1990年フィリピン地震によって飛び跳ねた石

Abstract

A large earthquake (M=7.8) struck the central area of Luzon Island on July 16, 1990. A number of stones were thrown out from the former sockets by severe shaking of the main shock. The dislodged boulders and cobbles were distributed within 30 km in north-south and about 10 km in east-west directions form the town of Rizal. The high accelerations of 2.6-3.3g were estimated from the dislodged distances of 45-58 cm, assuming that the seismic waves had a frequency of 3 Hz.By using the dislodged distances of 71 stones which were measured at various areas, the source of great vibration was determined at 7 km south-southeast of Rizal town. The displacement as well as calculated ground accelerations and velocities decrease with distance from this source. The result shows that the survey of thrown-out stones in the epicentral area is effective to estimate the high accelerations exceeding earth's gravity(1g).