HR: 1330h
AN: S52A-0130    [PDF]
TI: Directional Site Amplification Effect on Tarzana Hill, California
AU: * Graizer, V
EM: vgraizer@consrv.ca.gov
AF: California Geological Survey, 801 K Street, MS 13-35, Sacramento, CA 95814-3531 United States
AU: Shakal, A
EM: tshakal@consrv.ca.gov
AF: California Geological Survey, 801 K Street, MS 13-35, Sacramento, CA 95814-3531 United States
AB: Significantly amplified ground accelerations at the Tarzana Hill station were recorded during the 1987 Mw 5.9 Whittier Narrows and the 1994 Mw 6.7 Northridge earthquakes. Peak horizontal ground acceleration at the Tarzana station during the 1999 Mw 7.1 Hector Mine earthquake was almost twice as large as the accelerations recorded at nearby stations. The Tarzana site was drilled to a depth of 100 m. A low shear-wave velocity near the surface of 100 m/sec increasing to near 750 m/sec at 100 m depth was measured. The 20 m high hill was found to be well drained with a water table near 17 m. Modelo formation (extremely weathered at the surface to fresh at depth) underlies the hill. The subsurface geology and velocities obtained allow classification of this location as a soft-rock site. After the Northridge earthquake the California Strong Motion Instrumentation Program significantly increased instrumentation at Tarzana to study the unusual site amplification effect. Current instrumentation at Tarzana consists of an accelerograph at the top of Tarzana hill (Tarzana - Cedar Hill B), a downhole instrument at 60 m depth, and an accelerograph at the foot of the hill (Tarzana - Clubhouse), 180 m from the Cedar Hill B station. The original station, Tarzana - Cedar Hill Nursery A, was lost in 1999 due to construction. More than twenty events, including the Hector Mine earthquake, were recorded by all these instruments at Tarzana. Comparison of recordings and response spectra demonstrates strong directional resonance on the top of the hill in a direction perpendicular to the strike of the hill in the period range from 0.04 to 0.8 sec (1.2 to 25 Hz). There is practically no amplification from the bottom to the top of the hill for the component parallel to the strike of the hill. In contrast to accelerations recorded during the Hector Mine earthquake (high frequency part of seismic signal), displacements (relatively low frequency part of seismic signal) demonstrate almost no site amplification from the bottom of the hole to the surface at periods greater than 1.5 sec, in either direction. The directional effect at Tarzana hill seems to be azimuth dependent. Relatively higher amplification at the perpendicular component is produced for the earthquake sources located north of the station. We were not able to see any differences in hill response before and after development (a relatively small part of the hill was developed). The source of the site amplification that produces large motions at Tarzana is still under investigation with "the usual suspects" like topography and shear wave velocity profile not providing the explanation. New data recorded at Tarzana in recent years clearly show that the Tarzana effect is a very localized high-frequency effect observed only at the top of the hill. Drilling at Tarzana was co-funded by CSMIP and by the National Science Foundation through the Resolution of Site Response Issues from the Northridge Earthquake Project (ROSRINE).
DE: 7212 Earthquake ground motions and engineering
DE: 7294 Instruments and techniques
SC: Seismology [S]
MN: 2003 Fall Meeting