SEISMIC RESPONSE OF A HILL - THE EXAMPLE OF TARZANA, CALIFORNIA

The Northridge, California, earthquake that strongly shook the city of Los Angeles in January 1994, produced one of the highest ground accel erations ever recorded in an earthquake, at a site located on top of a small hill in Tarzana, about 6 km south of the epicenter. The subsequ ent study of aftershock recordings obtained by a dense seismic array d eployed on the hill a few days after the earthquake showed the existen ce of a strong amplification at stations located at the top of the hil l, relative to stations near the base (Spudich et al., 1996). Resonanc es and polarization rotations were also observed. We investigate in th is study the role that the topography of the site played on the observ ed ground motions and accelerations, To this aim, we perform numerical simulations and study the response of the three-dimensional topograph y of the site to incident shear waves polarized in different direction s. The method used is a boundary integral equation scheme in which the Green's functions are calculated by the discrete wavenumber method. T he results obtained show that the topography of the site, though quite gentle (the hill is less than 20-m high), strongly affects the ground motions in the frequency range between 2 and 15 Hz. Many of the obser ved characteristics of the seismic response at Tarzana are explained i n part by its topography: the consistent amplification of ground motio n at and near the top of the hill, the directional seismic response of the hill that results in a strong amplification of the ground motion transverse to the direction of elongation of the hill, the existence o f a fundamental transverse oscillatory resonance mode of the hill at 3 to 5 Hz, the rotation of the polarization of ground motion, and the s patial variation of amplification over the hill at the fundamental res onance mode. The seismic response of the topography, however, does not fully explain the amplitude of the effects observed. The three-dimens ional geological structure of the site must in some way amplify the ef fect of the topography to produce the observed seismic response. In sp ite of not being as strong as the observed effect, the topographic eff ect of the site is considerable. The ground motion is amplified by fac tors ranging from 30% to 100% at some locations in the frequency range from 2 to 15 Hz. Rapid spatial variations of ground-shaking intensiti es can take place over distance scales of a few tens of meters at high frequency. Finally, the results of the simulation indicate that the t opography of the site amplified the large east-west accelerations reco rded there during the Northridge mainshock by 30% to 40%.