An international research group consisting of Niigata University, Tokyo University of Marine Science and Technology, the University of Western Australia, Japan Agency for Marine-Earth Science and Technology, and the Danish Ultra-Deep Sea Research Center is conducting research on the ocean floor (off the coast of Miyagi Prefecture) that was raised during the 2011 Tohoku-Oki Earthquake (Great East Japan Earthquake). The world's first discovery of a 26m-long fault cliff.
The Great East Japan Earthquake occurred at the boundary between the ground in northeastern Japan and the subducting Pacific Plate. It is estimated that the rupture (slip) of the fault reached the sea floor of the Japan Trench, moving the nearby ground more than 50 meters to the east. Tsunamis are generated by sudden changes in the topography of the ocean floor, so it is important to understand the topographical changes at the ocean trench during an earthquake. However, there was no way to directly survey the ocean floor, which is over 6,500 meters deep, so the details were unknown.
This time, the research group conducted a study in the Japan Trench, approximately 7,500 meters deep off the coast of Miyagi Prefecture, which is located in the epicenter of the Great East Japan Earthquake, with a manned vehicle from a foreign private company with the ability to dive to a depth of 11,000 meters (full-depth specification capable of reaching any deep seabed). Undersea survey conducted using a submersible. On-site observation revealed that the uplift caused by the earthquake was 59 meters high, and on its eastern edge there was a near-vertical cliff with a drop of 26 meters (equivalent to a 7- to 8-story building), and the lower slope had a large amount of debris that appeared to have collapsed from the cliff. It was filled with rock masses.
Generally, the height of the fault scarp that appears in inland earthquakes ranges from several tens of centimeters to several meters, so the scale of the fault scarp caused by the Great East Japan Earthquake was unprecedented. A detailed analysis of the topography suggests that it was formed when the tip of the ground moved 80 to 120 meters eastward due to the fault, was suddenly lifted about 60 meters, and collapsed along the fault.
This time, the usefulness of a full-depth manned submersible has been demonstrated. Currently, there are no ultra-deep-sea probes or submersibles in Japan, but there are several plans for foreign research vessels to investigate the ocean trenches around Japan, and ultra-deep-sea research is expected to progress significantly.
Paper information:[Communications Earth & Environment] The submarine fault scarp of the 2011 Tohoku-oki Earthquake in the Japan Trench
