9416424 Darragh The aim of this project is to increase the understanding of site effects that may have caused the large shaking at Tarzana and the Santa Monica Freeway collapse near La Cienega during the 1994 Northridge, CA earthquake. The anticipated results at Tarzana will be useful for site characterization, topographic amplification and microzonation at similar sites. The anticipated results at I-10 will be useful for site characterization as well as design and construction practices of freeway structures at similar sites. The results will be shred with bridge designers. The study focuses on the site response at two California Strong Motion Instrumentation Program (CSMIP) stations: Tarzana and Baldwin Hills (located near the Santa Monica (I-10)freeway collapse at La Cienega). The objectives at both sites are to clarify: (1) the effects of different geology on site response by the acquisition and compilation of seismic, geologic and geotechnical data at the surface and from drill holes; and (2) the contribution of the site in strong motion records of the mainshock by using aftershock recordings of particular interest are the causes of the sustained and high amplitude shaking of over 1 g that was recorded at Tarzana, and the local site response effects that may have amplified seismic waves and caused damage at the collapsed I-10 freeway at La Cienega. The following methods are employed: 1.Site characterization through standard shear-wave and compressional-wave velocity measurements (surface and downhole) and geotechnical testing of drillhole samples damping, density and modulus reduction) from surface to bedrock (or to very stiff material). 2.Site response is estimated by calculating the ratio of the smoothed geometric-mean Fourier amplitude spectra from aftershocks recorded at both the strong motion station and nearby sites For stations located at similar distances and azimuths from the earthquake, the spectral-ratio method is olates the site response from source and path effects. 3.Site response predictions from one-dimensional linear and equivalent-linear models and a two-dimensional nonlinear model are compared to actual response from strong motion recordings to identify site effects and separate them from possible source and path effects. ***
