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Near-fault anisotropy following the Hector Mine earthquake

Elizabeth S. Cochran, John E. Vidale, & Yong-Gang Li

Published September 2003, SCEC Contribution #1081

We present anisotropy measurements from shear wave splitting along the Hector Mine rupture zone. Six major arrays were deployed in four locations in the year following the M7.1 Hector Mine earthquake. The dense station coverage, wide distribution of the arrays, and repeated deployments show a clear predominant fast direction and spatial variation of splitting along the fault but no resolvable temporal variations. We determined splitting parameters using an automated cross-correlation method, discarding fast directions with initial source polarizations near crack parallel or perpendicular directions. Only two of the four array locations give reliable measurements of anisotropy at depth. Fast directions and delay times are constant across the 1 km wide array length. However, some spatial variation of splitting is observed along fault strike. Delay times decrease from north to south, with greater splitting in areas of higher slip. A change in splitting parameters along fault strike likely reveals the orientation of cracking in the near-fault region during a major quake. Average fast directions are between fault parallel and the regional maximum compressive stress direction. We do not see temporal evolution in anisotropy; however, measured splitting suggests a heterogeneous stress field partially created during rupture that persists over at least a 1 year timescale.

Cochran, E. S., Vidale, J. E., & Li, Y. (2003). Near-fault anisotropy following the Hector Mine earthquake. Journal of Geophysical Research, 108(B9), 2436. doi: 10.1029/2002JB002352.