Study on the Nonlinear Site Response Using the Green’s Functions of a Near-Surface Layer Estimated for Weak Motions

Two noteworthy points shown in strong motion seismograms recently recorded in near source regions, are the transient response of tilt motion and the nonlinear response of near-surface layer. In the present study, we investigated the nonlinear response of near-surface layer by means of borehole array recordings. First, we estimated the Green’s function of near-surface layer for SH waves assuming that response of near-surface layer is linear for weak motion. Two Green’s functions are estimated for two different data sets which are constructed from the earthquakes occurred before the 2008 Iwate-Miyagi Inland Earthquake and its aftershocks, respectively. Furthermore, we estimated another Green’s function of near-surface layer using the coda part of the main shock of the 2008 Iwate-Miyagi Inland Earthquake. This Green’s function estimated using the coda part of main shock is signicantly different from the other two Green’s functions, although these two Green’s functions are practically identical. This fact reveals that the rigidity of surface layer reduced during the main shock and recovered after that. The coda part of main shock showed the reduced state of near-surface layer’s rigidity. Next, for explaining the strong motion part of main shock, we constructed a time-series model using the Green’s functions estimated using weak motion data. The nonlinear response of near-surface layer in the strong motion part is represented by the time-dependent pole conguration involving in the time-series model, which is constructed by an exponential auto-regressive model of second order. Hence, our constructed model is applicable to the whole process of main shock. Finally, we validated the practicability of model to the data involving the nonlinear response of near-surface layer by using borehole array recordings obtained at the IWTH25 station.