Fukushima Earthquake Moved Seafloor Half a Football Field

By Mark Fischetti

The March 2011 Tohoku-Oki earthquake that decimated Japan and its Fukushima nuclear reactors with a monster tsunami altered the seafloor off the country’s eastern coast much more than scientists had thought. Analysis released today in the journal Science indicates the ocean bed moved as much as 50 meters laterally and 16 meters vertically. The magnitude 9.0 quake occurred close to the nearby Japan Trench that runs north to south in the Pacific Ocean (dark blue line on the map below).

The trench exists because the oceanic Pacific Plate (dark blue on map below) is moving westward, hitting and bending down under the continental Okhotsk Plate (light blue) from which Japan rises (green, brown). This “subduction” action creates tension within the tectonic plates, which is occasionally released in the form of earthquakes.

Although measurements from satellites and seismic ground sensors had indicated the Okhotsk Plate moved after the 9.0 temblor on March 11, the extent of the movement was not clear. Researchers at the Japan Agency for Marine-Earth Science and Technology compared new seafloor maps made of the region this year with maps made in 1999 and were surprised by the extent of motion. For example, data along one transect (yellow marker, below) near the quake’s epicenter (black “x” on the map) indicated that the Okhotsk plate moved 50 meters east-southeast toward the trench.

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