Mount Shinmoedake, a part of the Kirishima Volcanic Complex on southern Kyūshū Island, began erupting slightly after 5.30 am Japan Standard Time on Wednesday 11 October 2017, for the first time since September 2011. The initial eruption only produced an ash column about 300 m high, but a later eruption, on Thursday 12 October, produced a column of that reached 1.7 km, and caused ashfalls in several neighbouring towns and cities. The events were not unexpected, as the Japan Meteorological Agency had been recording small earthquakes beneath the volcano since the begining of the month. Seismic activity beneath volcanoes can be significant, as they are often
caused by the arrival of fresh magma, which may indicate that a volcano
is about to undergo an eruptive episode.
Small plume on Mount Shinmoedake on 11 October 2017. Tomoaki Ito/Kyodo News/AP.
Japan has a complex tectonic environment with four plates underlying
parts of the Islands; in addition to the Pacific in the east and the
Othorsk in the North, there are the Philipine Plate to the south and the
Eurasian Plate to the West. Kyūshū Island lies at the northeast end of
the Ryukyu Island Arc, which sits on top of the boundary between the
Eurasian and Philippine Plates. The Philippine Plate is being subducted
beneath the Eurasian Plate, in the Ryukyo Trench, to the Southeast of
the Islands. This is not a smooth process, with the two plates
continuously sticking together then breaking apart as the pressure builds
up, leading to frequent Earthquakes in the region.
The movement of the Pacific and Philippine Plates beneath eastern Honshu. Laurent Jolivet/Institut des Sciences de la Terre d'Orléans/Sciences de la Terre et de l'Environnement.
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