Earthquake swarm started under the Miyake Island

 

  Abundant volcanic gas has still emitted from the summit of Miyakejima volcano since September 2000, though decreasing the extent; where vigorous eruptions had occurred during July to August 2000, being accompanied by subsidence of the summit area. Magma’s migration toward the Kozushima and Niijima islands, which generated the summit collapse, is considered to have stopped on 18 August 2000, though the seismic activity has still remained in a low level between the Kozushima, Niijima and Miyakejima islands.

    Earthquake activity spread toward the northwestern oceanic region. It includes five large earthquakes with the magnitude larger than 6.0 and a huge number of earthquakes. To get better resolution for the spatial and temporal changes in the activity, we conducted a series of pop-up and buoy telemetering ocean bottom seismometer observations. The epicenter distribution obtained strongly indicates a northwest-southeastern lineament (fig.2,3). The vertical cross-section shows two trends; the deeper (7-13km) distribution forms a very thin zone and the shallower (< 7km) distribution is much thicker. And it is clear that the distribution of the hypocenters migrated horizontally and vertically. This feature is important for understanding the behavior of the magma migration.