Vol.5 No.4 2013

Research paper−241−Synthesiology - English edition Vol.5 No.4 pp.241-250 (Mar. 2013) announcing the results to society and contributing in disaster mitigation. How the demand and request from society toward earthquake research changed before and after the earthquake is reviewed, and how AIST should respond to the social demands will be considered.2 Earthquake evaluation methodSince the 1995 Kobe Earthquake, the Headquarters for Earthquake Research Promotion (hereinafter, will be called Earthquake Headquarters) was established in the Agency of Science and Technology (currently Ministry of Education, Science, Sports, Culture, Science and Technology; MEXT). Under its leadership, earthquake research shifted its direction from scientific research to research that contributes to earthquake disaster prevention. One of the most important projects of the Earthquake Headquarters is the “Long-term Evaluation of Earthquakes.”[1]Based on the assumption that great earthquakes occur repeatedly in approximately the same place at the same scale, the earthquake that might occur in the future was predicted based on the information of the location, scale, and age of the past earthquakes. In this case, the reliability of the earthquake evaluation increased in accordance to the level of reliability of the information on past earthquakes.The most accurate record of earthquakes is the record detected by the seismometer, but such information is available only for about a hundred years (Fig. 1). The older 1 IntroductionThe ultimate goal of earthquake research is to mitigate the disastrous effects of earthquakes. Since the 1995 Kobe Earthquake (in the news media, Great Hanshin-Awaji Earthquake or Great Hanshin Earthquake), the emphasis of the earthquake research was placed on disaster prevention and mitigation. However, several earthquakes occurred in places where active faults were not recognized after the 1995 earthquake. In adition, effective warnings could not be issued in the 2011 Tohoku-oki Earthquake (in the media, Great East Japan Earthquake). But, our research group has been studying the tsunami generated by the Jogan Earthquake that devastated the Sendai Plain in AD 869. We are regretful that the Tohoku-oki Earthquake occurred before our study results could be reflected in the disaster prevention measures in the region. The difficulty of predicting the natural phenomenon has become very apparent. It is also true that this earthquake illuminated the weaknesses of earthquake research based on geophysics, and instead, shifted the focus to paleoseismology or the study of past earthquakes based on geology as conducted at AIST.In this paper, we describe AIST’s research method of reconstructing the Jogan Earthquake and tsunami that occurred in AD 869 by integrating the studies of history, geology, and geophysics. Also, we present the importance and issues of research for estimating the scale of a tsunami from the tsunami deposits that became clear in the 2011 Tohoku-oki Earthquake, and discuss the problems in - Significance of ancient earthquake studies and problems in announcing study results to society-To estimate the magnitude of the 869 Jogan tsunami (described in the historical record), we surveyed tsunami deposits and constructed a source-fault model by combining geological data with geophysical simulation. Although the 2011 Tohoku earthquake was larger than the earthquake estimated by our Jogan model, the 2011 earthquake proved that tsunami deposits are evidence of past giant tsunamis, and reliable warnings of future giant tsunamis. Our study results on the Jogan tsunami were submitted to the Headquarters of Earthquake Research Promotion, and in March 2011, the evaluation was near completion. However, the earthquake occurred just before issuing a warning against a giant tsunami. We need to announce our study of ancient earthquakes and tsunamis to society as quickly as we can so as not to repeat such a tragedy. Moreover, we have to concurrently carry out reliable studies based on rigorous surveys.Reconstruction of the 869 Jogan tsunami and lessons from the 2011 Tohoku earthquakeKeywords : Jogan earthquake, Tohoku-oki earthquake, tsunami deposits, announcement, disaster mitigation[Translation from Synthesiology, Vol.5, No.4, p.234-242 (2012)]Yukinobu OkamuraActive Fault and Earthquake Research Center, AIST 1-1-1 Higashi, Tsukuba 305-8567, Japan E-mail: Original manuscript received March 1, 2012, Revisions received June 4, 2012, Accepted June 8, 2012


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