Reconstruction of the 869 Jogan tsunami and lessons of the 2011 Tohoku earthquake

Abstract

To estimate the magnitude of the 869 Jogan tsunami (described in the historical record), we surveyed tsunami deposits and constructed a source-fault model by combining geological data with geophysical simulation. Although the 2011 Tohoku earthquake was much larger than the earthquake estimated by our Jogan model, there were similarities: the 2011 earthquake proved that deposits are evidence of tsunamis in the past, and reliable warnings of future giant tsunamis. Our study results on the Jogan tsunami were submitted to the Headquarters of Earthquake Research Promotion, and in March 2011, the evaluation was near completion. However, the earthquake occurred just before a giant tsunami warning could be issued. We need to announce our study results to society as quickly as possible so as not to repeat such a mistake. Moreover, we have to concurrently carry out reliable studies based on rigorous surveys.