Issues in Tsunami Countermeasures from the Viewpoint of Geotechnical Engineering

The 2011 off the Pacific coast of Tohoku Earthquake, with its epicenter off the Sanriku coast, measured the moment magnitude of 9.0, had a maximum seismic intensity of 7 in the northern part of Miyagi Prefecture, and impacted an area of 450 km. Consequently, a variety of unprecedented problems were made apparent. In particular, the human and property damage wrecked by the ensuing tsunami triggered our response for earthquake and tsunami resistance. In addition to conventional issues, such as earthquake resistance of buildings, disruption of lifelines, liquefaction of residential land and soil structures, and tsunami damage in coastal areas, there were new challenges, such as prolongation of earthquake disaster waste treatment. During the 10 years since the 2011 earthquake, tsunami countermeasures have been reexamined, and based on the concept of multiple protections, both tangible and intangible countermeasures have been advanced. This article addresses technical problems related to complex disasters, and includes the example of actual damage to a river levee in the Iwate Prefecture and the case of a building overturned by tsunami in Onagawa City, Miyagi Prefecture. It also discusses liquefaction caused by earthquakes and lists the points to be considered when selecting tsunami evacuation buildings to tackle future tsunami disasters.