Local Magnitude Tomography in California

Abstract

Lateral variation in crustal attenuation of California is calculated by inverting 25,330 synthetic Wood–Anderson amplitudes from the California Integrated Seismic Network (CISN) for site, source, and path effects. Two-dimensional attenuation ( q or 1/ Q ) is derived from the path term, which is calculated via an iterative least-squares inversion that also solves for perturbations to the site and source terms. Source terms agree well with initial CISN M L s, and site terms agree well with a prior regression analysis; q ranges from low attenuation at 0.001 ( Q =1000) to high attenuation at 0.015 ( Q =66), with an average of 0.07 ( Q =143). The average q is consistent with an amplitude decay function (log A ) for California when q is combined with a simple geometrical spreading rate. Attenuation in California is consistent with the tectonic structure of California, with low attenuation in the Sierra batholith and high attenuation at The Geysers, at Long Valley, and in the Salton trough possibly due to geothermal effects. Also, path terms are an order of magnitude smaller than site and source terms, suggesting that they are not as important in correcting for M L .