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Lessons learned from the March 11, 2011 Tohoku-oki earthquake and tsunamis

by Dr. Koji Okumura, Graduate School of Letters, Hiroshima University, Japan

Friday 24 February, 15h00
KAST 01.07, Kasteel Arenberg, Heverlee

Abstract:

The 2011 March disaster in Japan occurred on the edge between the known and the unknown. That means earthquake and tsunamis of unknown magnitude hit the area prepared for those of known magnitude. Knowledge on extreme natural hazards may derive from experience, history, and geology but incomplete. Confidence on the knowledge as well as lack of the knowledge resulted in severe disasters. The northern half of the damaged area – Sanriku rocky coast – was worldwide the best prepared for huge tsunamis, reflecting experiences with the 1896, 1933 and 1960 tsunamis. No associated tsunami deposits have been found to date. There was no historic or geologic information on tsunamis that might have exceeded these ones, so there was no preparation for the unknown. The coastal plains in south – Sendai and Ishinomaki areas – were rather poorly prepared only for a few meter high tsunami, like in 1896 or 1978. In these areas, there were few pieces of historic information on the much higher tsunamis of 1611 and 869, but they were too few for hazard countermeasures. Since 1990s geologic information on the 869 Jogan tsunami has been accumulated. The 869 tsunami deposits reached 2 to 3 km inland and up to 4 m above the present shoreline. Further south, at the Fukushima NPP, 869 tsunami deposits were found at 3.6 m above sea level, but a higher tsunami run-up was not taken into account.