天保4年 (1833) 山形沖地震とその津波の規模

抄録

It is reported by an old document that the Yamagata-Oki earthquake of Dec. 7, 1833 (Oct. 26, Tenpo 4), hitting the Yamagata, Niigata and Akita districts destroyed 475 houses in the Shonai plain and the tsunami killed about 150 persons. In this paper the distributions of the seismic intensity and the tsunami behavior along the Japan Sea coast are investigated with an addition of newly collected data. The magnitudes of earthquake and tsunami are compared with those of the 1964 Niigata and the 1983 Japan Sea earthquakes. The results are summarized as follows:
1) According to the intensity-distance diagram, the earthquake, magnitude is inferred as M=7.5, because the same intensity area is smaller than that of the 1983 Japan Sea earthquake and similar to that of the 1964 Niigata earthquake.
2) By judging from the tsunami height-distance diagram (HATORI, 1986), tsunami magnitude on the Imamura-Iida scale is m=2.5, which are somewhat larger than the former value. Extraordinary run-up heights at Wajima (tip of Noto peninsula) and Oki Islands located in far field may be caused by the refractive effect and the reflected waves from the continental coasts.