The 2011 Tohoku Earthquake caused a devastating tsunami along the shoreline from the Tohoku to Kanto
districts. Although many of the tide gauge stations along the Tohoku coast were saturated or damaged due
to the tsunami, two cabled ocean-bottom tsunami sensors installed off Kamaishi successfully recorded the
tsunami waveform just above the source rupture area. The records indicated a characteristic two-stage tsunami
development sequence: a smoothly increasing tsunami amplitude from 0 to 2 m during the first 800 s from the
earthquake origin time, and a short-period impulsive tsunami with a peak of more than 5 m in the following 200 s.
Such observations strongly suggest the lack of any sea floor upheaval at the stations during the earthquake, and
the occurrence of an extremely large slip in the shallow portion of the subducting Pacific Plate near the trench
axis. The source model derived from the offshore tsunami records indicates that a very large slip of 57 m occurred
off Miyagi near the trench axis, south of the rupture area of the 1896 Meiji Sanriku tsunami earthquake, and was
the major source of the highly destructive tsunami that subsequently developed.
Key words: The 2011 Tohoku Earthquake, tsunami, ocean bottom pressure gauges, tsunami simulation.
The 2011 Tohoku Earthquake caused a devastating tsunami along the shoreline from the Tohoku to Kantodistricts. Although many of the tide gauge stations along the Tohoku coast were saturated or damaged dueto the tsunami, two cabled ocean-bottom tsunami sensors installed off Kamaishi successfully recorded thetsunami waveform just above the source rupture area. The records indicated a characteristic two-stage tsunamidevelopment sequence: a smoothly increasing tsunami amplitude from 0 to 2 m during the first 800 s from theearthquake origin time, and a short-period impulsive tsunami with a peak of more than 5 m in the following 200 s.Such observations strongly suggest the lack of any sea floor upheaval at the stations during the earthquake, andthe occurrence of an extremely large slip in the shallow portion of the subducting Pacific Plate near the trenchaxis. The source model derived from the offshore tsunami records indicates that a very large slip of 57 m occurredoff Miyagi near the trench axis, south of the rupture area of the 1896 Meiji Sanriku tsunami earthquake, and wasthe major source of the highly destructive tsunami that subsequently developed.Key words: The 2011 Tohoku Earthquake, tsunami, ocean bottom pressure gauges, tsunami simulation.
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