Tsunamis of the Past

Abstract

Geology According to some seismic hazard maps, a large earthquake was not expected to occur offshore of the Tohoku region in Japan. The occurrence of the 2011 Mw 9.0 Tohoku-oki earthquake—one of the largest earthquakes in recorded history—and the resulting devastation after both the earthquake and the subsequent tsunami demonstrated that the model used to assess risk in that region may need serious revision. However, there were some signs that the region had the potential for a very large earthquake. Previous, although uncertain, historical accounts and sedimentological records of a large tsunami point to the occurrence of another large earthquake in 869 CE, often called the Jogan earthquake. Sawai et al. synthesized sedimentary data, including diatom assemblages and radiocarbon dates, of nearly 400 new and previously identified paleotsunami deposits from the Jogan and other historical earthquakes along the coast of Japan. By combining observations of coseismic subsidence in the sediment with tsunami simulation models, the authors estimate that the size of the Jogan earthquake was at least Mw 8.4, and the resultant tsunami reached at least 1.5 km inland. With a recurrence interval of 500 to 800 years, large earthquakes in this region of Japan do not occur frequently but are more likely to occur than predicted by previous hazard models. ![Figure][1] CREDIT: WIKIMEDIA COMMONS Geophys. Res. Lett. 10.1029/2012GL053692 (2012). [1]: pending:yes