抄録
Volcano Usu, which had been in a dormant state for 32 years since the formation of Showa Shinzan lava dome in 1943-45, became active beginning with local earthquake swarms and the following sequence of eruptions in August 1977. The volcanic activity has caused the remarkable upthrust deformation at the center of summit crater, accompanying numerous earthquakes. The uplift amounted to about 140m in relative height at the end of 1978. More than 150, 000 earthquakes including about 18, 000 events felt by inhabitants of Sobetsu Hot Spring Town (Δ=2-3km) had occurred by December 1978. In order to study the characteristics of this swarm activity, space-time-magnitude distributions are investigated in detail. The determination method of earthquake magnitude using entire coda amplitude traces is introduced for the 1023 data in August 1978. It is found that this method is powerful to estimate earthquake magnitude in wide range, when we can not utilize the maximum amplitude and/or the F-P time due to the inevitable continuous occurrence of a swarm.
The majority of the earthquakes is located within the small summit crater (1.7 by 1.8km), forming several clusters which encircle the major upthrust area. Distribution of shallow earthquakes (h<1.0km) in August 1978 shows a significant correspondence to the U-shaped zone of major fault traces which outline the upthrust area. Large earthquakes (M>3.8) are mainly located near the two corners of the U-shaped zone of major fault traces. Distribution of these large events are consistent with that of the localized stress concentration caused by a magma intrusion. In the period of October 1977 to February 1978, large earthquakes clustered mostly near the southeast corner, where the fault cut Ogariyama cryptodome and Ousu lava dome. This implies that these lava domes play an important role such as a barrier which can accumulate considerable stress in the upthrust movement. In July and August 1978, large earthquakes occurred near the northwest corner. This phenomenon may be caused by the stress redistribution due to the change of fracturing process in the development of the doming activity, and due to the eruptions from ‘Ginnuma Crater’. Magnitude-frequency distribution of earthquakes is quite distinct from cluster to cluster. In some cases where large earthquakes are clustered, small events are quite few in number and hence the distribution shows a remarkable peak. The considerable departure from Gutenberg-Richter's distribution may be interpreted by the concept of scattered barriers in different sizes and different strengths in the extraordinary heterogeneous medium beneath the summit crater.
