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Sensitivity of regional interseismic deformation to variations in active fault configuration of the southern San Andreas fault and San Jacinto faults

Hanna M. Elston, Michele L. Cooke, Scott T. Marshall, & Jennifer L. Hatch

Published August 14, 2019, SCEC Contribution #9594, 2019 SCEC Annual Meeting Poster #194

Several alternative active fault configurations have been proposed for the San Jacinto and southern San Andreas faults. These alternatives arise in regions where subsurface fault geometry is not well constrained (e.g., Cajon Pass and Indio Hills) and include different interpretations of the active traces of the San Andreas through the San Gorgonio Pass region. Beyer et al. (2018) tested six variations of northern and southern pathways of active faulting through the San Gorgonio Pass and found two best fitting models, each of which honor some, but not all of the geologic strike-slip rate data. Within the Cajon Pass, the San Andreas and San Jacinto fault traces converge to within 10 km without meeting; the faults may intersect at depth. Herbert et al (2014) found that connecting the two faults at depth didn’t alter slip rates enough to distinguish a better fitting model. Here, we expand on previous assessments of viable fault configurations that compared model results to geologic slip rates by also comparing results to both geodetic velocities and stress states inferred from over 50,000 focal mechanisms. Better constraints on the subsurface geometry of the southern San Andreas fault can improve our estimates of seismic hazard in the region.

For each of the alternative active fault configurations, we simulate interseismic deformation using forward Boundary Element Method (BEM) models that apply steady state slip rates below prescribed locking depths. Here, we implement a slip transition zone at the locking depth by prescribing half of the prescribed slip rate to elements whose centroid is within 2.5 km above a prescribed locking depth. We compare model velocities at GPS station locations and slip sense (Aphi) at focal mechanism locations to geodetic data and slip sense from focal mechanism inversions to determine the fault configuration most compatible with the data.

Model predicted GPS station velocities and focal mechanism Aphi values differ between alternative fault configurations, suggesting that these data may be used to distinguish between viable fault interpretations. The alternative fault configurations produce GPS residuals in the study area with differences that exceed the uncertainties of the measured station velocities. Modeled interseismic velocities indicate an active fault configuration excluding Mill Creek offers a better fit to data in our study area.

Key Words
San Andreas, San Jacinto, BEM, Interseismic

Elston, H. M., Cooke, M. L., Marshall, S. T., & Hatch, J. L. (2019, 08). Sensitivity of regional interseismic deformation to variations in active fault configuration of the southern San Andreas fault and San Jacinto faults . Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)