MAGNITUDE OF NONLINEAR SEDIMENT RESPONSE IN LOS-ANGELES BASIN DURING THE 1994 NORTHRIDGE, CALIFORNIA, EARTHQUAKE

The study of nonlinear site response has practical difficulties due to large ambiguities in isolating local response from other competing ef fects. We chose a sedimentary site LF6 in Los Angeles basin that (1) h as the closest reference rock sites available, compared to other stati ons, allowing an accurate estimation of local amplification, and (2) i llustrates clear resonance in the near surface. In our opinion, this c ase represents the least ambiguity in the identification of possible n onlinearity. The site responses during the Northridge, the 1987 Whittl er Narrows events and the Northridge aftershocks are compared. The sta tion shows a fundamental resonance-frequency change between the higher - and lower-amplitude motions in the entire ensemble of 17 events used . The net shear-modulus reduction during the Northridge event is a fac tor of 1.3 to 1.4 compared to the Whittler Narrows event and is a fact or of 1.7 compared to the aftershocks. This result provides guidance o f what to expect at other sites in the basin, where the nonlinear resp onse is less easy to characterize.