High-resolution imaging of the San Andreas Fault around San Gorgonio Pass using fault zone head waves and double-difference tomography, with implications for large earthquake ruptures

Pieter-Ewald Share, Yehuda Ben-Zion, Clifford H. Thurber, Haijiang Zhang, & Hao Guo

Published August 7, 2016, SCEC Contribution #6531, 2016 SCEC Annual Meeting Poster #120

We attempt to clarify the seismic velocity structure within and around the complex San Gorgonio Pass (SGP) “structural knot” of the San Andreas Fault (SAF) using fault zone headwaves (FZHW) and a new double-difference tomography code incorporating both event pairs and station pairs. The tomography provides information on the velocity structure at scales >1 km, while the FZHW give direct evidence for the existence and continuity of bimaterial fault interfaces. Arrival times of P and S waves generated by 9260 M>1 local events occurring from 1/1/2010 to 30/6/2015 within a 150 by 100 km region centered on the SGP and recorded by 143 stations are inverted for P and S velocity structures. Low velocity anomalies are observed, in particular at depths <6 km, SE of Cajon Pass between the SAF and San Jacinto Fault to the south, and at the intersection of the SAF/Mission Creek Fault and Pinto Mountain Fault to the north. Clear FZHW are found to be generated by 41 M>1.5 events within a 20 km zone around the SAF, and to propagate along two separate bilateral interfaces. The first extends from slightly NW of Cajon Pass to the SGP region and is associated with a slower seismic velocity on the SW side of the SAF. The second interface extends from slightly NE of SGP, ends in the Coachella Valley, and has an opposite velocity contrast. The low velocity tomographic anomalies correlate well with the inferred alternating slow sides of the SAF identified by the FZHW analysis. The observed FZHW, opposite velocity contrasts, and expected behavior of bimaterial ruptures have important implications for large earthquakes on the SAF in the region. The velocity contrast to the SE of SGP suggests that earthquakes on that section of the SAF tend to propagate to the SE. The opposite velocity contrast to the NW of SGP suggests that earthquakes on that section tend to propagate to the NW. The results also suggest that ruptures that enter the SGP area from either direction would likely be arrested by the reversal of the velocity contrast they would encounter with continuing propagation.

Citation
Share, P., Ben-Zion, Y., Thurber, C. H., Zhang, H., & Guo, H. (2016, 08). High-resolution imaging of the San Andreas Fault around San Gorgonio Pass using fault zone head waves and double-difference tomography, with implications for large earthquake ruptures. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)