Newly Developed Method of Predicting Slope Collapse Places Triggered by Faults Combined with γ-Ray and Magnetic Susceptibility Survey

Abstract

After experiences of the east Japan huge earthquake catastrophe, prediction of disasters of earthquakes and/or heavy rainfalls are requested more and more for safety and security life for residents. Many slope collapses were induced by existence of fault in top area of slope zones from geological and topographical surveys. To find the places of fault from field survey are very difficult before disaster, because almost surfaces are covered by sediments, plants and trees. The easy and simple method to determine the size and direction of faults were developed by authors combined with γ-ray and magnetic susceptibility survey. The method is applied on mountain ridges without thick sediments or weathered layers to remove a shielding effect of γ-ray divergence from faults. Checking intervals points of γ-ray intensities on the survey line can be narrowed to 10 cm. From two survey lines the width and direction of the faults can be determined. Case studies of slope failed places induced by faults by the method are introduced from (1) large scale slope failure by extrapolate fault existence in near mountain area (2) middle scale slope failure 1 year after confirmed fault in small scale of surface failure and (3) slope failure along fault predicted by the proposed method. From those case studies to find slope-collapse places, the proposed method is applicable not only for heavy rainfalls, but also for earthquakes.