2011年東北地方太平洋沖地震による茨城県神栖市，鹿嶋市の液状化発生域と砂利採取場分布の変遷との関係

抄録

 Extensive soil liquefaction and liquefaction-induced damage caused by the 2011 off the Pacific coast of Tohoku Earthquake was observed in Kamisu and Kashima Cities, Ibaraki Prefecture. The distribution of liquefied areas is investigated based on a field survey and interpretation of Google Earth images, and the distribution of past sand-gravel pits is revealed using time-series geospatial information (maps and aerial photos). A large number of sand-gravel pits were developed and refilled in this area from the latter half of the 1960s. Because the period from excavation to refill of many sand-gravel pits was only a few years, the locations of sand-gravel pits changed depending on the period, and sand-gravel pits were developed at numerous sites. GIS-based overlay analyses reveal that much of the liquefaction occurred in refilled sand-gravel pits, and subsequently in fill-up lands, former river channels, and ponds. In particular, refilled sand-gravel pits are highly susceptible to liquefaction. The appearance ratio of liquefaction in refilled sand-gravel pits indicates the same or slightly higher value than at reclaimed land and former river channels where liquefaction occurred during previous earthquakes. It seems that the high ground water table and the existence of thick (5-15 m) man-made sand fills induced liquefaction in these refilled sand-gravel pits. Because the excavation periods of individual sand-gravel pit were short (a few years) and the area was smaller than other land conditions (sand dune, flood plain, and former pond), it is difficult to detect past sand-gravel pits using only a single year of geospatial information, and the existence of refilled sand-gravel pits does not represent the land condition map and geomorphological map. However, many sand-gravel pits and iron mines on the alluvial plains of Japan were developed and refilled, and liquefaction in refilled sand-gravel pits was also observed at many other sites in the Kanto and Tohoku regions during the 2011 off the Pacific coast of Tohoku Earthquake. Therefore, numerous refilled sand-gravel pits with a high potential for liquefaction may not have been detected on many alluvial plains.