Scrutinizing Past Great Earthquakes and Tsunamis
－Tsunami disaster mitigation based on Geological Evidence－

Last Updated : April 9, 2021

　There are many similarities between the 2011 Off the Pacific Coast of Tohoku Earthquake and the historical great earthquake known as the 869 Jogan Earthquake. Geological records of the tsunami caused by the Jogan Earthquake are preserved in strata called tsunami deposits. Since 2004, the Geological Survey of Japan has been conducting geological surveys of these tsunami deposits to get further information on the extent of the area inundated by the Jogan Earthquake. The Survey has found that, in some areas, the tsunami extended 3 to 4 km inland from the coastline at the time. A tsunami simulation based on these data has shown that the Jogan Earthquake occurred in the plate boundary off the coast of Miyagi and Fukushima prefectures.
　In Sendai and Ishinomaki, most of the area inundated by the tsunami that followed the 2011 Off the Pacific Coast of Tohoku Earthquake overlaps with the area estimated to have been inundated by the tsunami from the Jogan Earthquake. If the possibility of recurrence of the Jogan Earthquake had been considered, then some precautionary measures might have been taken against the 2011 tsunami.

Knowing the impacts of tsunamis as historical events

　After the experience of the 2011 Great Earthquake Disaster, it is feared that the next large earthquake will occur along the Kuril Trench or in the Sagami Trough or Nankai Trough.
　The central and local governments are aiming to reduce the number of unexpected events in a future disaster by assuming the occurrence of the largest possible earthquake and tsunami. However, this is merely hypothetical, and it is uncertain whether such an earthquake and a tsunami have in fact occurred in the past. We distribute reliable data obtained by studying tsunami deposits and other nature traces by actual past events to the wider public. By developing and establishing surveys and research methods, we also aim to make it useful to both the government and private businesses.

Looking for lithofacies in a core sample obtained from Hirota Bay, Iwate Prefecture
(Photo by YASUTOMO Yasuhiro/GSJ)

Numerical simulation of the AD 869 Jogan earthquake tsunami, northeastern Japan