1983 年三宅島噴火に関連した地震活動

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Immediately after the 1983 eruption of Miyakejima, we set up temporary seismic stations on the island. This paper reports some characteristics of the seismic activity after the 1983 eruption and special features of wave forms of volcanic earthquakes observed at these stations. The seismograms recorded at Miyakejima are classified into three types as follows : (1) high-frequency earthquakes. These are classified into two subgroups : (a) high-frequency earthquakes having the same wave form as ordinary tectonic earthquakes with clear P and S phase ; (b) high-frequency earthquakes with no clear S phase. (2) low-frequency earthquakes which show dominant frequency of less than 10 or 15 Hz with no clear S phase. (3) low-frequency volcanic tremors which are also classified into two subgroups : (a) continuous tremor lasting several hours and (b) spasmodic tremor having a duration of a few minutes. The largest earthquake with M 6.2 occurred on October 3, about 7 hours after the biginning of eruption. The number of high-frequency and low-frequency earthquakes decreased gradually after the eruption and declined to only a few events per day on 5 November. On the otherhand. spasmodic tremor started to occur on 7 October. Such tremors were most dominant on 13 October and decayed gradually. Various wave forms of spasmodic tremor were recorded at temporary seismic stations. Hypocenters were determined for 48 high-frequency earthqaukes. Epicenters were concentrated within a small area of about 5 km in diameter, in the southwest part of Miyakejima. The focal depths were distributed between 2 km and 15 km. The push-pull distribution of P-wave first motions is very different from those of ordinary quadrant type earthquakes.
特定非営利活動法人日本火山学会の論文
1984-12-28

1983 年三宅島噴火に関連した地震活動

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