Researches on the tsunami deposits along the Nankai Trough:

Their achievements and upcoming challenges

Abstract

Paleotsunami studies within the Nankai Trough region started in the late 1990s and have contributed to our understanding of the tsunami history of this region over the past 6000 yr. Notably, the discovery of unusually thick tsunami deposits on the western Nankai coast suggests that exceptionally large tsunamis have occurred in this area. This has led to a hypothesis that there is a “hyperearthquake cycle” of 300-500 yr, which is distinct from the ordinary Nankai Trough earthquake cycle of 100-150 yr. However, the number of reliably documented tsunami deposits is still too small to accurately reconstruct the size and source area of each paleotsunami. In this review, we propose two main fields as future research directions for tsunami deposit research in the Nankai Trough region. The first involves verifying the presence or absence of great earthquakes during the historical period for which there is no written record of Tokai or Nankai earthquakes. Although the last three Tokai and Nankai earthquakes (i.e., after the 18th century) occurred in pairs with a short time interval between them (<2 yr), it remains unknown if the older Tokai and Nankai earthquakes also occurred in pairs. This is fundamental for improving our understanding of the fault rupture pattern along the Nankai Trough subduction zone. The second field of research involves quantification of the size of past tsunamis. Quantification of paleotsunami inundation height and area is needed to verify the “hyperearthquake cycle”, and to delineate the maximum tsunami size that should be adopted in hazard mitigation measures.