Large Teleseismic P Wavefront Deflections Observed with Broadband Arrays

Abstract

We measure the plane wavefront incidence azimuth for teleseismic P at large-aperture (ϳ50 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 20Њ. 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 Gräfenberg array in Germany and the NORSAR array in Norway, however, with much weaker amplitudes of ϳ5Њ. 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.

Quantifying Site Effects and Their Influence on Earthquake Source Parameter Estimations Using a Dense Array in Oklahoma

“…The high amplitude ground motions correlate with Pleistocene terrace deposits (Qt) at 2–4 Hz and shift to the youngest formation — the Holocene alluvium (Qal) — at increasing frequencies (Figure 3d), showing the depth sensitivity shifting to shallower layers in the subsurface with increasing frequency. A > 8 Hz amplification relates to a scale of less than 100 m near the surface (Al‐Shukri et al., 1995).…”

“…The first requirement is that there should be minimal variations across the array caused by path and source effects. The second requirement is the availability of higher frequency signals, ideally, above 4 Hz (Al‐Shukri et al., 1995), to characterize near‐surface site conditions in the same frequency range that is used to model source parameters. The earthquakes that best meet the first requirement are teleseismic earthquakes (>1,000 km), but the signal bandwidths of teleseismic events are generally below 1 Hz.…”

Theory and Observations - Seismic Anisotropy

“…The amplitude and direction of anisotropy inferred with this method were found to be consistent with other methods, in particular SKS splitting. Using arrays of stations to constrain the propagation direction and polarizations simultaneously shows however that long-period P-wave propagation may be affected significantly by local isotropic structure, so that a simple inference of anisotropy should be made with caution (Schulte-Pelkum et al, 2003).…”