Vol.5 No.4 2013

Research paper : Reconstruction of the 869 Jogan tsunami and lessons from the 2011 Tohoku earthquake (Y. Okamura)−243−Synthesiology - English edition Vol.5 No.4 (2013) 4 Reconstruction of the Jogan TsunamiThe research to reconstruct the 869 Jogan earthquake and tsunami (hereinafter, Jogan tsunami) involves the survey of tsunami deposits in the field, analysis of the geological samples, and construction of the tsunami source model. The greatest characteristic of our paleo-tsunami research is to combine geological survey of tsunami deposits and the geophysical method of tsunami simulation.[12] (Fig. 2)The distribution of tsunami deposit was revealed by detailed field survey. Because whether the tsunami deposit is preserved well or not depends on various natural conditions and the human activities, it is important to conduct coring as much as possible at the site (Fig. 3). The distribution area of tsunami deposits can be determined accurately by accumulating the data from many places. Also, since there may be several tsunami deposits of different ages with similar appearances, it is important to correlate each of the tsunami deposits by a detailed and careful survey. At the same time, surveys in a wide region as much as possible are necessary to increase the reliability of the tsunami source model.Yoshida [11] reviewed the description of the Jogan earthquake and tsunami in the historical record and pointed out the risk of a giant tsunami around Sendai. The tsunami deposits formed by the Jogan tsunami were reported in the Sendai Plain after 1990.[13][14] Using these survey results as reference, the survey area was widened from Ishinomaki to the northern part of Fukushima Prefecture to clarify the distribution of tsunami deposits, and the position of the coastline at the time was reconstructed (Fig. 4).In Ishinomaki, the deposits from the Jogan tsunami were Fig. 2 Progress of research on past giant earthquakesConstruction of tsunami source model and numerical calculationConstruction of Jogan tsunami model→ M8.4 or larger off the coast of Miyagi & FukushimaEarthquake disaster mitigationModel verification and identification of issuesEstimation of scale, recurrence, effectSystematic research on Miyagi-okiDevelopment of research & analysis methods2011 Tohoku-oki EarthquakeTsunami deposit research along Pacific coast of Hokkaido→Reconstruction of giant tsunamiHirakawa et al. 2000, Nanayama et al. 2000, AIST 2004869 Jogan EarthquakeMinoura & Nakaya 1991Abe et al. 1990Discovery of tsunami depositsYoshida 1906Historical recordGeophysical analysisDistribution of tsunami deposits in wide regionReconstruction of old coastline positionTsunami deposit dating → 500-600 year intervalsFossil diatom analysis → Subsidence during tsunamiGeological research & analysisGiant tsunami research in Pacific coast of TohokuImprovement of prediction accuracy Public relations, disaster prevention measuresFig. 3 Tsunami deposit research in Sendai PlainThe underground strata are collected using the geoslicer in the rice field after harvest.


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