Photo/Illutration A near-vertical fault scarp is seen on the seabed in the source area of the Great East Japan Earthquake. (Provided by Hayato Ueda)

Researchers using a submersible found dramatic evidence of the tectonic forces that came into play when the magnitude-9.0 Great East Japan Earthquake hit in March 2011: a nearly vertical cliff 26 meters high on the seabed off Miyagi Prefecture.

They surmised it was formed by a slip on the fault that triggered one of the strongest earthquakes on record. The site lies 7,500 meters below the sea surface.

The cliff is part of a ground uplift that is an estimated 60 meters in height.

The tip of the elevated ground collapsed during the tectonic movement and debris accumulated to form a slope beneath the cliff, the researchers said. 

Team members from Niigata University and other institutions said they are the first to have spotted the fault scarp formed on the seabed by the megaquake.

The Great East Japan Earthquake occurred along the Japan Trench, which marks the boundary between a tectonic plate on the side of the Japanese archipelago and the Pacific Plate, which dives beneath it.

The researchers used the Limiting Factor, a privately-owned U.S. manned submersible that can dive to a depth of about 11,000 meters, to study an area close to where an earlier study by the Japan Agency for Marine-Earth Science and Technology found what was likely a major seafloor uplift.

The fault scarp found on the ocean floor is nearly vertical. Its height is the equivalent to a seven- or eight-story building, which is considered exceptionally large.

A slope beneath the cliff was covered in debris, presumably pieces of the cliff that had crumbled down, the researchers added.

The height of the ground uplift was estimated at around 60 meters, which contributed to causing the towering tsunami that followed the earthquake.

Earlier studies led researchers to conclude that a fault near the trench continued to slide for about one minute starting from about 40 seconds following the onset of the quake.

“The ground rose at an alarming speed of 60 meters in a minute, or 1 meter per second, thereby forming this precipitous cliff,” said Hayato Ueda, an associate professor of geology at Niigata University, who led the research team. “We have thus found that something extraordinary was taking place on the trench bottom.”

Ueda added: “We hope to conduct more studies on the trench, partly to help improve the accuracy of tsunami forecasts.”

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