Saturation of Maximum Acceleration Near the Epicenter of Large Shallow Earthquakes Caused by Nonlinear Response of Surface Layers

We analyze accelerograms from large events (2000 Tottori (Mw=6.6) and 2003 Miyagi (Mw=6.1)) and small events (their foreshocks and aftershocks) recorded at KiK-net stations operated by National Institute of Earth Science and Disaster Prevention (NIED), Japan. Spectral ratio between accelerograms recorded on the ground surface and in the borehole (about 100m deep) at the same station shows amplification characteristics of surface layers. Spectral ratio (surface/borehole) for the large events at stations near the epicenter is very different from that for the small events, which is caused by the nonlinear response of surface layers. The peak frequency of the spectral ratio for the large events shifts towards the low frequency range. Also, reduction of the spectral ratio in the high frequency range is apparent for the large events. The difference between the spectral ratios for the large and small events becomes unclear at distant stations. The ratio of maximum accelerations (surface/borehole) plotted against the hypocentral distance also shows a difference between large and small events. The ratio of maximum acceleration for the large events decreases with the decrease in hypocentral distance while that for the small events is almost constant independently of the distance. The saturation of maximum acceleration near the epicenter of large earthquakes has been generally explained as the effect of finiteness of the earthquake source dimension. However, the present result suggests that the nonlinear response of surface layers is also one of the causes of saturation of maximum acceleration near the epicenter of large shallow earthquakes.