Finite-frequency P Wave Amplitudes and Travel Times Measured in the Western U.S.

We have developed a method to measure finite-frequency amplitude and travel time anomalies of teleseismic P waves. We use a matched filtering approach that models the first 20-30 seconds of a seismogram after the P arrival. Matched filters for waveform fitting are needed especially in the case of shallow earthquakes (less than 50~km deep): since the depth phases pP and sP arrive only a few seconds after the P pulse, recorded waveforms change as a function of distance and azimuth. Since the vast majority of earthquakes are shallow, this data holds the potential of assembling a database large enough for 3D body wave tomography. Given a set of broadband seismograms from a teleseismic event, we compute synthetic Green functions using published moment tensor solutions. We jointly deconvolve global or regional sets of seismograms with their Green's functions to obtain the broadband source time function (stf). The matched filter (or predicted waveform) of the kth seismogram is the convolution of the kth Green's function with the source time function. Amplitude anomalies are defined as the multiplicative factors that minimize the RMS misfit between matched filters and data. The above procedure is implemented in an iterative fashion, which allows for joint inversion of source time function, amplitudes, and a correction to the moment tensor. We run this inversion for many different assumed source depths to determine the most likely depth, as indicated by the overall RMS misfit, and by the non-negativity and compactness of the stf. We can account for source effects with azimuthal dependence by clustering global data sets based on cross-correlation. Clustering separates seismograms with different source characteristics into distinct groups, and we subsequently invert for one stf per group. Finite-frequency measurements are obtained by filtering broadband data and matched filters through a bank of passband filters. Travel-times are computed by time-aligning (cross-correlating) each seismogram with its matched filter, in every passband. Finite-frequency amplitudes are computed by minimizing the RMS misfit in each passband, as in the broadband case. We present amplitude and travel time statistics for a set of magnitude~5.5 to~7.0 events that were measured globally. We focus on the densely instrumented Western U.S. The LA~RISTRA array (deployed 1999-2001) serves as a particularly well-controlled test platform: we are able to check the accuracy of our method and to identify spatially coherent amplitude and travel time patterns across the tectonically active Rio Grande Rift/Colorado Plateau area. Time permitting, we will also present first inversion results for the LA~RISTRA region.