GEOLOGICAL EVIDENCE OF HISTORICALLY RECORDED TSUNAMIS IN SENDAI PLAIN, NORTHERN JAPAN

We report geological evidence for paleotsunamis in the past 1,200 years. In Sendai Plain, one of the devastated regions in the 2011 Tohoku earthquake (M=9.0), the written history of tsunamis begins with a disaster over 1,000 years ago. This event, so-called Jogan earthquake, registered geologically as a sand sheet near Sendai, as first reported in 1990’s by Abe et al. (1990) and Minoura and Nakaya (1991). After their works, between 2004 and 2010, we confirmed that the Jogan sand sheet and two of its predecessors extend along 100 km of coast and penetrate as much as 2 km inland from their contemporaneous shoreline. Numerical simulations for tsunami inundations suggest fault of the Jogan earthquake ruptured no less than 200 km by 100 km (Mw>8.4) (Sawai et al., 2012).

Stratigraphy above the Jogan sand sheet recorded an additional ~2cm-thick tsunami sand layer. The sand layer was observed in 1-m wide geoslice samples taken in northern part of the Sendai Plain. The sand is dominated by brackish-marine diatoms. It was tied in a row of ellipses, which reveals the loadcast structure resulted from rapid sedimentation on soft sediments. Radiocarbon dating permits correlation of the sand for AD 1454 Kyotoku, or AD 1611 Keicho earthquake. Distribution of the Kyotoku or Keicho sand layer shows that its run-up distance would be larger than tsunamis that occurred during the last 200 years except for the 2011 Tohoku tsunami.