Variation of Site Response at the UCSB Dense Array of Portable Accelerometers

Ten Kinemetrics model SSA‐2 Solid State Accelerographs were deployed in two dense arrays following the Landers‐Big Bear earthquake sequence. The two arrays were separated by approximately three km, the first at a shallow alluvial soil site and the second at a rock site. We examine the soil and rock sites in terms of spectral ratio and cross‐spectrum estimates of the site response. In order to construct an accurate representation of the motion in the horizontal plane, we treat the two horizontal components simultaneously as a complex signal. The Fourier transform of this complex signal represents the true motion in the horizontal plane as expressed in the frequency domain. The spectral ratio estimate is the ratio of this Fourier transform at the soil sites to the rock sites. The cross‐spectrum estimate is the ratio of the cross‐spectral density between the soil and rock sites to the power spectral density of the rock site. Spectral ratio estimates of site amplification are consistently higher than cross‐spectrum estimates. On average the soil sites show amplification factors on the order of 2 to 4 relative to the rock sites between the frequencies of 4 to 15 Hz. There are, however, large variations in the ground motion recorded at sites with separations as small as 80 m. These variations demonstrate that site response studies can be biased by the choice of location of the sensor at distances of 80 m. We conclude that in the analysis of site‐specific amplification both cross‐spectrum and spectral ratio techniques should be used along with ensemble averages over many events.