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.
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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