DIRECTIONAL TOPOGRAPHIC SITE RESPONSE AT TARZANA OBSERVED IN AFTERSHOCKS OF THE 1994 NORTHRIDGE, CALIFORNIA, EARTHQUAKE - IMPLICATIONS FOR MAINSHOCK MOTIONS

The Northridge earthquake caused 1.78 g acceleration in the east-west direction at a site in Tarzana, California, located about 6 km south o f the mainshock epicenter. The accelerograph was located atop a hill a bout 15-m high, 500-m long, and 130-m wide, striking about N78 degrees E. During the aftershock sequence, a temporary array of 21 three-comp onent geophones was deployed in six radial lines centered on the accel erograph, with an average sensor spacing of 35 m. Station C00 was loca ted about 2 m from the accelerograph, We inverted aftershock spectra t o obtain average relative site response at each station as a function of direction of ground motion. We identified a 3.2-Hz resonance that i s a transverse oscillation of the hill (a directional topographic effe ct). The top/base amplification ratio at 3.2 Hz is about 4.5 for horiz ontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill. This re sonance is seen most strongly within 50 m of C00. Other resonant frequ encies were also observed, A strong lateral variation in attenuation, probably associated with a fault, caused substantially lower motion at frequencies above 6 Hz at the east end of the hill. There may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill, causing particle motions (not spec tral ratios) at the top of the hill to be rotated about 20 degrees awa y from the direction transverse to the hill. The resonant frequency, b ut not the amplitude, of our observed topographic resonance agrees wel l with theory, even for such a low hill. Comparisons of our observatio ns with theoretical results indicate that the 3D shape of the hill and its internal structure are important factors affecting its response. The strong transverse resonance of the hill does not account for the l arge east-west mainshock motions. Assuming linear soil response, mains hock east-west motions at the Tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hil l. Probable variations in surficial shear-wave velocity do not account for the observed differences among mainshock acceleration observed at Tarzana and at two different sites within 2 km of Tarzana.