STRONG GROUND MOTIONS OF THE 1997 NORTHWESTERN KAGO-SHIMA PREFECTURE EARTHQUAKE AND SEISMIC HAZARD OF THE REGION

摘要

The earthquake of M 6.5 occurred in the northwestern part of Kagoshima prefecture in Japan on March 26 in 1997. After about 50 days, another earthquake of M 6.3 whose source region was adjoining occurred on May 13. Some reinforced concrete school buildings and a lot of wooden houses suffered severe damage. This region is in low seismicity in Japan and there was no record of historical seismic disaster. Many strong ground motions were recorded by the new type seismographs that were settled in high density by the JMA and the NIED after the 1995 Hyogoken-Nanbu Earthquake. Two source regions were under the Mt. Shibi, and so the nearest seismographs were at seven stations in three cities and a town that surround the source regions in different directions and were about 10 km apart from the epicenters. The high peak ground accelerations such as 728 cm/s~2 in Izumi or 902 cm/s~2 in Miyanojyo were observed. These motions with the high peak ground accelerations were observed at the stations that were at right angles to the faults and their response spectra show that they contain predominant high frequencies. The seismic hazard analyses by the statistical method and by the probabilistic method show that the return period of these peak ground accelerations in the region is estimated as 3500 years or 1100 years.