Focal spot imaging based on zero lag cross-correlation amplitude fields in a dense seismic array: application to the San Jacinto fault zone

Gregor Hillers, Philippe Roux, Michel Campillo, & Yehuda Ben-Zion

Published March 21, 2016, SCEC Contribution #6235

We image the subsurface below a dense seismic array based on focal spot properties. The focal spots are the zero lag amplitude fields of wave number filtered noise cross correlations computed between all stations in a dense deployment straddling the Clark branch of the San Jacinto fault zone. The emergence of focal spots in the cross correlation functions is linked to time-reversal processes. However, in contrast to preceding seismological applications using reversed propagation, the refocusing done here is re-constructed from dense array data around the focal point. Local medium properties are inferred from the spatially variable size and shape of the surface wave focal spot. By finding the distance of the first zero crossing of the damped Bessel function-shaped amplitude vs. distance distributions, the method provides simultaneous estimates of wave speed, attenuation, and anisotropy without solving a tomographic inverse problem. Images of the frequency dependent seismic velocity distributions are consistent with independent estimates from a far-field Rayleigh wave tomography. We observe an inverse relation between attenuation and seismic velocity, and a fault parallel alignment of fast propagation directions, with greater structural complexity SW to the fault. The results imply a complex fault zone structure including a waveguide NE to the fault that is continuous across the observed depth range, and a low-velocity structure to the SW associated with a shallow sedimentary basin.

Citation
Hillers, G., Roux, P., Campillo, M., & Ben-Zion, Y. (2016). Focal spot imaging based on zero lag cross-correlation amplitude fields in a dense seismic array: application to the San Jacinto fault zone. J. Geophys. Res., 121, 8048-8067.