The Mikawa district, the southern part of Aiti Prefecture, suffered serious damages in the earthquakes that shook the locality twice successively on December 7, 1944, and on January 13, 1945. We made detailed studies on the distribution of damaged houses and the difference of the manner of their distribution in the two respective shocks. The aim of the present paper is to find out, through such studies, what extent, of close relation there exists between seismic destruction and the condition of the ground. Now we will represent the seismic destruction rate (Y) by Y=(D+0.5H)|M, where M is the total number of the dwelling houses in a certain area, D and H the numbers of totally destroyed and partially destroyed houses in that area. As a unit of the area within which Y was to be determined, the minour division (Aza) of each town or village was taken. The geographical distributions of Y with respect to the first and second earthquakes are shown in Figs.2 and 3. As shown in these figures the value of Y differs greatly with the locality ; in the hilly north and east Y is very small, while in the coastal regions in the south-west and in the zone along the River Yanagi and Old Yahagi Y has a large value. Although this general feature is common to the two occasions, closer comparison of the two figures will reveal many points o difference in them. Especially, in Fig.3, the e icenter of the second earthquake was seated so near the affected areas that the value of Y varied considerably with the epicentral distances of the respective sections. It was desirable to eliminate the factor, contained in Y, that was due to the difference of the epicentral distancies. We picked up several sections that were located on a firm hilly ground, and tried to show, so to speak, by a standard curve, the relation between the Y of such sections and the epicentral distance Δ. The curve is represented by Y_c=59.8e^<-0.0986Δ>, Y_c being understood to represent the standard value of Y. By computing the value of Y/Y_c we can arrive at a sort of measure of seismic destruction in any locality separated from the effect of the epicentral distance. Assuming that the difference of the value of Y/Y_0 with the respective locality results exclusively from the uneven quality of the ground, we suggest for this value of S=Y/Y_c the name of the vulnerability rate of the ground of the locality. The epicenter of the first shock was situated so far from the Mikawa district that the difference of the distance of the various sections of the district from the epicentre was of little significance. Therefore the figure representing the distribution of Y of the first shock may well be considered to represent that of S also. if the values of S represents the true vulnerability rate of the ground accurately enough, the manner of distribution of S in the first shock (namely, Fig.2) must be identical with that of the second shock (namely, Fig.6) in every detail. Comparing the two figures closely, we find that such is not the case. The distribution of the ratio of R=s_2/s_1, in Fig.7. where s_1 and s_2 are values of the first and second shocks, shows that R is by no means a constant value throughout the whole region. This must largely be due to the inaccuracy of the data reported by the town or village authorities, but, admitting that there are errors in the data, still they do not fully account for the large values of R noticeably concentrated in the zone along the River Yahagi and Old Yahagi. In the present writer's view, this Indicates that seismic damages grow abruptly when seismic strength exceeds a certain degree. Fig.8 that shows the relation between s_1 and s_2 may give some support to this point of view. This fact may well be worth bearing in mind in discussing the problem of earthquake proof structures.

雑誌名

東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo

巻

24

号

1-4

ページ

87 - 98

発行年

1948-06-30

ISSN

00408972

書誌レコードID

AN00162258

フォーマット

application/pdf

日本十進分類法

453

出版者

東京大学地震研究所

出版者別名

Earthquake Research Institute, University of Tokyo

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