Vol.5 No.4 2013
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Research paper : Reconstruction of the 869 Jogan tsunami and lessons from the 2011 Tohoku earthquake (Y. Okamura)−247−Synthesiology - English edition Vol.5 No.4 (2013) before the earthquake. We presented our study at academic meetings, and the contents drew media attention and were covered by the newspaper and TVs. However, it did not lead to consciousness raising or disaster prevention measures of the region. In addition, we conducted lectures to the public in Miyagi Prefecture a few times. Although the number of audiences was small, we might have been able to change the awareness for earthquakes if we continued the lectures. However, warning to society about the risk of a giant tsunami is difficult work beyond the ability of the researchers at AIST, and we believe that using the communication resources centered at the Earthquake Headquarters is the most effective and efficient way. Because the Earthquake Headquarters officially evaluate earthquakes, it was probably difficult for the local governments to actively promote disaster prevention measures before the final evaluation was issued from the headquarters.The system where the Earthquake Headquarter, representing the government, informs the earthquake risk to society is very important and mandatory. Because this is the official information, it takes time to complete reliable evaluations based on careful examination. If some research results showed the possibility of serious damage by an unknown earthquake or tsunami, we believe that we need to inform the risk to the public through some brief objective evaluation.After the earthquake, since the concern and interest of society for earthquakes increased, the opinions of many researchers on tsunami deposit research and earthquakes were easily reported by the media before sufficient investigations were done. I think there are both advantages and disadvantages in releasing the brief research results to society. The advantage is that the information is released quickly to society. It is possible to minimize the possibility of being too late, as it was in 2011. On the other hand, it should not be forgotten that the reliability of prompt report has not been confirmed, and the mass media likes to report the information of larger tsunamis. If survey results were shown to be not true, earthquake researches will lose confidence. To avoid such confusion, we hope for the official organization for quick objective evaluation.7 Changes in society before and after the earthquakeSince several inland earthquakes had been occurring during the several years before the 2011 Tohoku-oki Earthquake, the awareness in the Japanese society for earthquakes was high, but the awareness for tsunamis was low. We have to persuade society that there is a risk of giant tsunamis and to take action for the disaster prevention measures, therefore we need to show convincing evidences of giant tsunamis. This is why Fig. 7 Source fault model for Jogan tsunami proposed before the earthquake (model 10)[17][18]Since this model was created based on the data from Ishinomaki Plain to northern part of Fukushima Prefecture, it could have spread further north and south.Fig. 8 Inundation zone calculated by the Jogan tsunami model and the inundation zone of the 2011 Tohoku-oki Earthquake[19]The tsunami inundation zone based on the distribution of the Jogan tsunami deposits and the inundation zone of 2011 match approximately.Plate boundary typePlate boundary typeNormal fault typeNormal fault typeTsunami earthquake typeTsunami earthquake typeIshinomaki PlainSendai PlainOdakaUkedoDepthDepth0 km0 kmJapan TrenchJapan TrenchInundation zone of the tsunami by the 2011 Tohoku-oki Earthquake according to Geospatial Information Authority of JapanInundation zone of the tsunami by the 869 Jogan Earthquake estimated from the distribution of tsunami deposits(Model 10 according to Namegaya et al. 2010)Estimated coastline during the 869 Jogan EarthquakeLimit of sand deposit in 2011 tsunamiLimit of mud deposit in 2011 tsunami5 km0Sendai CityNatori CityNatori RiverSendai AirportIwanuma City

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