Seismic Wave-Field Observations at a Dense, Small-Aperture Array Located on a Landslide in the Santa Cruz Mountains, California

A rectangular (4 by 5) array of short-period three-component seismometers with 15-m spacing was deployed to record several U.S. Geological Survey calibration explosions detonated around the Santa Cruz Mountains. The array was located at a site where an earlier station had recorded frequency-dependent polarized site resonances for aftershocks of the 1989 Loma Prieta earthquake. The site is on a hillside believed to be a landslide structure, with the near surface consisting of poorly sorted sediments and weathered rocks with dipping subsurface layers. The primary objective was to explore the site effects in this complex three-dimensional soft-rock environment, characteristic of much of the Loma Prieta source region. The direct P waves from four nearby (15 to 20 kin) explosions at easterly azimuths from the array show counterclockwise arrival azimuth anomalies of 30 ° to 50 ° . These deflections are attributed to the presence of more than one dipping velocity contrast beneath the array, with dips of from 10 ° to 50 ° and dip directions generally toward the south. One such boundary may correspond to the landslide slip surface, and the presence of dipping velocity contrasts underlying the site is probably responsible for some of the observed directional site resonance. A slowness vector analysis demonstrates that arrivals early in the P coda have similar azimuthal anomalies, while later scattered arrivals come from many azimuths. Particle motions indicate that the more coherent arrivals in the coda are comprised of scattered P waves and Rayleigh waves, probably associated with scattering from the rough topography in the region. The coda displays greater spatial coherency along the hill strike than down the slope, consistent with a wedge-shaped landslide. The overall wave-field spatial coherence, CCC(f, Ax), decreases with increasing frequency, f, and spatial offset, Ax, and on average can be well represented by CCC(f, Ax) = e -cfAx, with c = 0.6 k m 1 H z 1 for the vertical P wave in the first 1-sec window. This behavior is comparable to that found for previously studied hard-rock locations.