1983 年三宅島噴火後の微小地震活動

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After the 1983 eruption of Miyakejima on Oct. 3, 1983, we carried out a temporary seismic observation at Miyakejima island from Oct. 6 to Nov.1. Four seismic stations were installed in the island. All the data were telemetered to and recorded at a key station. We have observed about 3400 seismic events in this period. The hypocenter determination has been done very precisely and extensively. This is resulting from the adequate configuration of seismic stations which were surrounding the epicentral region and from the high time-resolution due to telemetering observation system. Major results are summarized as follows ; (1) The seismic events are classified into four different categories as short-period earthquakes (Type 1), long-period earthquakes (Type 2), isolated volcanic tremor (Type 3), and continuous volcanic tremor (Type 4). The characteristic period of seismic waves for events of each category is about 0.1, 0.5, 1.0 and 0.8 s, respectively. The amplitude decay of seismic coda for events of each category is also different from one another. (2) Frequencies of both the short- and long-period earthquakes decreased gradually. The continuous volcanic tremor was observed in the morning of Oct. 7 for about five hours. A swarm of isolated volcanic tremor started on Oct. 9, and was gradually activated and attained to the peak activity on 13th. (3) The released seismic energy during the observation period was estimated to be about 3×10^<16> erg. The energy partition of the short- and long-period earthquakes and the isolated and continuous volcanic tremors were about 1/2, 1/20, 1/10 and 1/3 of the total, respectively. (4) Hypocenters of short-period earthquakes are distributed in four characteristic regions. Two major regions are located along the old caldera rim. The focal depths of these earthquakes are from 1 to 6 km below the sea level. One of the regions is very close to the newly formed fissure. The other is about 1 km NW of the former. These activities are probably controlled by the local stress strongly related to the structure of the volcano and the movement of underground magma. The third region which is not so active as the formers is around the summit in the depth of about 3 km. The fourth region is to the south-west of the island. Focal depths of earthquakes in the last region are rather deep as much as 15 km. (5) Hypocenters of some long-period earthquakes were determined. They were mostly distributed around the summit in depths shallower than 5 km.
1984-12-28

1983 年三宅島噴火後の微小地震活動

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