Broadband modeling of local earthquakes

Abstract

Three-component broadband waveforms of two small earthquakes near Upland, California, recorded on the Pasadena broadband, high dynamic range instrument, were modeled to obtain useful Green's functions for this path and to examine the sensitivity of the synthetic seismograms to perturbations of the crustal model. It was found that a simple layer over a half-space model is an adequate approximation of the upper crust along this profile. In particular, the waveforms are controlled by a relatively slow, 4km-thick surficial layer (α=4.5km-s-1, β=2.6km-s-1) over a faster layer (α=5.9km-s-1,β=3.5km-s-1). The relative amplitudes of direct and multiple S indicate that the main shock occurred at a depth of 6km, while the aftershock occurred at a depth of 8 to 9km. Sensitivity analyses indicate that for distances <50km and for periods longer than 1sec, the synthetic seismograms are not very sensitive to perturbations of the deep crustal structure. Generally, it was found that it is possible to reproduce local waveforms at frequencies up to 1Hz without a complete knowledge of fine structural detail. -from Authors