Local wave propagation near the San Jacinto fault zone, southern California: Observations from a three-component seismic array

We present results from a study conducted to characterize the P and S wave trains and codas for local earthquakes observed near the San Jacinto fault zone, southern California. The data were recorded by a small-aperture, three-component seismic array and analyzed using a statistical array processing algorithm that provides estimates of the propagation direction, apparent wavenumber, and polarization characteristics for plane wave arrivals observed at the array. The P codas for these local earthquake data are similar in that each is composed of predominantly P waves traveling in a direction subparallel to the path of the direct P arrivals. These P coda exhibit a character consistent with propagation in a waveguide as opposed to propagation in a medium with randomly distributed isotropic scattering sources. The S codas differ from the P codas in two dramatic ways: (1) the S codas are composed of predominantly S waves, and (2) the S coda waves arrive from more random/spatially uniform directions. The dramatic differences between the P and S codas provide compelling evidence that P and S coda are the product of distinctly different scattering mechanisms and therefore that the P and S codas provide information about different source characteristics and/or structural features. A third important observation concerning the S coda is that the direction-of-arrival spectra for the S codas differ from event to event considerably more so than do those of the P codas. This observation is at odds with the conventional wisdom and suggests that the character of the S coda reflects a potentially complex interaction among several factors including, but not limited to, the source depth, the source distance, and the source radiation pattern.