A 26-meter-high fault cliff formed in the Japan Trench by the Great East Japan Earthquake — Niigata University and collaborators undertake manned submarine assessment

A 26-meter-high fault cliff formed by the 2011 Great East Japan Earthquake was discovered at a depth of approximately 7,500 meters in the Japan Trench. An international research group consisting of Associate Professor Hayato Ueda of the School of Natural Sciences at Niigata University, Visiting Professor Hiroshi Kitazato of the School of Marine Resources and Environment at Tokyo University of Marine Science and Technology, and researchers from the University of Western Australia, Japan Agency for Marine-Earth Science and Technology, and the Danish Center for Hadal Research Satellite Office discovered the fault cliff during their manned submersible survey of the ocean floor in the Japan Trench close to the coast of Miyagi Prefecture, the epicenter of the 2011 Great East Japan Earthquake. The discovery was published in the academic journal Communications Earth & Environment.

From August to September 2022, an international research expedition was conducted on board the deep-sea vessel Pressure Drop, where both an unmanned ocean floor observation vehicle and a manned submersible were used to examine the geology and deep-ocean creatures in the ocean trenches around Japan.

During the submersible survey, the researchers observed and took video images of the ocean floor conditions while traversing the terrain uplifted by the earthquake. They recorded the ocean floor topography while measuring the distance and water depth. Consequently, a near-vertical cliff with a drop of 26 meters was found at the eastern edge of a 59-meter-high uplifted landform. The lower slope was filled with large amounts of rock mass that appeared to have collapsed from the cliff. This terrain did not exist before the earthquake and was observed 11 days after the event. From this, the researchers speculate that the cliffs discovered this time were also formed as a result of the uplift during the earthquake.

Ueda said, "Observations from the ship merely showed that there was uplift, and the details were unclear. By conducting an actual manned dive, we were able to perform a survey tailored to the location, and a fault cliff was discovered."

Since the height of fault scarps that generally appear in inland earthquakes ranges from tens of centimeters to several meters, it is clear that the fault scarp caused by the Great East Japan Earthquake was much larger. From a detailed analysis of the topography, this cliff is estimated to have formed when the tip of the ground, which had already moved 80−120 meters eastward because of the fault, was suddenly lifted by approximately 60 centimeters and collapsed along the fault.

Ueda further explained, "The shaking of the earthquake in the ocean trench was thought to have lasted one minute, and the ground was then lifted by one meter in approximately one second."

This is the world's first reported observation and recording of a fault scarp in the ocean floor that appeared as a result of the displacement of the subduction boundary of an oceanic plate; that is, a subduction zone earthquake. This survey included a study of the biota. The organisms on the raised ocean floor plateaus were found to be different from those below the ocean floor.

Kitazato said, "We speculate that when the upheaval occurred, the organisms on the surface were blown away, and other organisms then started to live there. Unfortunately, the on-board manipulator was broken and so a detailed investigation was not possible. In the future, we hope to collect samples to determine how the uplift caused by the earthquake has changed the biota."

Journal Information
Publication: Communications Earth & Environment
Title: The submarine fault scarp of the 2011 Tohoku-oki Earthquake in the Japan Trench
DOI: 10.1038/s43247-023-01118-4