1999 年 52 巻 1 号 p. 11-24
The distribution of seismic intensity I=VII (very disastrous) for the 1995 Hyogoken-Nanbu earthquake was reported by the Japan Meteorological Agency (JMA). It was the first announcement since the seismic intensity in JMA scale had been revised in 1949 to include the highest class of I=VII. Originally the seismic intensity of I=VII was defined as “strong ground motion with collapse more than 30% of wooden houses”, which was based on destructive damage at the 1948 Fukui earthquake. During the last half century, aseismic design of wooden houses has progressed especially due to the popularization of the standard building code published in 1950. Recent cities like Kobe include various kinds of buildings in seismic performance from modern earthquake-resisting structures to old and vulnerable residences. Therefore the seismic intensity of I=VII should be recognized as “with collapse more than 30% of less aseismic wooden houses such as those built before 1950”. From these points, it must be confirmed whether the increase of the seismic performance of buildings was taken into account in the reported distribution of I=VII. First we review the term of “collapse of houses”. Then relationship between the collapse rate of houses and the overturning acceleration of tombstones is investigated and analyzed using damage data obtained from the 1995 Hyogoken-Nanbu earthquake. The analysis result and its comparison to the relationships for past earthquakes show that the average of the seismic performance has increased by 40-60% from 1948 to 1995, and that the collapse rate of 10% at the Hyogoken-Nanbu earthquake corresponds to that of 30% at the Fukui earthquake for the same overturning acceleration. Comparing the reported distribution of I=VII to the area with collapse more than 10%, historical continuity of the seismic intensity in JMA scale is discussed.