Large Teleseismic P Wavefront Deflections Observed with Broadband Arrays

Abstract

We measure the plane wavefront incidence azimuth for teleseismic P at large-aperture (∼50 km) broadband arrays. The incidence azimuth is determined by crosscorrelation of the P arrivals on the vertical component seismograms filtered in successive frequency bands. The periods considered range from 10 to 35 sec. At the Anza array in southern California, the plane wave direction is deflected from the great circle azimuth of the event by up to 20°. In addition, we find a surprisingly strong frequency dependence of the same magnitude and a striking antisymmetric pattern of the deflection as a function of backazimuth, whereas the curvature of the wavefront is small. Similar characteristics are found at the Grafenberg array in Germany and the NORSAR array in Norway, however, with much weaker amplitudes of ∼5°. We ascribe the behavior at Anza to structure in the lower crust and uppermost mantle beneath the array, given that the observations are only a function of source backazimuth and not of source depth and source mechanism, that the wavelengths under consideration range from 50 to 270 km, and that the sign of the deviation is opposite to that predicted from shallow crustal structure and Moho topography. We are able to reproduce the magnitude and frequency dependence of the wavefront deflection using finite difference numerical modeling of plane wave propagation through simple 2D structures.