VARIABILITY OF SITE RESPONSE IN THE SAN FERNANDO VALLEY FROM THE 1994 NORTHRIDGE EARTHQUAKE USING AFTERSHOCK DATA AND SEISMIC REFLECTION MODELING

We present results from two related studies describing the variability of site response observed in aftershock recordings in the San Fernando Valley and possible structural causes for these observations. Spectral ratios from aftershock records, taken with respect to a low-amplitude reference site document the variation in site amplification in the frequency range 2 to 6 Hz, both spatially and with backazimuth to the source. Relative site-response estimates between nearby stations demonstrate that preferred directions of higher motion can exist even in areas with no surface topographic effects. A correlation exists between late arrival times of P and S waves and larger site amplification in Sherman Oaks and Northridge. These observations, along with waveform modeling and seismic reflection profiles, suggest that sedimentary structures such as folds and buried basins in the upper 2 km, as well as topography on the sediment-basement interface, can focus energy in spatially restricted areas at the surface. As such, these structures may play a dominant role in the modification of local ground motions. Significant variations in site response over short distances (up to a factor of 2 over 200 m) are not explained simply by differences in surficial geology or shallow S-wave velocity. P-wave seismic reflection data were acquired across high-damage areas in Sherman Oaks to determine whether shallow (less than 1-km depth) geologic structures contributed to the dramatic localized damage resulting from the Northridge earthquake. We believe these data reveal a geologic structural geometry in the upper 600 m that contributed to the increased earthquake ground shaking in the high-damage areas south of and along the Los Angeles River. A low-velocity sub-basin, overlying a southward-thinning wedge of basin sediments, may be up to 150 m thick beneath the channelized Los Angeles River. Two-dimensional elastic and SH-mode finite-difference modeling suggests that a peak horizontal-velocity amplification factor of two-and-greater can be explained in the high-damage area above the sub-basin by the imaged structural geometry.