A joint research group led by Professor Zhao Da Peng of Tohoku University surveyed the entire area of Kyushu, targeting the epicenter area of the 2016 Kumamoto earthquakes. For the first time, the three-dimensional underground structure around the "Beppu-Shimabara Graben" was elucidated with high resolution.This clarified the mechanism by which Kyushu, which had been a mystery until now, splits from north to south, and the cause of the 3 Kumamoto earthquake.
The Kumamoto earthquake (magnitude 2016) that occurred on April 4, 16 caused enormous damage mainly in Kumamoto and Oita prefectures.The Kyushu region is characterized by a prominent crustal rift called the "Beppu-Shimabara Rift Zone" that runs vertically in the northeast-southwest direction.The existence of the Beppu-Shimabara rift zone suggests that the entire Kyushu region has been pulled north and south, and the north and south sides have begun to divide.However, Kyushu is located in a complicated place where the Philippine Sea plate is subducting from the southeast side, and there is also a geological boundary called "Okinawa Trough" on the southwest side and "Median Tectonic Line" on the east side. , Beppu-The origin of the Shimabara rift zone was unknown.
This time, the research group applied the "seismic tomography method" to image the underground structure to a large amount of seismic wave propagation time data recorded by a high-density and high-precision seismic observation network installed in the entire Kyushu area and surrounding islands. bottom.In this way, the three-dimensional underground structure (P-wave / S-wave velocity structure) under Kyushu centered on the Beppu-Shimabara trench zone can be obtained in detail, and the physical quantity (Poisson's ratio) sensitive to the presence of underground magma and water can be obtained. The distribution was also examined.
As a result, it was found that this rift zone was formed by a combination of three factors: a hot mantle ascending current under an active volcano, an extension of the Okinawa Trough to the north, and an extension of the Median Tectonic Line to the west.It was also clarified that the inhomogeneous structure of this rift zone and the behavior of water directly under the epicenter affected the occurrence of the 2016 Kumamoto earthquake.
Paper information:[Scientific Reports] Tomography of the 2016 Kumamoto earthquake area and the Beppu-Shimabara graben
