Nonlinear site response at liquefiable sites: Insights from downhole seismic observations

Abstract

The nonlinear behavior of shallow soil plays an important role in modifying surface ground motions. We analyze the nonlinear site response at liquefiable sites using the compiled long-term ground motion data (ranging from 1992 to 2020 with over 2000 earthquake events) at four high-quality downhole arrays that have liquefied during at least one earthquake. Two nonlinear event parameters, the percentage of nonlinearity (PNL) and the frequency shift parameter (fsp), are used to quantify the modification of site response of strong events to that of weak events. Site-specific regression analyses are performed to correlate nonlinear event parameters to incident motion parameters. Results indicate that site response at liquefiable sites varies from linear to nonlinear to liquefied states as the incident motion intensity changes, and the occurrence of liquefaction could enhance the amplification of low-frequency and very-high-frequency ground motions. As strong-motion observations for liquefiable sites are very limited, the findings of this work are of significance for further study of strong-motion seismology at liquefiable sites.