長野市地付山地すべりとその前兆としての極微小地震

Abstract

A landslide generated over one hour from about 17h, 26 July 1985, on a slope of Jizukiyama, locating in the northern part of Nagano city. This slide had approximately a width of 350m, a length of 700m, and a volume of 3.5×106 cubic meters. Of downward movement of soil and rock divided into two directions in the foot of landslide, the one destroyed 50 houses in a residental area, and the other an old people's home including 5 buildings, where 26 persons were killed.Since 1981, a distribution of cracks and settlements had been in progress on road of toll highway constructe on the slope of Jizukiyama. In particular, the area of soil movement begun to enlarge from spring in 1985. While rainy day continued in a longer period of 10 June to 20 July, and the total amounted to about 520mm. After heavy rain fall on 20 July, a small scale of mud flow occurred in an extent of 25m in width and 150m in length, of the central part of landslide area as a precursor.An observation of acoustic emission (called here microearthquake) was made in the landslide area by using horizontal and vertical seismometers of 1 sec period. In spite of short interval from 23 to 25 July, microearthquakes were observed. They were interpreted as radiating from the landslide area because P-S interval had small value of 0.05 to 0.2sec. The wave forms were characteristic of being identifiable for P and S phases, a proportional relation of P-S interval to duration of ground motion, and downward direction in the initial motion of the vertical component.Moreover, a few microearthquakes were recorded on 26 July at the Shinetsu Seismological Observatory, Tokyo University, which was located at a distance of 1.1km south-west of the landslide area. The amplitudes of ground motion were larger in a comparison with those microearthquakes in the landslide area on 23 to 25 July. On the other hand, the arrival times were 08h37m26s, 08h37m43s, and 15h12m01s respectively for three events, in advance of the occurrence of landslide. Simultaneously with these microearthquakes, a part of retaining wall falled away, which had been constructed along the highway, in the central part of the landslide area.During the landslide, a succession of microtremors were recorded over a few minutes for four times, of which wave forms were quite different from those of isolated microearthquakes. The train of microtremors were generated due to exciting the ground surface when massive soil and rock moved downward.Therefore, it has been considered that the microearthquakes were produced due to slippage fractures in rocks beneath the landslide area, and the stability abruptly decreased on account of enlargement of the fracture zone preceding the landslide. If such observations are made hereafter, some problems of landslide may be elusidated in view of the fracture mechanism in rocks and the prodiction of the occurrence.