A Study of Earthquakes in Relation to Volcanic Activity (I)

Earthquakes That Occurred during the Formation Period of a New Dome “ Showa-Shinzan ” and Their Forerunners near Volcano Usu

抄録

Showa-Shinzan, one of th e Belonite-Volcanoes, born (1943-1945)in the eastern part of the Volcano Usu (in the South-western part of Hokkaido, Japan) is famous in the history of volcanoes in the world.
This volcano erupted in a farm-land and further developed a lava dome (Spine) in the central part. It is a Pelée-type one, and its developing process was clearly observed for the whole period.
Some remarkable earthquake swarms occurred during the volcanic activity, and the major ones among them were recorded by seismographs even at a station about 800 km apart from the volcano.
The author investigated the relations betwee n the occurrence of earthquakes and the formation of Showa-Shinzan and tried to find some clues to the volcanic mechanism. In this first paper, after an investigation of the characteristics of earthquake swarms recorded on the seismographs in Muroran, Mori and Sapporo (epicentral distance: 25-69 km) and the relation of the volcanic activity therewith, some interesting results were obtained:
1) Daily earthquake frequency diagram for the whole period of twenty three months were prepared for the first time.
2) These earthquakes were clearly divided into the following three stages:
( i ) Pre-volcanic stage: In this stage there was no eruption, and remarkable earthquakes occurred around Volcano Usu frequently. The relation between the maximum amplitude (A) in these earthquake swarms and their frequency (N) was expressed by
NA^m = const.
where m is about 1.8, similar to that of the tectonic earthquake. Moreover, the migration of the hypocentres of these earthquakes showed an intimate connection with the course of the magma intrusion.
(ii) Stage of eruptive activity: A violent eruption occurred in the central part of the rising area. The extremely shallow earthquakes during the period of six months including just before and through the eruptive activity were proved to have been originated at the area adjacent to the newly formed crater.
(iii) Birth and developmen t of lava dome (spine): The lava dome grew and developed without eruption, accompanied with numerous earthquakes. The earthquakes, which differed greatly from those at the former two stages, began to occur almost simultaneously with the end of the eruption and the frequencies of the earthquakes during the following twelve months corresponded closely to the speed of the development of the lava dome. The value of m in this case was 3.5.
3) Process of magma intrusion, increase of magma viscosity a n d their relationship with the feature change of earthquake swarms were investigated. The possibility of predicting volcanic activity Volcano Usu by seismograph at stations up to approximately sixty or seventy kilometers from the volcano is suggested.