Seismicity during the 1986 Eruption of Izu-Oshima Volcano

Abstract

During the 1986 eruption of Izu-Oshima volcano, a drastic change of pattern in seismicity is observed. About one day after the summit eruption on Nov. 15, a very active earthquake swarm began at the northern part of the island. The correlation between the activitiy of the eruption and that of the earthquake swarm shows the stress concentration around the depressurized magma reservoir. After the fissure eruption on Nov. 21, hypocentral distribution dramatically changed. Just after the eruption, earthquakes began to propagate from the both tips of the fissure to NW and SE directions. The propagating velocities to NW and SE directions are 11 m/min. and 16 m/min., respectively. This is interpreted as the propagation of the change in the stress field owing to the fissure eruption. However, these two groups are very different from each other. The focal mechanisms of the events at the northwesten part are almost strike slip types, which are consistent with the typical stress field around Izu-Oshima Island. No clear subsidence is observed near the epicentral area. On the other hand, those of southeastern part are almost normal fault types, which shows NE-SW tension. The subsidence in the southeastern part takes its maximum just above the region of high seismicity.