1984 Volume 29 Issue TOKUBE Pages S36-S44
A swarm of earthquakes, precursory indication of the eruption, began to be recorded at Miyakejima Weather Station at 1358 on 3 October, 1983, although none was recorded at any other seismic station of JMA until the onset of the eruption. On the other hand, another swarm of earthquakes that resumed after the onset of the eruption was caught by seismographs around Miyakejima. Precursory earthquakes were grouped into two types, i.e., high frequency earthquakes and low frequency ones, according as their predominant frequencies were higher than 2.5 Hz or not. It is unpromissing to try to determine hypocenters of precursory events, but, both the first motions of 5 high frequency earthquakes and the particle motions of 2 low frequency ones in the horizontal plane inidicate that their sources were located to the SW of the seismograph, probably on the island. Gradually increasing continuous tremors started immediately after the earthquake at 1522 which had a predominant frequency of about 1.4 Hz from initial motion through coda. The following continuous tremor had almost the same predominant frequency. Major eruptive activity probably began with this low frequency earthquake. The magnitudes of two large precursory earthquakes were estimated to be about 3.0 by applying the relation between the magnitudes of post-eruption earthquakes and their maximum amplitudes or duration times of vertical component at Miyakejima Weather Station. However, this estimation was not appropriate because earthquakes of such size were large enough to be recorded at seismic stations other than Miyakejima. The seismograph at the sea bottom off Omaezaki (named “TK1OBS” in the seismological bulletin of Japan Meterological Agency), about 180 km W of Miyakejima, detected post-eruption earthquakes of magnitude about larger than 2.5, but did not record any pre-eruption earthquakes. The background noise on 3-4 October had remained at a similar level of 0.02 milikine, which corresponds to the expected maximum velocity of the vertical component on TK1OBS when an earthquake of magnitude 2.4 occurs at Miyakejima. Therefore, precursory earthquakes seem to be of magnitude less than 2.4.
