On 20 December 2013 the Japan Meteorological Agency detected an increase in seismic activity beneath Aso (or Asosan) a volcanic caldera on central Kyūshū Island, Japan. This grew steadily for the next week, prompting the agency to raise the alert level on the mountain from 1 to 2 (on a scale of 1 to 5) on 27 December. The frequency and intensity of the tremors dropped sharply on 2 January 2014, however this drop was accompanied by a sharp rise in the amount of sulphur dioxide being emitted from the caldera, reaching 1200 tonnes per day on 2 January, and rising to 1500 tonnes per day on 10 January. Seismic activity resumed on 12 January, with a series of tremors that grew steadily from 8.00 am to 7.00 pm Japan Standard Time. On 13 January at 12.15 pm a small eruption occurred on Mount Nake, the one of five small volcanos within the central caldera, producing a plume that rose 600 m above the summit and drifted to the south, producing small ashfalls downwind of the summit.
The central summits of the Aso Caldera, seen from the caldera rim. Wikipedia.
The Aso Caldera is the largest in Japan, and one of the largest in the world, being approximately 25 km in diameter. The caldera is thought to be the result of a series of four massive eruptions, the first of which took place around 300 000 years ago, and the last around 90 000 years ago. The caldera contains five smaller summits in a complex at its center, the highest of which Mount Taka, rises 1592 m above sea-level. These are also a series of hot springs within the caldera, which is a popular tourist resort and part of the Japanese Geoparks Network. Despite the site's dramatic history modern eruptions tend to be quite small, and there are no records of any historical fatalities connected with the volcano.
Terrain map of the Aso Caldera and surrounding area. Google Maps.
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. As it is drawn into the interior of the Earth, the tectonic plate is partially melted by the heat of the Earth's interior, and liquid magma rises up through the overlying Eurasian Plate to form the volcanos of the Ryukyu Islands and Kyūshū.
See also Magnitude 5.4 Earthquake in Ibaraki Prefecture, Japan, Volcano creates new island in the west Pacific, Tokyo shaken by Magnitude 5.4 Earthquake on the Bōsō Peninsula, Magnitude 6.8 Earthquake off the east coast of Japan and Eruption and pyroclastic flow on Sakurajima, Kyushu.
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