There is practically no amplification from the bottom to the top of the hill for the component parallel to the strike of the hill. 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). More than twenty events, including the Hector Mine earthquake, were recorded by all these instruments at Tarzana.

The original station, Tarzana - Cedar Hill Nursery A, was lost in 1999 due to construction. 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. After the Northridge earthquake the California Strong Motion Instrumentation Program significantly increased instrumentation at Tarzana to study the unusual site amplification effect. The subsurface geology and velocities obtained allow classification of this location as a soft-rock site. Modelo formation (extremely weathered at the surface to fresh at depth) underlies the hill. The 20 m high hill was found to be well drained with a water table near 17 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 Tarzana site was drilled to a depth of 100 m. 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. 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. Directional Site Amplification Effect on Tarzana Hill, California

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