Trench-normal variation in observed seafloor displacements associated with the 2011 Tohoku-Oki earthquake

Seafloor geodetic measurements using the GPS/acoustic technique just above the rupture area of the 2011 Tohoku-Oki earthquake revealed strong trench-normal variation in the horizontal crustal displacement associated with the earthquake. One set of observations, at a site only 50 km away from the trench, yielded the most trench-ward data ever reported with an accuracy better than 1 m. It showed a coseismic displacement of up to 31 m, whereas another set of observations 100 km away from the trench showed a 15-m displacement. These horizontal data, as well as vertical data from pressure gauges and tsunami observations, strongly indicate that the seismic slip reached the trench. They are also likely to contribute quantitatively to further investigations that reveal the coseismic behavior of the frontal wedge of the overriding plate in the source region along the Japan Trench.