WHAT WE LEARNED FROM THE 2011 MEGATHRUST EARTHQUAKE (M9.0) ALONG THE JAPAN TRENCH: PALEOSEISMOLOGICAL PERSPECTIVES

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. Northern half of the damaged areas, Sanriku rocky coast area was the best in the world prepared for huge tsunamis reflecting experiences in 1896, 1933, and 1960 tsunamis. But there was no historic and geologic information on tsunamis that might have exceeded recent ones and 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 tsunamis like in 1896 or 1978. In these areas, there were few pieces of historic information on much higher tsunamis in 1611 and 869, but they were too few for hazard countermeasures. Since 1990s geologic information on 869 Jogan tsunami has been accumulated by Minoura et al. (2001), Sawai et al. (2007) and Shishikura et al. (2007). They studied surface geology of the coastal lowlands and found tsunami deposits distributed over almost entire lowlands. The 869 tsunami deposits reached 2 to 3 km inside and up to 4 m above present shoreline. Satake et al. (2008) simulated 869 tsunami using the geologic record and proposed two models. One is 100 km long and 100 km wide with 10 m slip, the other is 200 km long and 100 km wide with 7 m slip both offshore Sendai in the middle of 2011 source area. The estimated magnitude is Mw 8.1 to Mw 8.4. The extent of the fault plane is constrained only by the limitation of geologic data. Along the Sanriku coast in north no tsunami deposits of recent tsunamis have been found. In farther south, 869 tsunami deposits were found near Fukushima NPP at 3.6 m above sea level but much higher tsunami run-up was not estimated.