Serious disaster occurred due to the Izu-hanto-oki earthquake of May 9, 1974.
Investigation-team of 10 engineers was delegated from Public Works Research Institute, Ministry of Construction, to observe the damage and obtain the new information about the earthquake.
The magnitude was 6.8, the epicenter was 5km west from Iro-zaki, 20km beneath the surface.
Actural fault was found at Iro-zaki, whose horizontal displacement was 30cm, vertical was 20cm, and strike was N70°W, dip was 80°S. Along this fault, buildings were damaged by displacement, besides ground vibration.
A great number of slope-failure and land-slide was occurred, and most of damage due to the earthquake was caused by that, and this fact is the characteristic of the Izu-hanto-oki earthquake. Another type of damage was caused by the settlement of loose fine sand layer, and that was observed in Iruma district.