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Institute of Geophysics and Planetary Physics University of California at San Diego, La Jolla, California 92093
Department of Geological Sciences Indiana University, 1005 10th St., Bloomington, Indiana 47405
Department of Geological Sciences The University of South Carolina, Columbia, South Carolina 29208
University of Alaska, Fairbanks, 903 Koyukuk Dr., POB 757320, Fairbanks, Alaska 99775-7320
BP Exploration Inc., 8332 Leamont Dr., Houston, Texas 77072
Abstract
Analysis of data collected by a high-frequency array experiment conducted at Pinyon Flat in southern California provides strong evidence that the high-frequency wave field from local earthquakes at this hard-rock site are strongly distorted by near-surface scattering. The seismic array we deployed consisted 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 element grid array with a nominal spacing of 7 m centered over the borehole sensors. The remaining 22 seismometers were laid out in two 11-element linear arrays 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 waves. This reveals systematic variations of particle motion across the array in which the particle motion tracks tilt drastically away from the backazimuth expected for an isotropic medium. These variations, however, are frequency dependent. Below around 8 Hz, the particle motions become virtually identical for all stations. At progressively higher frequencies, the wave-field particle motion becomes increasingly chaotic. Frequency-wave-number analysis of these data provide quantitative measures of the same phenomena. We find that direct wave f-k spectra are bathed in a background of signal-generated 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 indicating the wavelength of this "noise" on the scale of tens of meters and less. Refraction measurements we made on two lines crisscrossing the array reveal that the weathered layer velocities are highly variable and define a very strong wave guide. Measured surface P-wave velocities varied from 400 to 1300 m/sec, and velocities at depth of approximately 15 m varied from 1600 to 2700 m/sec. Previous measurements in the boreholes showed that the intact granite below about 65 m depth has a velocity of approximately 5400 m/sec. These results demonstrate the extreme velocity contrast and degree of velocity heterogeneity of the near surface at this site. We conclude that all the observations we made can be explained by strong scattering of incident body-wave signals into a complex mishmash of body-wave and surface-wave modes in this heterogeneous near-surface wave guide.
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