The research group of Associate Professor Keiichi Tadokoro of Nagoya University, Professor Mamoru Nakamura of Ryukyu University, and Visiting Professor Masataka Ando of Shizuoka University has a place (fixed area) where the plates are strongly fixed along the Ryukyu Trench to the south of the main island of Okinawa. We newly discovered that there is something from the results of observations of submarine crustal movements.
Trench-type earthquakes such as the XNUMX off the Pacific coast of Tohoku Earthquake (Great East Japan Earthquake) and the Nankai Trough earthquake occur when the long-standing plate fixation area is destroyed at once.The movement of the seafloor at this time lifts the seawater, causing a tsunami.For long-term evaluation of subduction-zone earthquakes and tsunami assumptions, it is essential to grasp the state of adhesion between the plates.
Therefore, in order to measure crustal movements closer to the trench than the Ryukyu Islands, the research team conducted about 60 km south of the main island of Okinawa at two locations (2300 km and 2900 km from the Ryukyu Trench, respectively) at a depth of 2 to 55 m. A survey of submarine crustal movements was conducted for 70 years.
As a result, it became clear that they were moving toward the main island of Okinawa at both points.The magnitude of the movement was 6.3 cm / year near the trench and 2.1 cm / year at the other location.These movements mean that the land-side plate is being dragged by the subduction of the Philippine Sea plate, which is evidence of interplate sticking.Furthermore, it was found that there is a fixed area of the plate boundary in this sea area over at least 130 km in length × 20 to 30 km in width (maximum width 60 km).It can be said that this fixed area stores its energy (strain) toward the next earthquake.
A recent study reported that in 1791, an M8-class subduction-zone earthquake occurred off the southern coast of the main island of Okinawa, causing a tsunami of 11 m to flood Yonabaru.The fixed area discovered this time overlaps with the area that is said to have caused this tsunami, and there are concerns about the occurrence of subduction-zone earthquakes and tsunamis in the future.
Paper information:[Geophysical Research Letters]: Interplate coupling state at the Nansei-Shoto (Ryukyu) Trench, Japan, deduced from seafloor crustal deformation measurements
