Observations of basin ground motions from a dense seismic array in San Jose, California

We installed a dense array of 41 digital seismographs in San Jose, Californ ia, to evaluate in detail the effects of a deep sedimentary basin and shall ow sedimentary deposits on earthquake ground motions. This urban array is l ocated near the eastern edge of the Santa Clara Valley and spans the Evergr een sedimentary basin identified by gravity data. Average station spacing i s 1 km, with three stations initially spaced 110 m apart. Despite the high- noise urban environment, the stations of the array successfully triggered o n and recorded small local earthquakes (M 2.5-2.8 at 10-25 km distance) and larger regional events such as the M 5.0 Bolinas earthquake (90 km distanc e), M 4.6-5.6 earthquakes near Mammoth Lakes (270 km distance), M 4.9-5.6 e vents in western Nevada (420 km distance) and the M 7.1 Hector Mine earthqu ake (590 km distance). Maps of spectral ratios across the array show that t he highest amplitudes in all frequency bands studied (0.125-8 Hz) are gener ally observed at stations farther from the eastern edge of the Santa Clara Valley. Larger spectral amplitudes are often observed above the western edg e of the Evergreen Basin. Snapshots of the recorded wavefield crossing the array for regional events to the east reveal that large, low-frequency (0.1 25-0.5 Hz) arrivals after the S-wave travel from south to north across the array. A moving-window, cross-correlation analysis finds that these later a rrivals are surface waves traveling from the south. The timing and propagat ion direction of these arrivals indicates that they were likely produced by scattering of incident S waves at the border of the Santa Clara Valley to the south of the array. It is remarkable that the largest low-frequency pha ses at many of the valley sites for regional events to the east are basin s urface waves coming from a direction about 70 degrees different from that o f the epicenters. Basin surface waves emanating from the eastern edge of th e valley are also identified by the crosscorrelation analysis.