(5)The West Saitama Earthquake (September 21, 1931, M 6.9)


This earthquake occurred in a very shallow location in western Saitama Prefecture. It generated strong ground motion, and seismic intensity 5 in JMA scale were observed over a wide area in the Kanto region (Fig.5-23). The damage was concentrated in areas with comparatively soft ground along the Arakawa and Tonegawa Rivers in Saitama and Gunma Prefectures (Fig.5-24). The proportion of houses collapsed to the total ones was small, but there were 16 fatalities in all (Fig.5-25).

Perceptible aftershocks disappeared after about 3 weeks. The largest aftershock M 5.6 occurred about two weeks after the main shock (Fig.5-26).

The West Saitama Earthquake is thought to be a left-lateral strike-slip. It occurred in the northwestern margin of the Kanto Plain fault zone along the border between Gunma and Saitama Prefectures. No fault appeared at the surface, however. Cracks appeared in the area where the ground motion was stronger. Effusions of groundwater and earth and sand, as well as muddied well water, were observed over a wide area.