Refined thresholds for non-linear ground motion and temporal changes of site response associated with medium-size earthquakes

We systematically analyse non-linear effects and temporal changes of site response associated with medium-size earthquakes, using seismic data recorded by the Japanese Strong Motion Network KIK-Net. We apply a sliding-window spectral ratio technique to surface and borehole strong motion records at six sites, and stack results associated with different earthquakes that produce similar peak ground acceleration (PGA). In some cases we observe a weak coseismic drop in the peak frequency when the PGA is as small as ∼20–30 Gal, and near instantaneous recovery after the passage of the direct S waves. The percentage of drop in the peak frequency starts to increase with increasing PGA values. We also observe a coseismic drop in the peak spectral ratio for two sites. When the PGA is larger than ∼60 Gal to more than 100 Gal, we observe considerably stronger drops of the peak frequencies followed by logarithmic recovery with time. The observed weak reductions of peak frequencies with near instantaneous recovery likely reflect non-linear response with essentially fixed level of damage, while the larger drops followed by logarithmic recovery reflect the generation (and then recovery) of additional rock damage. The results indicate clearly that non-linear site response may occur during medium-size earthquakes, and that the PGA threshold for in situ non-linear behaviour is lower than the previously thought value of ∼100–200 Gal.