Earth Planets Space, Vol. 63 (No. 7), pp. 815-820, 2011
Yushiro Fujii1, Kenji Satake2, Shin'ichi Sakai2, Masanao Shinohara2, and Toshihiko Kanazawa2
1International Institute of Seismology and Earthquake Engineering (IISEE), Building Research Institute (BRI), 1-3 Tachihara, Tsukuba, Ibaraki 305-0802, Japan
2Earthquake Research Institute (ERI), University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
(Received April 9, 2011; Revised June 6, 2011; Accepted June 7, 2011; Online published September 27, 2011)
Tsunami waveform inversion for the 11 March, 2011, off the Pacific coast of Tohoku Earthquake (M 9.0) indicates that the source of the largest tsunami was located near the axis of the Japan trench. Ocean-bottom pressure, and GPS wave, gauges recorded two-step tsunami waveforms: a gradual increase of sea level (∼2 m) followed by an impulsive tsunami wave (3 to 5 m). The slip distribution estimated from 33 coastal tide gauges, offshore GPS wave gauges and bottom-pressure gauges show that the large slip, more than 40 m, was located along the trench axis. This offshore slip, similar but much larger than the 1896 Sanriku "tsunami earthquake," is responsible for the recorded large impulsive peak. Large slip on the plate interface at southern Sanriku-oki (∼30 m) and Miyagi-oki (∼17 m) around the epicenter, a similar location with larger slip than the previously proposed fault model of the 869 Jogan earthquake, is responsible for the initial water-level rise and, presumably, the large tsunami inundation in Sendai plain. The interplate slip is ∼10 m in Fukushima-oki, and less than 3 m in the Ibaraki-oki region. The total seismic moment is estimated as $3.8 × 1022 N m (Mw = 9.0).
Key words: 2011 Tohoku Earthquake, slip distribution, tsunami waveform inversion, ocean-bottom pressure gauge, GPS wave gauge.