The future of tsunami research following the 2011 Tohoku-oki event

In this paper we summarize the regional setting, our previous understanding of historical and pre-historical tsunamis on the Pacific coast of Tohoku, Japan, prior to the 2011 Tohoku-oki tsunami, and our current understanding of the sedimentological, geochemical and paleontological features of the onshore and offshore deposits of the event. Post-tsunami surveys revealed many new insights, such as; (1) the maximum extent of the sand deposit is sometimes only 60% of the inundation distance, (2) the inundation limit can be estimated by geochemical analysis even a few months after the event, (3) a minor amount of marine sediment was transported inland by the tsunami on the Sendai and adjacent plains with the major sediment sources being from beach and dune erosion or vented sediments from liquefaction, although nearshore and offshore surveys revealed that there was a significant amount of sediment transport on the seafloor, (4) coarse gravel deposits (~. 1. m in thickness) were usually thicker than the sand ones (~. 30. cm in thickness), and (5) beach erosion was minimal in some places while severe in others. Another important aspect of this event is that it was a large, infrequent, tsunami that took place where possible predecessors (e.g., AD869 Jōgan) were already known to have occurred based on historical and geological evidence. The AD869 Jōgan tsunami deposits are noticeably similar to the 2011 Tohoku-oki sands, therefore suggesting that the Jōgan and its source mechanism may have been larger than previously thought. While we have learned many lessons from the 2011 Tohoku-oki event, more research is needed to provide reliable tsunami risk assessments around the world.