Ground deformations associated with the 1977–1978 eruption of USU volcano, Japan

Abstract

Abstract The activity of Usu volcano situated at southern Hokkaido is characterized by the persistent occurrence of earthquake swarms and remarkable ground deformations including the formation of lava domes, both of which are due to the high viscosity of its magma. After the pumice eruptions of August 1977, the central part of the summit crater bounded by a U-shaped fault upheaved day after day at the maximum rate of 1 m/day. Although the rate has decreased, the total upheaval reached to about 180 m as of September 1981 resulting in formation of a new cryptodome. The doming at the summit has caused the northeastern rim to thrust towards the northeast about 180 m, and consequently caused corrugation of the ground at the northeastern foot of the volcano. The two kinds of the deformations, upheavals and thrusts, are closely related with each other. To examine the relationship between the earthquake occurrences and the deformations, a quasi-continuous observation of the distance from the northeastern rim of the summit crater to the foot of the volcano, was carried out by an optical distancemeter for 44 hours. It was found that the thrusts were episodic and accompanied by relatively large earthquakes. This supports that the earthquake swarms are caused by stick-slip motions of frictional sliding at the planes of the doming faults.