Effects of Surface Geology on Seismic Motion

This study presents observations of nonlinear soil behavior evidence at several KiK-net sites. First, we compare the site responses performed with 1) the recordings from the aftershocks and 2) the recordings from the main event of the 2011 Tohoku earthquake at four sites. Then, to understand the influence of soil parameters on nonlinear effects, we extend the results to the whole network. We compute the linear (from recordings with surface PGA<20 gals) to nonlinear (from recordings with surface PGA>200 gals) site responses ratio. We study the dependency of this ratio with respect to Vs30, the predominant frequency and the resonance frequency of the linear site response. Finally, to understand the influence of ground motion intensity on nonlinear effects, we compare site responses computed from recordings with different PGA ranges at four KiK-net stations. We define a new parameter fnlS which represent the frequency from which non-linear effect is significant; ie the frequency from which the linear to non linear ratio (lower 68% confidence limit) is above one. We find that fnlS correlate well with Vs30, f0 and fpred and for most of sites fnlS is in between f0 and fpred.The fact that the predominant frequency or higher modes are more de-amplified than the fundamental resonance frequency suggests that nonlinear soil behavior occurs at shallow depths and that only subsurface investigations of dynamic soil parameters might be enough to characterized the nonlinear behavior of the soil column.