Tsunami source of the 2011 Tohoku earthquake detected by an ocean-bottom magnetometer

Tsunami-induced magnetic signals resulting from the 11 March 2011 Tohoku earthquake (M 9.0) were identified from an examination of the record of an ocean-bottom electromagnetometer emplaced 50 km east of the Japan Trench. The impulsive variation of the vertical magnetic field component directly indicates the tsunami wave height and arrival time at the station. The impulsive variation of the horizontal magnetic components indicates east–southeast as the tsunami-propagation direction. These data indicate that the tsunami source was located along the Japan Trench at approximately 39.0°N latitude, which is about 100 km northeast from the main rupture area. Joint analysis of the observed magnetic variations and offshore sea-level data supports this source location and indicates that the tsunami origin time was about 1 min after the earthquake origin time. In addition, the magnetic field variation and the sea level change indicate that the source area was elongated in the trench direction and had a narrow width.