2018 Volume 127 Issue 3 Pages 305-323
Kambara Jishinyama (earthquake-mound) located on the west bank of lower reach of the Fujikawa river, is widely believed to be a mound that was tectonically formed at the time of the 1854 Ansei Tokai earthquake. Using old maps and aerial photogtaphs, geomorphological changes around Kambara Jishinyama before and after the earthquake are examined. The Fujikawa river frequently flooded and the course on its west bank changed especially after construction of the Karigane-zutsumi (big bank) in order to protect farmland on its east bank. The area around the lower reach of the river was surveyed in 1803 for the Dai Nihon Enkai Yochizu large-scale map, which is the so-called Ino-Daizu. On that map, the river was at almost the same location as its present course. The historical road map (Kaido-Ezu) of Tokaido, which was the trunk road connecting Edo and Kyoto, illustrated in the same period as Ino-Daizu, shows that the Fujikawa river shifted its course close to the foot of river terraces at the west bank. Due to lateral erosion of the river, part of the Tokaido between the towns of Iwabuchi and Kambara collapsed several times. Subsequently, the road was diverted to the new route via Shinzaka as shown on the 1:20,000 scale topographic map published in 1890. A micro-landform classification map of the alluvial lowland of the west bank of the Fujikawa river based on interpretations of aerial photographs taken in 1952 and 1953 reveals that Kambara Jishinyama was located on one of the former mid-channel bars in the braided channels of the river before the 1854 Ansei Tokai earthquake. The earthquake caused a large landslide that dammed the Fujikawa river for a short period at the foot of Shiratori-yama to the north of Iwabuchi. The discharged flood water changed the river course close to the present stream. Geomorphic evidence for tectonic uplift does not exist around Kambara Jishinyama. The Koike river, a small stream flowing in the former main stream of the Fujikawa river, abandoned at the time of the Ansei Tokai earthquake, concordantly flows into the present main stream of the Fujikawa river showing that co-seismic uplift did not take place at the west bank. We conclude that Kambara Jishinyama was not tectonically formed by the earthquake, but is a product of the river course change.