FIRST-GENERATION SITE-RESPONSE MAPS FOR THE LOS-ANGELES REGION-BASED ON EARTHQUAKE GROUND MOTIONS

Ground-motion records from aftershocks of the 1994 Northridge earthqua ke and mainshock records from the 1971 San Fernando, 1987 Whittler Nar rows, 1991 Sierra Madre, and 1994 Northridge earthquakes are used to e stimate site response relative to a rock site for the urban Los Angele s area. Site response is estimated at 232 mainshock and 201 aftershock sites relative to a low-amplitude site in the Santa Monica Mountains. Average amplification values are calculated for the frequency bands: 1 to 3, 3 to 5, and 5 to 7 Hz. These bands are chosen based on limitat ions in aftershock recording equipment at lower frequencies and reduce d significance to the building inventory at higher frequencies. Site a mplification factors determined at the instrumented locations are grou ped by the surficial geology and contoured to produce a continuous spa tial estimation of amplification. The maps in this article represent t he first attempt to produce estimates of site amplification based on o bservations of ground motion for such a large areal extent of the Los Angeles region. These maps are expected to evolve as more data become available and more analysis is done.