SHALLOW VELOCITY STRUCTURE AND POISSONS RATIO AT THE TARZANA, CALIFORNIA, STRONG-MOTION ACCELERATOR SITE

The 17 January 1994, Northridge, California, earthquake produced stron g ground shaking at the Cedar Hills Nursery (referred to here as the T arzana site) within the city of Tarzana, California, approximately 6 k m from the epicenter of the mainshock. Although the Tarzana site is on a hill and is a rock site, accelerations of approximately 1.78 g hori zontally and 1.2 g vertically at the Tarzana site are among the highes t ever instrumentally recorded for an earthquake. To investigate possi ble site effects al the Tarzana site, we used explosive-source seismic refraction data to determine the shallow (<70 m) P- and S-wave veloci ty structure. Our seismic velocity models for the Tarzana site indicat e that the local velocity structure may have contributed significantly to the observed shaking. P-wave velocities range from 0.9 to 1.65 km/ sec, and S-wave velocities range from 0.20 and 0.6 km/sec for the uppe r 70 m. We also found evidence for a local S-wave low-velocity zone (L VZ) beneath the tup sf the hill, The LVZ underlies a CDMG strong-motio n recording site at depths between 25 and 60 m below ground surface (B GS), Our velocity model is consistent with the near-surface (<30 m) P- and S-wave velocities and Poisson's ratios measured in a nearby (<30 m) borehole. High Poisson's ratios (0.477 to 0.494) and S-wave attenua tion within the LVZ suggest that the LVZ may be composed of highly sat urated shales of the Modelo Formation. Because the lateral dimensions of the LVZ approximately correspond to the areas of strongest shaking, we suggest that the highly saturated zone may have contributed to loc alized strong shaking. Rock sites are generally considered to be ideal locations for site response in urban areas; however, localized, highl y saturated rack sites may be a hazard in urban areas that requires fu rther investigation.