关键词: 大地切片调查法, 活动断层, 第四纪, 松散沉积物采样法, 日本

Abstract: In order to mitigate the disaster in urban area caused by an earthquake under the city, it is very important to detect and locate the distribution of active faults in urban area, and to keep away from these active faults in future construction. Many techniques and methods for detecting active fault have been developed in the past century. Nevertheless, some methods are not suitable to being applied in urban area. It is necessary, therefore, to develop and apply some other new methods for detecting active fault in urban area. This paper introduces Geo slicer method, a newly developed technique in Japan for detecting active fault. A Geo-slicer is made of steel and has a simple structure composed of a sampling box and its shutter, and very easy to be used in the field. The Geo-slice can be taken to a laboratory for close examination or can be displayed at a meeting or even stored for future re-examinations. Moreover, this technique enables us to carry on high-resolution analyses not only in active fault research but also in many other fields with less expenditure of time, labor and money, such as the researches of liquefaction, landslide and fluviolacustrine or littoral sediments. Three examples show the application of this technique to the studies of active fault, liquefaction caused by earthquake, and littoral sediment. The first example is the Geo-slicer investigation on the Hayward Fault of San Andreas Fault system east of the San Francisco Bay. The object of the investigation is to detect fault trace 3m under ground surface, where investigation by trenching is difficult. As a result, four geo slices revealed fault traces and a firstly discovered paleo earthquake under a 3m deep trench. The second is the geo-slicer investigation of liquefaction caused by the 2000 M 6.6 Totori earthquake in Japan. The geo slices taken in Oone Island of Shimane Prefecture, Japan showed the structure of liquefaction in 3.5m deep infilled sandy soil behind a mole. The third is the geo-slicer investigation of grouted sediment near the mouth of the Columbia River, the United States. Those geo-slices revealed liquefaction relics not only caused by 1700 Cascadia earthquake but also by other older events. Finally, the advantages and shortcomings of this method have been summarized. The advantages include the following five aspects: it may decrease the earthwork to 1/100 of a trench and reserve effectively the geologic information; can be applied to weak strata where trenching is difficult; can be applied to detect active fault in urban area; multi geo-slicer and 3D-geo-slicer can be taken in a narrow area; can be conserved for future re-examinations. The shortcomings are also summarized as several aspects, such as difficult to apply to the gravel composed of big boulders and highly viscous clay sediment, and higher expenditure in developing countries.

Key words: Geo-slicer method, active fault research, Quaternary research, unsolidated sediment sampling method, Japan