Assessing the magnitude of the 869 Jogan tsunami using sedimentary deposits: Prediction and consequence of the 2011 Tohoku-oki tsunami

In this paper, the spatial distribution and sedimentological features of the 869 Jogan tsunami deposit along the Pacific coast of Japan are reviewed to evaluate deposit-based estimates of the magnitude of the Jogan tsunami and the use of tsunami deposits in the prediction of the 2011 Tohoku-oki earthquake and tsunami. Inundation of the Sendai Plain and the offshore wave sources of both tsunamis are compared. The Jogan tsunami deposit is ubiquitous on the coastal plains of Sendai Bay, whereas, to date, it is only identified in a few locations along the Sanriku and Joban Coasts. This resulted in an underprediction of the size of the wave source of the Tohoku-oki tsunami. The inland boundary of the inundation area of the Tohoku-oki tsunami on the Sendai Plain is approximately equivalent to that of the Jogan tsunami, although many sedimentological and geomorphologic factors make a direct comparison of the tsunamis complicated and difficult. The magnitude of the Jogan earthquake (Mw = 8.4), which was derived from the tsunami deposit inland extent and numerical inundation modeling, was too small to predict the magnitude of the Tohoku-oki earthquake (Mw = 9.0–9.1) and tsunami. Additional research is needed to improve deposit-based estimates of the magnitudes of past tsunamis and to increase the ability to use tsunami deposits, in conjunction with inundation modeling, to assess future tsunami hazards.

► The Jogan tsunami deposits were found mainly on the Sendai Plain, and rarely found in the Sanriku and Joban Coasts.
► The inland extent of the Jogan tsunami deposit is approximately the same as the inundation area of the Tohoku-oki tsunami on coastal plains.
► The wave source inferred from the alongshore distribution of the Jogan tsunami deposit is quite narrow comparing to the focal region of the 2011 Tohoku-oki earthquake.