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)−242−Synthesiology - English edition Vol.5 No.4 (2013) records exist as written historical records. The oldest historical record of an earthquake is the description of the earthquake that occurred in AD 599.[2] However, the records from the distant past are sparse and the information volume is low, and in many cases, the information is insufficient to estimate the magnitude of past earthquakes. The historical earthquake records with sufficiently high quality and volume are those during the Edo Period (AD 1603~1868) or later. The greatest advantage of historical records is that the date of occurrence and the place affected by the earthquake damage can be known fairly accurately, and it has been used widely as past earthquake records. The geological records such as tsunami deposits and active faults offer information about earthquakes before historical records, and the crustal movements and tsunamis caused by earthquakes are left in the topographies and geological strata. The records of the giant earthquakes and tsunamis left in nature include some errors in terms of date, but the greatest advantage is that they offer information on past earthquakes and tsunamis for the period of several thousand years.In general, inland earthquakes occur by slip of active faults and the recurrence intervals of the slip events are long, such as thousand years or more. Although the latest slip events may be known from historical records, many events remain unrecorded in history, and thus we must rely on the slip records left in nature. On the other hand, subduction zone earthquakes often have occurrence intervals of decades to two hundred years, and therefore, information of multiple occurrences and scales can be obtained from the historical records.[3] For Tokai and Nankai Earthquakes, nine occurrences have been recorded in history since over a thousand years ago.[2] For the Japan Trench, many earthquake have been recorded during the Edo Period and later, and their magnitudes were 7 to 8. The subduction zone earthquakes have been evaluated based on such historical records.[4]The evidences for giant tsunamis unrecorded in history were found in the Pacific coastal area of east Hokkaido. The tsunami deposits were distributed wider and further inland than the inundation zone of the tsunami known in history, in the Pacific coastal area of Tokachi, Kushiro, and Nemuro regions.[5][6] In these areas, earthquakes of around magnitude 8, such as the Tokachi-oki Earthquake and Nemuro-oki Earthquake, have occurred at intervals of several decades to hundred years, but it was inferred that giant tsunamis were probably caused by multi-segment earthquakes in which multiple earthquakes occur simultaneously.[7]-[9] The last multi-segment earthquake is estimated to have occurred in the 17th century, but there is no historical record in Hokkaido because the region has a short historical record. Through the survey and analysis of the tsunami deposits, the earthquake that caused the giant tsunami was estimated to have a magnitude of about 8.5 and a recurrence interval of about 500 years. In these studies, it was shown that the scale of the past giant earthquakes and tsunamis could be reconstructed from the geological data, and that much larger earthquakes and tsunamis may occur even in areas where the subduction zone earthquakes of about magnitude 8 occur repeatedly. This hypothesis was proven correct in the 2004 Indian Ocean Earthquake.3 Limit of the conventional earthquake scenario for the Tohoku regionThe subduction zone earthquakes that occurred in the Pacific coast of the Tohoku region were evaluated and predicted based mainly on the historical records during the Edo Period and later. Except for the Sanriku offshore area, these earthquakes had the scale of magnitude 7~8, and no earthquake of magnitude 9 had been recorded. In the Sanriku coastal area, the 1611 Keicho Sanriku Tsunami, the 1986 Meiji Sanriku Tsunami, and the 1933 Showa Sanriku Tsunami were known as destructive tsunamis. People’s awareness for tsunamis was high in this region and some measures were taken accordingly. In contrast, the awareness against a giant tsunami in the coastal area of the Sendai Plain and further south was extremely insufficient, although it was known that the coastal area of Sendai Plain was damaged severely in the 1611 Keicho Sanriku Tsunami,[10] and there was a historical record of a giant tsunami in AD 869.[11] The latter one was known as the Jogan Earthquake. The Nihon Sandai Jitsuroku that was written at the court of Kyoto during the Heian Period (AD 794~1185) describes the fact that there was a great earthquake in Mutsu-no-Kuni (current Tohoku region). The ground shakes were so severe that people could not remain standing, many buildings collapsed, and the tsunami flooded vast inland areas. It is believed that this is a description of the disaster which occurred in Tagajo area which was the capital of Mutsu-no-Kuni at that time, however, the scale of the tsunami and the distribution of damage remained unclear. Fig. 1 Periods during which the information about past earthquakes exists and the intervals of earthquake occurrencesOnly the geological record can solidly cover the period that is longer than the intervals of giant earthquake occurrences. Note: Earthquake recurrence intervals show the approximate interval of occurrence of the different types of earthquakes, and do not show the age of occurrence of the earthquake.Active fault (inland earthquakes)Earthquake reccurrence intervalMulti-segemnt earthquakesSubduction zone earthquakes of about M8Tsunami deposits, active faultsGeologicalrecordsSeismograph10,000 years3000Years1000Years400Years100Years10YearsHistoricalrecords(Note)

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