Subduction zone paleoseismology along the Pacific coast of northeast Japan — progress and remaining problems

Abstract

This paper reviews recent progress in tsunami geology and the history of coastal uplift/subsidence along the Pacific coasts of Hokkaido and Tohoku, facing the Kuril and Japan Trenches, respectively. In the last 20 years, research by tsunami geologists in northeast Japan has focused on extending the record of tsunamis beyond the range of historical documents in the region, which span too little time to reveal much of the earthquake history over the last millennia.Along the coast of Hokkaido, recurrent sandy deposits are evidence of historical and prehistoric large tsunamis. At Kiritappu marsh, for example, at least 15 identified tsunami deposits occurred between 200 and 6,000 years ago. The spatial distribution of the most recent prehistoric tsunami deposit, that of the so-called 17th-century earthquake and tsunami, exceeds historical and recent tsunami inundations in eastern Hokkaido. Numerical simulations of this event require a multi-segment fault model with variable slip (Mw > 8.8) encompassing both the Tokachi-oki and Nemuro-oki regions to reproduce the observed deposit distribution in eastern Hokkaido. The recurrence interval of such multi-segment earthquakes along the southern Kuril Trench is estimated to be 100–800 years based on tephrochronological and radiocarbon evidence. Tsunami deposits are also associated with historical earthquakes in 1843 and 1894 in eastern Hokkaido and with the 1640 Komagatake eruption in western Hokkaido. The 17th-century earthquake probably triggered postseismic coastal deformation. Diatom-based transfer functions suggest rapid subsidence before the event and gradual uplift afterward, and historical volcanic ash layers indicate that gradual postseismic uplift occurred over decades. A simple dislocation model can explain such gradual uplift as an aseismic fault extension after the 17th-century event.In the Tohoku region, a tsunami deposit associated with the 869 Jogan earthquake has been identified, and the inundation area of this event has been constrained based on the lateral extent of the tsunami deposit and its stratigraphic position beneath the historical To-a tephra (915 CE). Land-level changes at Odaka, Fukushima prefecture, associated with the Jogan event have been estimated based on changes of fossil diatom assemblages below and above the tsunami deposit. Numerical simulations based on geological evidence and observations of the 2011 Tohoku tsunami deposit suggest that the Jogan event was a plate-boundary rupture at least 200 km long along the Japan Trench (Mw > 8.3–8.6). The 1454 Kyotoku tsunami is also considered to have been generated by a rupture area including the Miyagi-oki region (part of the Jogan rupture). It remains unclear whether the 869, 1454, and 2011 tsunamis were similar in size and source; if they were, the recurrence intervals of such events near Sendai are less than 600 years and more regular than those for multi-segment earthquakes along the Kuril Trench.