Geophysical characteristics of dacite volcanism — the 1977–1978 eruption of Usu volcano

Abstract

Abstract The 1977–1978 eruption of Usu volcano is discussed from the geophysical standpoint as a classic example of dacite volcanism. The activities of dacitic volcanoes are characterized by persistent earthquake swarms and remarkable crustal deformations due to the high viscosity of the magmas; the former include shocks felt near the volcanoes and the latter accompany formation of lava domes or cryptodomes. The hypocenters of the earthquakes occurring beneath Usu volcano have been located precisely. Their distribution defines an earthquake-free zone which underlies the area of doming within the summit crater. This zone is regarded as occupied by viscous magma. The domings within the summit crater forming the cryptodomes have amounted to about 160 m. In addition to uplift they showed thrusting towards the northeast. As a result, the northeastern foot of the volcano has contracted by about 150 m. The relation between crustal deformation and earthquake occurrence is examined, and it is found that the abrupt domings are accompanied by the larger earthquakes ( M = 3–4.3). Both the seismic activity and the ground deformation are shown to have a unique and common energy source. The energy of activities of Usu volcano consists of the explosive type, the deformation type and the seismic type; the second and the third are in parallel with each other in discharges, and both energies are complementary to the explosive energy. The explosive energy and the seismic energy have been calculated for an explosion sequence, and it is concluded that the deformation energy is about 10 times greater than the seismic energy. The discharge rate of the seismic energy and the upheaval rates of the cryptodomes have continued to decrease since the outburst of the eruption, except for a small increase at the end of January 1978. Eruptions are governed not only by the supply of the energies but also by the depth of the magma, which has gradually approached the surface. The last eruption occurred in October 1978; however, the crustal deformations and the earthquake swarms are still proceeding as of January 1980, albeit at a lower rate of activity.