Magnitude 6.9 Earthquake off the east coast of Kyūshū Island, Japan.

The Japan Meteorological Agency recorded a Magnitude 6.9 Earthquake at a depth of about 30 km, beneath the Hyūganada Sea, off the coast of Miyazaki Prefecture on Kyūshū Island, Japan, slightly before 9.20 pm Japan Standard Time (slightly before 12.20 pm GMT) on Monday 13 January 2025. One person was slightly injured by the event, due to falling on some stairs, minor damage to several buildings reported, and train services stopped temporarily while lines were checked. A tsunami warning was issued after the initial event, with concerns a wave as high as one metre could be triggered, although in the event a wave only 20 cm high was reported in Miyazaki Port.

The approximate location of the 13 January 2025 Hyūganada Sea Earthquake. USGS.

Japan has a complex tectonic situation, with parts of the country on four different tectonic plates. Kyūshū Island lies on the margin where the the Eurasian Plate is overiding the Philipine Plate, with the Pacific Plate being subducted beneath the Okhotsk Plate to the east. This is not a smooth process; the rocks of the two plates constantly stick together, only to break apart again as the pressure builds up, causing Earthquakes in the process.

The movement of the tectonic plates beneath Japan. University of Wisconsin Eau Claire.

Earthquakes along subductive margins are particularly prone to causing tsunamis, since these often occur when the overlying plate has stuck to the underlying plate, being pulled out of shape by its movement. Eventually the pressure builds up to far and the overlying plate snaps back, causing an Earthquake and a tsunami. 

Simplified graphic showing tsunami generation along a convergent margin. NASA/JPL/CalTech.

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Eruption on Mount Aso, Japan.

Mount Aso, a volcanic caldera on central Kyūshū Island, Japan, erupted at about 11.45 am local time on Wednesday 20 October 2021, according to the Japan Meteorological Agency. The eruption produced a column of gas and ash about 3.5 km high from its No. 1 Nakadake Crater, a pyroclastic flow was observed on its western flank, and cinder falls recorded as far as 900 m from the crater. The eruption occurred without warning, and 16 climbers were on the volcano when it happened, although all have been accounted for and are uninjured.

 

An ash column over Mount Aso, Kyūshū Island, Japan, on 20 October 2021. AP.

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 centre, 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.

 

Eruption on Mount Aso, Japan, on 20 October 2021. Tikara Zukeyama/Instagram.

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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