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Seismic imaging of the Southern California plate boundary around the South-Central Transverse Ranges using double-difference tomography and fault zone head waves

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

Published August 14, 2018, SCEC Contribution #8547, 2018 SCEC Annual Meeting Poster #101

We image seismic velocity structures within the South-Central Transverse Ranges using fault zone head waves (FZHW) and a new double-difference tomography algorithm incorporating both event and station pairs. The tomography provides information on velocity structure at scales >4 km, whereas FZHW give direct evidence for the existence and continuity of bimaterial fault interfaces. Tomographic images of velocities within a 222 km x 164 km region are obtained using >1,000,000 P and S arrival times (picked with an automatic detection algorithm) and differential times associated with >10,000 local events from 1/1/2010 to 30/6/2015 recorded by >250 stations. Early P waveforms from the same dataset (augmented by events from earlier years) are analyzed for the presence of FZHW.
The resulting tomography models include low velocities along major fault segments and across-fault velocity contrasts. Analyses of FZHW reveal bimaterial interfaces at the core of several of these fault segments. Extensive faulting and Pelona schist manifest as low velocities throughout the San Bernardino basin. High velocity granitic rocks are juxtaposed in this basin to the SW and NE, forming continuous bimaterial interfaces in the northern San Jacinto fault zone (SJFZ) and the San Andreas Fault (SAF) NW of San Gorgonio Pass (SGP). The bimaterial interface along the northern SJFZ extends through and perhaps slightly beyond Cajon Pass. The results also show a bimaterial interface across the SAF SE of SGP that is offset from the fault surface trace to the NE at depth. We interpret this to be a dipping section of the southern SAF (estimated dip=57°), which separates granitic rocks in the SW from gneisses and schists in the NE. The flip in velocity contrast polarity across the SAF through SGP imaged with FZHW, complemented by abrupt west-to-east changes in elastic properties observed in the tomography models, have implications for large earthquake ruptures in this area.

Similarly large P and S datasets employed in the inversion lead to high quality VP/VS estimates for the region. These models show very high VP/VS anomalies near shallow damaged rock, whereas fault zones exhibit either low (<1.73) or high (>1.73) VP/VS characteristics at greater depth. Regional-scale low and high VP/VS values are related to relative abundances of igneous or metamorphic rocks. Near-fault VP/VS anomalies at depth are likely associated with variations of crack densities and fluid content.

Share, P., Guo, H., Thurber, C. H., Zhang, H., & Ben-Zion, Y. (2018, 08). Seismic imaging of the Southern California plate boundary around the South-Central Transverse Ranges using double-difference tomography and fault zone head waves. Poster Presentation at 2018 SCEC Annual Meeting.

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