Vol.2 No.2 2009

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Research paper : A study of paleoearthquakes at archeological sites (A. Sangawa)−86−Synthesiology - English edition Vol.2 No.2 (2009) became possible to identify the dates of individual artifacts fairly accurately.Hardly any attention was paid to the “traces of earthquakes” that must have been found in the process of the archaeological excavations. Even if the earthquake traces showed up, most were neglected as items unrelated to archaeology, without knowledge that they were products of earthquakes. Even if the archaeologist noticed that they might be related to earthquakes, the survey methods were unaltered and the traces remained untouched, except in very few cases.In July, 1987, I moved from the Tsukuba Science City to the Geological Survey Osaka that was located in the government building of Chuo-ku, Osaka. Many archaeological excavations were conducted in the Kansai area, since this was the seat of central government in ancient and medieval periods, and public interest in archaeology was high. I frequently visited the archaeological sites where the earthquake traces were found and participated in the excavation. I learned the survey methods and dates of artifacts from the archaeologists in charge of the excavation, and in exchange, taught them the basic knowledge of earthquakes.In November, 1987, I presented the case studies and the basic survey methods of earthquake traces in a lecture of the Kodaigaku (Ancient Study) Research Group, Osaka[5]. Many participants enthusiastically reported that they had come across earthquake traces in past excavations. Immediately after, with advice from an archaeologist, I named this study “earthquake archaeology,” and officially declared its establishment at the Japanese Society for Scientific Studies on Cultural Property and the Japan Archaeological Association in spring of 1988[6][7]. In the following year, I presented the basic survey methods for earthquake archaeology in the journal of the Society of Archaeological Studies[8].By using the name “earthquake archaeology,” people involved in the archaeological excavations became aware that earthquake traces were subjects of archaeology. There increased the number of cases where the dates were narrowed down and the effects of earthquakes on the people were investigated when earthquake traces were found in the course of an archaeological excavation. It became routine to include a detailed description of earthquake traces in the archaeological site reports[9].At the Nara National Research Institute for Cultural Properties (currently part of the National Institutes for Cultural Heritage), there is a system of training new research methods for archaeologists in charge of excavations from local governments and research centers for buried cultural properties throughout Japan. From 1989, I started lecturing on the survey methods for earthquake traces.4 Characteristic of the studyAreas with rich traces of people’s existence buried underground and buildings (such as kofun or mound tomb) valuable as cultural heritages are designated “archaeological or historical sites” based on the Law for the Protection of Cultural Properties established in 1950. An archaeological site contains abundant remains such as residences and structures as well as artifacts such as plates, bowls, burial goods, and agricultural tools. When a site is to be destroyed by development, archaeological excavation is conducted prior to the construction.In Japanese archaeology, date sequencing for remains and artifacts has been studied thoroughly so that when an earthquake trace is found in an archaeological excavation, the date of the earthquake that left the trace can be narrowed down by looking at the sequential relationship of the remains and artifacts for which the dates have been confirmed. Particularly, archaeological sequencing through remains and artifacts as well as absolute dating are well-established for the 2,000 years after the Late Yayoi Period, and narrowed-down dates of the earthquake traces can be obtained.Since the Japanese Archipelago is subject to severe tectonic activities, the submerging areas are covered by sediments carried by rivers and ocean, and plains and basins are formed. Our ancestors set up residence in flat places close to the water, and majority of the archaeological sites are concentrated in plains and basins. Older remains and artifacts are buried in the lower layers.When a major earthquake strikes, the area with weak foundation suffers most, and traces of liquefaction are seen in Kitoge Nishikaido and Kizugawa Riverbed sites. Liquefaction drew attention when a modern city suffered great damages in the Niigata Earthquake that occurred in 1964, and severe damages occurred to lifelines in the Great Hanshin Awaji Earthquake of 1995. When sand is carried up to the surface with underground water due to liquefaction, sand boils are formed.In the soft sand layer deposited underground, there are spaces between the sand grains, and with strong tremors, the sand grains juggle around to reduce the space, which in turn compacts the sand layer. The underground water in the space is compressed, resulting in increased water pressure, and water spews out onto the surface along with sand as they tear the layers above. Figure 3 is a schematic diagram of a trace of sand boiling, and the layer torn by the sand boil was deposited before the earthquake while the layer covering the sand boil was deposited after the earthquake. The dates of the two layers are determined by the remains and artifacts in each layer. As in this figure, assuming that the topmost layer torn by the sand boil (before the earthquake) is dated to the

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