Borehole P- and S-wave velocity at thirteen stations in Southern California

INTRODUCTION The U.S. Geological Survey (USGS), as part of a program to aquire seismic velocity data at locations of strong-ground motion in earthquakes (e.g. Gibbs, et al., 2000), has investigated thirteen additional sites in the Southern California region. Of the thirteen sites, twelve are in the vicinity of Whittier, California and one is located in San Bernardino, California. Several deployments of temporary seismographs were made after the Whittier recorded significant aftershock data. These portable sites with the exception of Santa Anita Golf Course were co-sited with strong-motion recorders. were part of an array of digital strong-motion stations deployed from "bedrock" in Whittier to near the deepest part of the Los Angeles basin in Norwalk. Although development and siting of this new array (patially installed at the time of this writing) was generally motivated by the Whittier Narrows earthquake, these new sites (with the exception of HOO) were not part of any Whittier Narrows aftershock deployments. A similar new digital strong-motion site was installed at the San Bernardino Fire Station during the same time frame. Velocity data were obtained to depths of about 90 meters at two sites, 30 meters at seven sites, and 18 to 25 meters at four sites. Lithology data from the analysis of cuttings and samples, was obtained from the two 90-meter deep holes and from five of the shallower holes to supplement the velocity interpretation. The two 90-meter boreholes (SB1, CPB) have been instrumented with borehole seismometers for continuous monitoring of earthquake activity (Rogers, et al., 1998). No drill samples or cuttings were obtained from the other six sites but driller's logs were scanned for major changes noted there. The velocity models at those sites were interpreted using only the measured data and major changes in the driller's log as noted above. The sites are shown in Figure 1 and listed in Table 1, which gives references to information regarding the strong-motion data. Several hundred strong-motion records of the main-shock were written by this moderate size earthquake (ML = 5.9) making it important from a scientific and engineering prospective (Brady et al., 1988, Shakal et al., 1988).