Paleo-seismites in the Nara-Heian period in the central part of Echigo Plain : Liquefactions of historical earthquake in the Niigata Prefecture,Japan : Part2

Recently, several paleo-earthquake induced liquefaction structures from the Nara-Heian period (approx. 9th century) have been discovered in the Echigo Plain. During this period, a wake of concentrated large-scale earthquakes may have occurred in this area. In this report, liquefaction and rupture structures confirmed at the Yoshida and Tsubame archaeological sites in the central area of the plain are described. Liquefaction structures dated to be approx. 7-8th century have been confirmed at Kosuwamae A site, approx. after 7-8th century at Kosuwamae B and Sankakuda sites and approx. 9th century at Ohashi and Kitashowaki sites. At the Kosuwamae B site, liquefaction occurred at two different periods and at the Ohashi site a 9th century liquefaction can be confirmed with two stages of sand eruption indicating the main earthquake followed by the aftershock. At the Kitasho-waki site, there is a 7-8m wide rupture zone with several sand dykes where the bedding completely destroyed. It is thought that an earthquake fault must be near by or possibly a massive tremor must have occurred. Up to now, several 9th century liquefaction structures have been discovered in the northern and southern areas of the Echigo Plain. With new discoveries in the central area, there seem to be a possibility that they may be distributed throughout the plain