Station SHIP

Questions for Tadashi Ishikawa

Seafloor Geodesy and the 2011 Tohoku-Oki Earthquake

Japan has the largest scientific GPS network in the world (Figure 1). Operated by the Geographical Survey Institute, it can precisely measure crustal deformation rates in Japan. However, it cannot measure how the ground deforms on the sea floor. Since many plate boundaries are offshore, this means one cannot directly measure deformation there with GPS. However, Japan recently began sea floor geodesy experiments (Figure 2). This is a way to measure fault motions offshore.

In traditional geodesy, the location of the GPS antenna above a ground marker is measured. In sea floor geodesy, a monument on the sea floor is defined relative to four acoustic transponders (Figure 3). The distance between the transponders (Figure 4) and a ship is then calculated. This is combined with GPS measurements on the ship that calculate its position with respect to the GPS network onland. Small interseismic deformation rates been successfully measured offshore Japan (Figure 5) using this technique. This new method's unique contribution came in 2011, when the first sea floor geodesy measurements of coseismic deformation were made. More than 20 meters of displacement were observed for the great Tohoku-Oki earthquake (Figure 6).

Please read more about Displacement Above the Hypocenter of the 2011 Tohoku-Oki Earthquake in Science. And watch the youtube video of how the ground in Japan moved, as measured by GPS.