Vol.2 No.2 2009

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Research paper : A study of paleoearthquakes at archeological sites (A. Sangawa)−89−Synthesiology - English edition Vol.2 No.2 (2009) were found at the Kawanabe Site in Wakayama City and the Sakafuneishi Site of Nara Prefecture.On the other hand, there are some records of the Tokai earthquake in 1498, but no historical record of the Nankai earthquake exists. However, after 1989, the traces of liquefaction attributed to about the 15th century were found one after the other at the Azono Site, Shimanto City, Kochi Prefecture of Shikoku as well as at sites in Itano-cho, Tokushima Prefecture[9][17], and it can be seen that there was a Nankai earthquake that rocked the entire island of Shikoku.When the earthquake traces of both earthquakes are entered into the timetable along with the dates of earthquakes known from written records, it can be seen that the great earthquakes of the Nankai Trough occur at fairly regular intervals at the same time or in sequence.5.4 Investigating the total picture of the Keicho Fushimi EarthquakeOn the other hand, many traces of inland earthquakes were found around the Osaka Plain. Because the majority of the traces show that the medieval layers were torn but were covered by layers of the Edo period, they are suspected to be due to the Keicho Fushimi Earthquake of 1596, as in the Kizugawa Riverbed Site (Fig. 6). There are several types of earthquake traces. Traces of large-scale liquefaction can be seen in the alluvial lowlands with high groundwater level in southern Kyoto Basin, as in Kizugawa Riverbed Site and Uchisato Haccho Site (Fig. 7). Also, at Sumiyoshi Miyamachi Site in the southern foot of the Rokko Mountain Range, traces of lateral flow in which the ground slid sideways due to liquefaction were found. In the Imashirozuka Tomb of Takatsuki City and the Nishimotomezuka Tomb of Kobe City, although the mounds were deformed due to landslides, from the date of the layer covered by the soil from the collapsed mound, it was known that the trace was of the Keicho Fushimi Earthquake.For this earthquake, the chronicles of temples and shrines and aristocrats’ diaries describe that the tower of the Fushimi Castle collapsed in Kyoto, the temples Toji, Daikakuji, Tenryuji, and Nisonin were destroyed, houses of Osaka and Sakai were damaged, and the fires consumed the fallen buildings in Hyogo (current Kobe). As will be described later, from the trench excavation of active faults, it was found that this earthquake was caused by the Arima-Takatsuki Fault Zone and the active faults of Awajishima.In the case of the Keicho Fushimi Earthquake, the active fault was identified and the damages to castles, temples, and houses were determined from written records. Adding the ground disaster that could be learned from earthquake traces at the archaeological sites, the overall picture of the earthquake can be seen from three perspectives.5.5 New findings on liquefactionSince the Niigata Earthquake of 1964, liquefaction came into the spotlight, as sand boils that pour out on to the surface can be observed by anyone immediately after the earthquake. However, there was lack of knowledge about the original sand layer that supplied the sand boils, as well as for the mechanism of sand boil rising to the ground. Knowledge was limited to speculations based on results of underground boring. However, when liquefaction traces were found at archaeological sites, observation of the cross section of layers could be made by excavating the ground, and the following basic facts that were unknown before were clarified.For example, the depth of the sand layer that supplied sand to the sand boil when liquefaction occurred was only tens of cm to 2 m deep, and it was shallower than originally thought. Also, liquefaction was thought to occur in the sand layer, but many cases were found where the gravel layer containing large proportions of gravel liquefied and rose through the sand vein. Figure 8 shows the trace of liquefaction observed at the Hariehama Site at the bottom of Lake Biwa. Liquefaction occurs in the sand layer containing plenty of Fig. 7 Traces of liquefaction of the Uchisato Haccho Site (excavation of Yawata City Education Board, photography by Sangawa).010 cmSand jetGround surface at time of earthquakeFig. 8 Traces of liquefaction at the Hariehama Site.The liquefaction occurred in the gravel layer, but large gravels did not reach the ground surface.

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