Re-examination of the damage distribution and the source of the 1828 Sanjo Earthquake in central Japan

The Sanjo earthquake occurred in the mid-Niigata Prefecture, Japan, at around 6 or 7 a.m. on 18 December, 1828, and caused serious damage especially in the town of Sanjo. The epicenter and source fault were previously investigated by Usami (2003) and Uetake et al. (2005). Usami (2003) also estimated the magnitude as 6.9. According to historical records, the number of completely collapsed houses is 2418 in Sanjo town, in the flood plain of Shinano river, and 305 in Yoita town, on the western edge of Nagaoka plain. These two towns were the large population centers in those days. However, it is erroneous to conclude that the source of this earthquake is around Sanjo or Yoita town from the number of collapsed houses, because large population towns yield more collapsed houses. Therefore, we investigate damages in the other areas, and also estimate damage ratio in order to specify the focal region of the Sanjo Earthquake. We selected three reliable historical records which describe the original population and number of houses, as well as casualties or damages caused by this earthquake. The first document, compiled by Kuwana feudal clan, records the numbers of total houses and damaged houses at 54 villages. The second historical document seems to be based on a report which compiled damages and casualties to a public office of the Nagaoka feudal clan, and records the numbers of houses and of non-collapsed houses. The third document describes the number of houses and damages of each house (burned, collapsed or partially collapsed) in each region of Yoita town. The collapse ratio of houses at each village estimated from the above three historical documents is then converted into Japanese seismic intensity scale based on Usami (1986)'s table, i.e. Seismic intensity 7 (XII on MM scale) : 81 - 100 % collapse ratio of houses. Seismic intensity 6 (X - XI on MM scale): 71 - 80 % collapse ratio of houses. Seismic intensity 5+ (IX on MM scale) : 1 - 70 % collapse ratio of houses. Seismic intensity 5- (XIII on MM scale): 0 % collapse ratio of houses. The estimated focal region of the Sanjo Earthquake is neither in Sanjo town, the worst-damaged area, nor Yoita town. The collapse ratio of houses and seismic intensity distribution revealed in this study indicate that the focal region of the 1828 Sanjo Earthquake is estimated in the Higashiyama hill located to the east of Nagaoka plain, which is now in the south part of Mitsuke City. Our study supports that the earthquake source fault of the 1828 Sanjo Earthquake was not along the western edge of Nagaoka plain, but possibly along the eastern edge of the plain which separates the flood plain and the Higashiyama hill.