NONLINEAR SEDIMENT RESPONSE DURING THE 1994 NORTHRIDGE EARTHQUAKE - OBSERVATIONS AND FINITE SOURCE SIMULATIONS

We have addressed the long-standing question regarding nonlinear sedim ent response in the Los Angeles region by testing whether sediment amp lification was similar between the Northridge earthquake and its after shocks. Comparing the weak- and strong-motion site response at 15 sedi ment sites, we find that amplification factors were significantly less for the main shock implying systematic nonlinearity. The difference i s largest between 2 and 4 Hz (4 factor of 2), and is significant at th e 99% confidence level between 0.8 and 5.5 Hz. The inference of nonlin earity is robust with respect to the removal of possibly anomalous sed iment sites and how the reference-site motion is defined. Furthermore, theoretical ground-motion simulations show no evidence of any bias fr om finite source effects during the main shock. Nonlinearity is also s uggested by the fact that the four sediment sites that contain a clear fundamental resonance for the weak motion exhibit a conspicuous absen ce of the peak in the strong motion. Although we have taken the first step of establishing the presence of nonlinearity, it remains to defin e the physics of nonlinear response and to test the methodologies pres ently applied routinely in engineering practice. The inference of nonl inearity implies that care must be exercised in using sediment site da ta to study large earthquakes or predict strong ground motion.