Vol.5 No.4 2013
Research paper : Reconstruction of the 869 Jogan tsunami and lessons from the 2011 Tohoku earthquake (Y. Okamura)−244−Synthesiology - English edition Vol.5 No.4 (2013) found 5 km inland from the current coastline. At least two layers of tsunami deposits below and above the 869 tsunami deposits were confirmed. The Ishinomaki Plain expanded gradually over the past several thousand years, and the coastline was about 1.5 km inland from the current coastline during the Jogan earthquake.In the Sendai Plain, the tsunami deposits were distributed 4 km or more inland from the current coastline and the coastline during the Jogan earthquake was located about 1~ 1.5 km inland. It was estimated that there were three or more inundations before the Jogan tsunami, and at least one after the Jogan tsunami.The date of tsunami occurrence and the crustal movement due to an earthquake can be estimated by analyzing the deposits obtained in the fieldwork. To estimate the date, fragments of fossil plants were removed from the peat layers just above and below the tsunami deposit, and radioactive carbon 14C was measured in the fragments. In this dating method, large errors may occur depending on the measured samples, thus it is important to choose an appropriate material without contamination of younger and older materials. Based on the carbon dating, we inferred that the tsunami deposits were formed at intervals of approximately 500 years. By analyzing the fossil diatoms in the peat, we presumed that subsidence occurred during the Jogan and the precedent tsunamis in northern Fukushima.The epicenter and the magnitude of the earthquake that caused the tsunami that inundated the deposit distribution zone were estimated by numerical simulation. Since it was difficult to determine the position of the source region only from the distribution zone of tsunami deposits, different types and scales of earthquakes were assumed based on the earthquakes that occurred in the past along the Japan Trench(Fig. 5), and then the inundation zones were calculated, and compared with the distribution zone of tsunami deposits Coastline during the Jogan tsunamiCoastline during the Jogan tsunamiCoastline during the Jogan tsunamiCoastline during the Jogan tsunami3 km3 km0Coastline at the timeIshinomaki PlainSendai PlainPlaces where deposit from Jogan tsunami was foundPlaces where deposit possibly from Jogan tsunami was foundPlaces where deposit from Jogan tsunami was foundPlaces where deposit possibly from Jogan tsunami was found38°30’E38°00’E141°30’E141°00’E38°00’E38°30’E141°00’E141°30’EIshinomakiMatsushimaShiogamaSendaiIwanumaSomaFig. 4 Distribution area of Jogan tsunami deposits in the Ishinomaki and Sendai Plains and the coastline at the timeDetailed researches of the deposits were conducted along several survey lines to clarify the distribution area. At the same time, the position of coastline during the earthquake was confirmed.