Abstract

We study very shallow (<200 m) S-wave anisotropy effects on local earthquake seismograms recorded by a vertical array of accelerometers at Garner Valley (Southern California). Using seismic events recorded by sensors buried at depth in the crystalline bedrock (220 m), in weathered crystalline rocks (22 m), and in sediments (15, 6, and 0 m), we analyze the S-wave vertical travel times and polarizations at different depths. Hodograms plotted for horizontal components at S onset exhibit a clear linear and constant polarization independent of both earthquake location and mechanism. This alignment varies with depth and is N 325° ± 13° at 220 m while N 25° ± 20° at 22, 15, 6, and 0 m. (a) For the deep propagation (>220 m), the S-wave direction of polarization at 220 m is identical to the observation obtained on a crystalline outcrop at the KNW USGS stations, 7 km away. Studies conducted on this site concluded to a rock fabric anisotropy due to the alignment of minerals and/or microcracks along the lineation direction. (b) For the shallow (<220 m) propagation, we observe a N 0° ± 20° fast-axis anisotropy with an 8 ± 2% magnitude between 220 and 22 m. The associated Sfast-Sslow travel-time delays roughly correspond to half the dominant period of the signal, and the subsurface linear polarization is interpreted as a degenerated ellipse. The north ± 20° anisotropy fast axis coincides with the maximum horizontal compressive stress associated with the San Andreas fault system. The shallow anisotropy extension corresponds to slow velocities associated with altered granite. It suggests either a stress-induced anisotropy due to pore/crack deformation in an altered (e.g., hydrothermally) medium, or the superposition of two crystalline units having different lineation directions. These data show that in presence of slow 0- to 200-m subsurface layers, shallow anisotropy strongly affects the earthquake surface records in the usual frequency range, below 30 Hz, used in short-period seismology.

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