Scattering of teleseismic body waves under Pasadena, California

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Abstract
Teleseismic receiver functions for structure under Pasadena, California (PAS) are derived from azimuthally distributed teleseismic P waves recorded on Benioff 1–90 instrumentation. The broadband three‐component Benioff 1–90 system is peaked at a 1‐s period and allows resolution of major crustal interfaces from large Ps conversions seen in the receiver function data. The observed body wave data are quite complex, showing exceptionally large Ps conversions and scattered waves on horizontal components. Radial and tangential motions are of equal magnitude and show major off‐azimuth converted Ps waves, suggesting large‐scale crustal heterogeneity beneath the station. Stochastic simulations of one‐dimensional plane layered structure show that geologically unreasonable one‐dimensional models are required to fit the data. The observed coda decay yields a scattering Q estimate of 239 at a 2‐s period using an energy flux model for a propagating plane wave interacting with a scattering layer over a homogeneous half‐space. Observed and synthetic coda decay follows the theoretical exponential decay predicted by the model and is due entirely to diffusion of coda energy out of the layer into the half‐space. PAS coda is compared to coda from deep teleseisms recorded at State College, Pennsylvania, and it is seen that scattering is more severe at PAS, as reflected in higher coda levels and slower decay rate. Consideration of energy partitioning and coda amplitude suggests that much of the coda consists of scattered surface waves. Analysis of a major Ps conversion arriving 3 s after direct P indicates that a major crustal discontinuity at about 20 km depth dips at moderate angles to the north under the San Gabriel Mountains. This interface probably represents the crustal tectonic boundary between the Transverse Ranges and the Los Angeles Basin.