Vol.2 No.2 2009

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Research paper : A study of paleoearthquakes at archeological sites (A. Sangawa)−87−Synthesiology - English edition Vol.2 No.2 (2009) 7th century and the lowest layer covering the sand boil (after the earthquake) is of the 8th century, it can be known that this sand boil is a trace of a major earthquake that occurred from the 7th to 8th centuries.In Japan, there are written records for the past thousand or more years, and there are many references to damages by earthquakes. After the Noubi Earthquake of 1891, the Imperial Earthquake Investigation Committee was created, and the collection of earthquake records was started as one of its activities. This work continues to the present[10]-[12], and interdisciplinary research on historical earthquake materials are done by the Society of Historical Earthquake Studies. Since many of the disaster records include the time and date of the earthquakes, the time at which the earthquake traces were left can be obtained by comparing the written records against the traces at the sites.When the earthquake traces are confirmed, the records of temples and shrines and the diaries of aristocrats which mention the earthquakes are also confirmed. In the periods before the Edo Period when there is markedly less number of written records, great earthquakes may not have been documented, but the gaps in history can be filled through earthquake traces at the archaeological sites. Also, before the Kofun Period where there is no written record, the earthquake traces lead to discovery of earthquakes.By comparing the records of historical earthquakes, the accuracy of dates of earthquake traces is increased. In turn, presence of earthquake traces raise the reliability of historical earthquakes in written records, and the history of earthquakes can be traced to periods without written records.5 Outline of research resultsMajority of the archaeological excavation in Japan are conducted due to development, and therefore, where and what kind of earthquake traces are found is dominated by chance. Progress of research depends on the discovered traces. I shall describe some of the results obtained so far[13]-[16].5.1 Earthquakes in periods without written recordsAs an example where an earthquake in periods without written records was found, I shall describe the region around Lake Biwa. As mentioned before, earthquake traces of the Jomon Period was found at the Kitoge Nishikaido Site. After this find, the Shiga Prefecture Cultural Properties Protection Association conducted an excavation of the Hariehama Site, which is a lake-bottom site, 250 m off the coast of Shinasahi-cho, Takashima-gun, located in the northwestern area of Lake Biwa. When the lake bottom was dug for about 1 m, the ground on which the people of Middle Yayoi Period lived was revealed, and remains of furrows, agricultural tools, and fallen willow trees were found. This ground was covered by a sand boil that poured out from the underground sand layer. It is thought that the land on the lakeshore submerged due to an earthquake, and liquefaction occurred due to severe seismic tremors. In the alluvial lowlands around Lake Biwa, traces of liquefaction thought to have occurred in the same period were found at several sites (Fig. 4), and it is highly likely that a great earthquake during the middle of the Yayoi Period caused part of the lakeshore to submerge as the area around Lake Biwa was shaken severely.5.2 Earthquakes described in the NihonshokiThe word “jishin (earthquake)” appears for the first time in the Nihonshoki (Chronicles of Japan). In the “7th Year of the Reign of Emperor Tenmu (679 AD),” there is a specific description of the damages caused by the Tsukushi Earthquake: “The earth was torn and the width of the rift was about 2 jo (about 6 m) and the length more than 3,000 jo (about 10 km), and many houses fell in all villages.” However, the Nihonshoki contains several alterations of historical facts and imitations of Chinese history, and verification was necessary for the reference to the Tsukushi Earthquake.Since the birth of earthquake archaeology in 1988, traces of Sand layer7th Century8th CenturySand jetGround surfaceGround surfaceFig. 3 Schematic diagram of liquefaction trace.The yellow part in the diagram is the sand.H010 kmHira Moutain RangeLake BiwaOhtsuTakashimaHikoneFig. 4 Traces of earthquakes around Lake Biwa.The red lines represent active faults (the hatching sides are relatively submerged, while the arrows show the direction of lateral shift). Green dots represent the sites from which medieval to modern (Edo period) earthquake traces were found. Red are sites from which earthquake traces of Yayoi Period were found. Brown is the Kitoge Nishikaido Site where the Jomon Period earthquake trace was found. H is Hariehama Site.

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