Regional wave propagation in southern California and Nevada: Observations from a three‐component seismic array

Abstract

We present results from a study conducted to characterize regional wave propagation in southern California and Nevada. The data are from events whose paths sample the southern Coast Ranges, the southern Sierra Nevada Mountains and the southern Basin and Range. These regional earthquake data were recorded by a small‐aperture three‐component seismic array located at the Piñon Flat Observatory, southern California. The data are analyzed using a statistical array processing algorithm that provides estimates of the propagation direction, frequency, wavenumber, and particle motion polarization characteristics of the coherent arrivals observed at the array. Perhaps the most striking feature shared by these data is the extent to which the P, S and Lg wave trains are composed of coherent, forward scattered/multipathed energy. The nature of the P, S and Lg wave trains suggests that spatially anisotropic scattering plays an important role in regional wave propagation in southern California and Nevada, and that scattering sources are located not on elliptical shells whose foci are the source and the receiver but instead in a limited volume subparallel to the path of the direct arrivals. The high‐amplitude, coherent nature of numerous of the Pg and Lg coda waves suggests that strong scattering in the source zone plays an important role in shaping the Pg and Lg wave trains. While the P, S and Lg wave trains for these regional events share some general characteristics they also exhibit fairly significant differences. These differences illustrate the important role regional and source zone structure play in shaping the wave field, and the need to account for both complex scattering and three‐dimensional propagation effects in regional waveform modeling. The coda for these regional data do not begin to exhibit a character consistent with the random scattering model for coda generation until the arrival of the surface waves. The fact that much of the surface wave coda cannot be effectively modeled as plane waves suggests that it is not the product of scattering from distant heterogeneities.