Evaluation of Ground Motion Intensity at Wakago Area in Niijima Island during the Earthquake on July 15, 2000

In order for better understanding of ground motion intensity in the near-field region, the damage survey of tombstones and questionnaire seismic intensity survey are conducted at Wakago and Honson areas in Niijima Island for the earthquake on July 15, 2000 (Mw6.0). The relations of damage indexes on overturned tombstones and questionnaire seismic intensity to ground motion intensities are examined. As a result, the ground motion intensity in Wakago area is estimated as the J. M. A. seismic intensity of 6.1, peak ground acceleration of about 800cm/s2 and peak ground velocity of about 80-100cm/s. The synthesized seismograms calculated from the existing fault model are compared with the observed strong motion records, and the agreement of calculations and observations is not good. Therefore the fault parameters are modified from the existing fault model. The modified fault model is closer to Niijima Island as compared to the earlier one. The synthesized seismograms calculated from this model show better agreements with the observed ground motion records. According to the synthesized seismogram calculated fromm the modified fault model, in Wakago area, the ground motion around the period of 1sec is dominant in north-south direction. This may be mainly due to the effect of the fault rupture.