Interseismic strain localization in the San Jacinto fault zone

Eric O. Lindsey, Valerie J. Sahakian, Yuri Fialko, Yehuda Bock, Sylvain D. Barbot, & Thomas K. Rockwell

Published November 2014, SCEC Contribution #1929

We investigated interseismic deformation across the San Jacinto fault at Anza, California where previous geodetic observations have indicated an anomalously high shear strain rate. We present an updated set of secular velocities from GPS and InSAR observations that reveal a 2-3 kilometer wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two. GPS occupations of an alignment array installed in 1990 across the fault trace at Anza allow us to rule out shallow creep as a possible contributor to the observed 15 strain rate. Using a dislocation model in a heterogeneous elastic half space, we show that a reduction in shear modulus within the fault zone by a factor of 1.2-1.6 as imaged tomographically by Allam & Ben-Zion (2012) can explain about 50% of the observed anomalous strain rate. However, the best-fitting locking depth in this case (10.4 ± 1.3 km) is significantly less than the local depth extent of seismicity (14-18 km). We show that a deep fault zone with a shear modulus reduction of at least a factor of 2.4 would be required to fully explain the geodetic strain rate, ssuming the locking depth is 15 km. Two alternative possibilities include fault creep at a substantial fraction of the long-term slip rate within the region of deep microseismicity, or a reduced yield strength within the upper fault zone leading to distributed plastic failure. Both cases are consistent with a fault segment that is reaching the end of the
interseismic period.

Lindsey, E. O., Sahakian, V. J., Fialko, Y., Bock, Y., Barbot, S. D., & Rockwell, T. K. (2014). Interseismic strain localization in the San Jacinto fault zone. Pure and Applied Geophysics, 171(11), 2937-2954. doi: 10.1007/s00024-013-0753-z.