Near-surface scattering effects observed with a high-frequency phased array at Pinyon Flats, California

Analysis of data collected by a high-frequency array experiment conducted a t Pinyon Flat in southern California provides strong evidence that the high -frequency wave field from local earthquakes at this hard-rock site are str ongly distorted by near-surface scattering. The seismic array we deployed c onsisted of 60, 2-Hz natural frequency, three-component sensors deployed in a three-dimensional array. Two of the sensors were located in boreholes at 150 and 275 m depth. The other 58 sensors were deployed in an areal array above these boreholes. Thirty-six of these were deployed in a 6-by-6 elemen t grid array with a nominal spacing of 7 m centered over the borehole senso rs. The remaining 22 seismometers were laid out in two Ii-element linear ar rays radiating outward from the grid. Coherence calculations reveal a rapid loss of coherence at frequencies over 15 Hz at all but the shortest length scales of this array. Three-dimensional visualization techniques were used to closely examine the spatial stability of particle motions of P and S wa ves. This reveals systematic variations of particle motion across the array in which the particle motion tracks tilt drastically away from the backazi muth expected for an isotropic medium. These variations, however, are frequ ency dependent. Below around 8 Hz, the particle motions become virtually id entical for all stations. At progressively higher frequencies, the wave-fie ld particle motion becomes increasingly chaotic. Frequency-wavenumber analy sis of these data provide quantitative measures of the same phenomena. We f ind that direct wave f-k spectra are bathed in a background of signal-gener ated noise that varies from 10 to 30 dB down from the direct arrival signal . This signal-generated noise appears to be nearly white in wavenumber indi cating the wavelength of this "noise" on the scale of tens of meters and le ss. Refraction measurements we made on two lines crisscrossing the array re veal that the weathered layer velocities are highly variable and define a v ery strong wave guide. Measured surface P-wave velocities varied from 400 t o 1300 m/sec, and velocities at depth of approximately 15 m varied from 160 0 to 2700 m/sec. Previous measurements in the boreholes showed that the int act granite below about 65 m depth has a velocity of approximately 5400 m/s ec. These results demonstrate the extreme velocity contrast and degree of v elocity heterogeneity of the near surface at this site. We conclude that al l the observations we made can be explained by strong scattering of inciden t body-wave signals into a complex mishmash of body-wave and surface-wave m odes in this heterogeneous near-surface wave guide.