AN ATTEMPT BEFORE 2011 TO ESTIMATE EXTRAORDINARY TSUNAMIS ALONG JAPAN TRENCH (Invited Presentation)

Written records tell of an unusually large tsunami near Sendai in July 869. This so-called Jogan tsunami also registered geologically as a sheet of sand near Sendai, as first reported in 1990 by Abe et al. (1990) and Minoura and Nakaya (1991). Building on their reconnaissance study, our new work 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 shorelines. In addition, our work showed that one of the plains had subsided during the Jogan earthquake and during a preceding earthquake as well. Modeling the inundation and subsidence, we reported a preliminary estimate that the Jogan earthquake attained magnitude 8.4. The estimate was a minimum because the full length of Jogan rupture was unknown. To forewarn of the 2011 earthquake of magnitude 9.0, with a fault rupture 400 km long, geological evidence for inundation and subsidence needed to be found and correlated along additional parts of the Tohoku coast. The 2011 disaster took place before much work had been done on this difficult task.