Insights on nonlinear soil behavior and its variation with time at strong-motion stations during the Mw7.8 Kaikōura, New Zealand earthquake

Abstract To the end of investigating the potential nonlinear soil behavior at 44 strong motion stations during the 2016 Mw7.8 Kaikōura, New Zealand earthquake, we analyzed the modifications of the site response under the strong mainshock recording based on the horizontal-to-vertical spectra ratio, including the shift of the predominant frequency, and three nonlinearity indicators, DNL, ADNL, and PNL. Significant modifications to site response were observed at more than half of the stations, where the predominant frequency generally shifted to the lower one, and the DNL, ADNL, and PNL values were generally greater than 4.0, 0.3, and 10%, respectively. The strain proxy represented by PGV/Vs30 was found in high level as the nonlinearity indicators were high. The deformation proxy may be a better indicator predicting the nonlinear soil behavior at sites with available shear-wave velocity. After the nonlinear soil behavior occurred, the soil structure completely recovered and the nonlinearity vanished after a period of time. The recovery process was dependent on the ground motion intensity immediately after the strong ground shaking, and the soil condition.