The Losing Stream on a Fan as a Clue to the Discovery of a Concealed Active Fault

Abstract

In recent years, large inland earthquake damages have been reported in areas where active faults have not been certified, and active fault surveys in the plain have become an issue. The new clues are needed because there is a limit to the discovery of active faults that are not accompanied by ground displacement due to only tectonic relief. Even a slight accumulation of flexure, the thickness of the aquifer may change significantly due to vertical displacement due to active faults, and this is considered to be one of the factors that contribute to the formation of losing stream in the fan. In this study, the geophysical surveys of microtremor survey, two-dimensional microtremor array survey, and voltage difference method electrical survey were conducted at the losing stream site of the Koto River Fan in central Kagawa prefecture, and the hydrogeological structure was comprehensively analyzed. As a result, it was found that the large change in aquifer thickness is likely to reflect the flexural structure of a concealed active fault of the Busshozan fault （tentative name）. In this case, it shows that the losing stream in the fan is a clue to discover concealed active faults.