Vol.2 No.2 2009

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Research paper : A study of paleoearthquakes at archeological sites (A. Sangawa)−85−Synthesiology - English edition Vol.2 No.2 (2009) Through this process, we could now see the overall picture of the active faults in Japan. The researchers were now encouraged to move to the next step from diverse perspectives, such as research on the “mechanism of earthquake occurrence at active faults,” “detailed investigation of the characters of individual active faults,” and “consideration of fault activities and history of geomorphology.”3 New research developmentsWhen the compilation of the 1:500,0000 Neotectonic Maps was almost complete, I started to pay attention to archaeological sites. This happened coincidentally.I visited a town history library of the Imazu Town Hall, Takashima-gun (currently Takashima City) in spring of 1986 to collect historical materials on Lake Biwa. The excavation of Kitoge Nishikaido Site was being conducted by the Imazu-cho Education Board, and I had an opportunity to talk to the archaeologist who conducted the excavations. He found a strange crack filled with sand at the excavation site, and asked me whether it may be related to past earthquakes.The Kitoge Nishikaido Site was identified as the cemetery of the Jomon to Yayoi periods. I immediately visited the site, and observed a crack of about 1 m width, running straight and filled with sand. By digging the ground and looking at the cross section of the geological layers, it was found that the sand emerged from the underlying sand layer that rose and filled the crack. This was a trace left when the area was hit by a strong seismic tremor, the underground sand layer liquefied, the layers covering it were ripped apart, and a sand boil erupted onto the ground surface.As shown in Fig. 1, the old graves from the Jomon Period were ripped apart while the new graves were dug into the sand boils. The ripped graves were built before the earthquake, while graves on top of the sand boil were built after the earthquake. There were two kinds of graves found at the site: dokoubo (hole grave) where bodies were buried in holes dug into the ground, and dokikanbo (earthenware coffin grave) where the bodies were placed in coffin jars and buried. The graves were dated using the earthenware buried with the body and the coffin jars. Based on these informations, the earthquake occurred in the Late Jomon Period or Shigasato “a” Period according to archaeological sequencing, or about 3,000 years ago[3].I found great interest in the fact that the traces of earthquake was found in an archaeological excavation and that I was able to identify an earthquake occurrence that was not recorded in writing, and decided to look for earthquake traces in other archaeological sites. When I made an inquiry to the Kyoto Prefecture Research Center for Archaeological Properties, I obtained information that similar formations were found at the Kizugawa Riverbed Site in Yawata City, Kyoto, where excavations were being carried out at the time.Large-scale liquefaction had occurred at the Kizugawa Riverbed Site, and cracks (sand vein) of about 1 m width crisscrossed the ground surface (Fig. 2). The sand boil that erupted from the cracks tore the layers of Nanbokucho and Muromachi periods, but was covered by the Edo Period layer. Therefore, these were traces of an earthquake that occurred at the end of the 16th Century.In fact, there are many written records of Keicho Fushimi Earthquake that occurred on September 5, 1596 (5th year of Bunroku, leap month July 13 of 1st year of Keicho) and caused great damages to Kyoto. It is written that the houses were leveled in the villages of Yawata where the Kizugawa Riverbed Site is located. Hence, I was able to find physical evidence of a recorded earthquake at the Kizugawa Riverbed Site[4].Japan was enjoying steady economic growth in 1986, and transportation network and residences were constructed actively. The cultural properties buried underground would be destroyed due to these developments, and many archaeological surveys were conducted prior to construction. In the course of this process, archaeological materials were accumulated, sequencing of artifacts such as pottery progressed, and it 01 m12345TrenchFig. 1 Traces of earthquake found at the Kitoge Nishikaido Site (information added to Reference[3]).1. Modern stake marks. 2. Dokoubo (hole-shaped grave). 3. Dokikanbo (coffin jar grave). 4. Remains from the Yayoi Period. 5. Sand vein.The area marked “trench” was excavated to reveal that this was a trace of liquefaction.Fig. 2 Traces of liquefaction at the Kizugawa Riverbed Site.The white line that stretches diagonally is the sand vein (excavation of Kyoto Prefecture Research Center for Archaeological Properties, photography by Sangawa).

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