1998 年 50 巻 4 号 p. 485-505
Two types of earthquakes have occurred in and around Japan, which have resulted in severe damages. They are inter-plate earthquakes in subduction zones around the Japan Islands, and intra-plate earthquakes in the upper crust beneath the mainland of Japan. The faults associated with intra-plate earthquakes usually exist within a depth of 20km. Six intra-plate earthquakes causing a toll of more than 1, 000 lives have occurred since the end of the 19th century. The faults responsible for these events were estimated from the trains of surface fault breaks and the measured crustal deformations. Three intra-plate earthquakes with magnitude M larger than 7, which killed more than 200 people, have also occurred in the same period. Their causative faults have also been estimated. Many reports of damage surveys of the above nine intra-plate earthquakes were examined to investigate conditions necessary for generating strong ground motion with seismic intensity I=VII (very disastrous) in the Japan Meteorological Agency (JMA) scale, and to elucidate characteristics of strong ground motion with I=VII. According to the original definition of the JMA scale, I=VII (very disastrous) corresponds to “Collapse of more than 30% of wooden houses”. The data on damage to wooden houses for the nine events show that the area of I=VII extends up to 5km on both sides of the fault in the case of faults through mountainous region. On the other hand, in the case of earthquake faults lying underneath basins, where sediments have accumulated from the Late Pleistocene to Holocene, the area of I=VII extends over a wider area and occasionally fills the whole basin. The ground condition is seen to be an important factor in generating strong ground motion with I=VII. A similar result is obtained from an examination of the 1923 Kanto earthquake near source region with the thrusting fault at a depth shallower than 20km. The Kanto earthquake is one of the typical disastrous inter-plate earthquakes in Japan. The data on the directions of simple bodies that have overturned and collapsed wooden houses in the proximity of earthquake faults indicate that systematically larger ground motions occur in a direction normal to the strike than parallel to it irrespective of the type of faulting. The effective periods for these simple bodies and wooden houses are estimated to be in the range of 0.3sec to 1.5sec. These results indicate that the dominant strong ground motion in the direction normal to the strike plays an important role in generating the severe damages in regions hit by intensity I=VII.