Time and Space Distribution of Coseismic Slip of the 2011 Tohoku Earthquake as Inferred from Tsunami Waveform Data

Kenji Satake, Yushiro Fujii(IISEE), Tomoya Harada, Yuichi Namegaya）(AIST)

Bulletin of Seismological Society of America,103,No2B 1473-1492(2013)

Abstract

A multiple time window inversion of 53 high‐sampling tsunami waveforms on ocean‐bottom pressure, Global Positioning System, coastal wave, and tide gauges shows a temporal and spatial slip distribution during the 2011 Tohoku earthquake. The fault rupture started near the hypocenter and propagated into both deep and shallow parts of the plate interface. A very large slip (approximately 25 m) in the deep part off Miyagi at a location similar to the previous 869 Jogan earthquake model was responsible for the initial rise of tsunami waveforms and the recorded tsunami inundation in the Sendai and Ishinomaki plains. A huge slip, up to 69 m, occurred in the shallow part near the trench axis 3 min after the rupture initiation. This delayed shallow rupture extended for 400 km with more than a 10‐m slip, at a location similar to the 1896 Sanriku tsunami earthquake, and was responsible for the peak amplitudes of the tsunami waveforms and the maximum tsunami heights measured on the northern Sanriku coast, 100 km north of the largest slip. The average slip on the entire fault was 9.5 m, and the total seismic moment was 4.2×10²²  N·m (M_w 9.0). The large horizontal displacement of seafloor slope was responsible for 20%–40% of tsunami amplitudes. The 2011 deep slip alone could reproduce the distribution of the 869 tsunami deposits, indicating that the 869 Jogan earthquake source could be similar to the 2011 earthquake, at least in the deep‐plate interface. The large tsunami at the Fukushima nuclear power station is due to either the combination of a deep and shallow slip or a triggering of a shallow slip by a deep slip, which was not accounted for in the previous tsunami‐hazard assessments.