Regarding the 1855 Ansei Edo Earthquake (M 6.9), there are two ideas: first, it has been caused by active fault movement close to the surface; second, it has occurred on the upper surface of the Philippine Sea Plate.The Ansei Edo Earthquake is thought to have occurred in a shallow or moderately deep area near the mouth of the Arakawa River (Ara Kawa). It generated strong ground motion over a wide area in the Kanto region, and its ground motion is estimated to have been the equivalent of seismic intensity 6 in JMA scale in eastern Tokyo, northwestern Chiba Prefecture, and eastern Saitama Prefecture (Fig.5-20 and Fig.5-21). This earthquake caused extensive fire damage over a wide area. Liquefaction of the ground is also known to have occurred in several locations. There were more than 7,000 fatalities in all. While there was no report of the tsunami generated by this earthquake, a large number of aftershocks are thought to haUve occurred (Fig.5-22). Historical data reveal that several phenomena occurred before the earthquake, including the spring of groundwater and ground rumbling.