On December 17, 1987, an earthquake of magnitude 6.7 occured at a depth of 58 km, near Boso Peninsula, in Pacific Ocean.
Many slope failures were induced by the earthquake shock, concerned with the loose sand (SimousaUpland, the northern part of peninsula) and “soft rock ” (Kazusa-Hills, the central part of peninsula).
The geology of the latter is Plio-Pleistocene sedimentary rocks which consists mainly of mudstone, sandstone, and their alternations. The slope failures occured especially on valley cliff which consists of thick mudstone and the slope stability was controlled by tectonic joint system with north-northeast to south-southwest trend, high dip angles.
The isoseismal was estimated from frequency of overturned tombstones. Both the local high seismic intensity and exposure of jointed mudstone are recognized as major factors in concentration of slope failure in Kazusa-Hills.
In about 100 place, geometry of slope failures were measured by 5 m yard-stick. The frequency of failed volume conforms to log-normal distribution. The shape of failed source area seems affect to volume of slope failure. Motion of failing mass accelating on a slope can be explained approximately with the mass point model, introducing “Representative (ideal) length” of failing mass. Geometrical data show that larger mass can be transported more distantly as expected by the above-mentioned dynamic model.