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3-D upper crustal velocity structure of the Coachella Valley, Southern California: results from the salton seismic imaging project

Rasheed Ajala, Patricia Persaud, Joann M. Stock, Gary S. Fuis, John A. Hole, Mark R. Goldman, & Daniel S. Scheirer

Published August 14, 2018, SCEC Contribution #8575, 2018 SCEC Annual Meeting Poster #103

The Coachella Valley is host to the southernmost section of the San Andreas fault (SAF), which is generally considered to be overdue for a large magnitude (M≥7) earthquake. To improve seismic risk assessments in this region, accurate knowledge of the Coachella Valley sedimentary basin structure and fault structure is required. We inverted first P-wave arrival times from local seismicity (39,998 events) and shots (251 land/sea explosions) from the 2011 Salton Seismic Imaging Project to produce a 3-D Vp model of the Coachella Valley and the surrounding mountain ranges. Several inversion and resolution tests were performed to ensure that the best parameters were used in the modeling procedure and that the final model is representative of a smooth version of the actual structure. Our model has a grid spacing of 0.5 km with good resolution (~50 rays/cubic km) to 9 km depth. The top ~3 km of the model conforms with the surface geology. Sediment thickness in the valley ranges from a maximum of ~4 km at the northern end of Salton Sea and thins to ~2 km at the northern end of the valley. The basin asymmetry is evident from the persistence of low velocities on the eastern side of the valley compared to the western side. Estimated basement depths in the Coachella Valley using a 4.5 km/s isovelocity surface reveal multiple sub-basins related to upward steps in basement toward the NW. Lateral velocity contrasts subparallel to and offset from mapped surface faults provide fault geometry information, including evidence supporting a northeast-dipping SAF. Basement in the valley is highly heterogeneous and comprises high velocity (≥6.4 km/s) regions that we interpret to be mylonitic and schistose rocks from the Peninsular Ranges and Eastern Transverse Ranges, respectively.

Ajala, R., Persaud, P., Stock, J. M., Fuis, G. S., Hole, J. A., Goldman, M. R., & Scheirer, D. S. (2018, 08). 3-D upper crustal velocity structure of the Coachella Valley, Southern California: results from the salton seismic imaging project. Poster Presentation at 2018 SCEC Annual Meeting.

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