2018 Volume 18 Issue 5 Pages 5_12-5_32
We estimated the seismic intensity distribution of the 1945 Mikawa earthquake (M6.8) from the original sheets of questionnaire survey conducted by the Earthquakes Research Institute of the University of Tokyo. We excluded 25 sheets without information for estimating seismic intensities, and carefully examined 149 original questionnaire sheets. Because multiple seismic intensity values are obtained for various kinds of damage and detailed human behaviors at each survey point, representative values of seismic intensities are necessary to make a seismic intensity distribution map. Mean of the top three seismic intensity reports is adopted as the representative seismic intensity at each survey point, because it hardly saturates and its distribution is similar to that reported by the Central Meteorological Observatory. The new seismic intensity distribution map clarifies some characteristics of the 1945 seismic ground motion. (1) Significant high intensities (IK 10∼11) in Katahara Town located in the source region. (2) High intensities (IK 6∼8) extending from the source region to northwestern Aichi Prefecture and southwestern Gifu Prefecture due to the amplification in the Nobi Plain. (3) High intensities (IK 6∼7) distributed in the western Shizuoka Prefecture, while low intensities in most of Mie Prefecture. (4) Low intensities in Shiga and Fukui Prefectures where high intensities were recorded by the 1944 Tonankai earthquake. No questionnaire survey point existed in the totally damaged area probably because researchers could not send questionnaire sheets or retrieve answered sheets from that area. (5) The reason for low intensities in the surrounding Prefectures may be due to the facts that respondents could not assess accurate seismic intensities because of the midnight occurrence of the 1945 earthquake, the preceding 1944 Tonankai earthquake already caused damage, and aftershocks of 1944 earthquake were frequently noticed.
