 km/s) between the east and west branches of the 1D models, there is a strong possibility of an east-dipping interface along the profile. This is confirmed in our 2D model, which has a shallowly NE dipping basement that deepens eastward towards the Banning Fault at ~2 km depth and shoals abruptly near the eastern end of the line. Based on high velocity zones (~6.1 km/s) derived from traveltimes at 9-20 km from the western end of the line, we included an offset from ~2 km to 4 km depth near the middle of the line. The 2D model fit is greatly improved when this high-velocity zone is included. If fault-related, this offset could have occurred on the Garnet Hill Fault if it extends southward in the subsurface. Additional information on the geometry of near-vertical faults was obtained from the pre-stack line migration method of Bauer et al. (2013). These findings are compared to our 2D tomographic results to produce a comprehensive picture of the structural complexity near the profile.

Hernandez, A., Persaud, P., Bauer, K., Stock, J. M., Fuis, G. S., Hole, J. A., & Goldman, M. R. (2015, 09). Profile of Shallow Crustal Structure across the San Andreas Fault Zone, Coachella Valley based on Controlled-Source Data from the Salton Seismic Imaging Project (SSIP). Poster Presentation at SCEC Annual Meeting.