Earthquake-induced deep-seated landslides are prone to occur at gravitationally deformed and loosened slopes. These slopes need to be identified for landslide susceptibility mapping and landslide risk assessment. Surface morphology is an effective factor for finding the slopes, but is not effective for estimating the degree and the area of loosening. Subsurface geophysical investigations are quite useful for determining subsurface geological structures in regions where loosening has occurred. In this study, a method was developed for regionally mapping gravitationally deformed and loosened slopes as earthquake-induced deep-seated landslide susceptible zones by differentiating from sound bedrocks in flexural toppled slopes. Using the helicopter-borne electromagnetic resistivity data, ''average ruggedness of resistivity'' was proposed as an index parameter for differentiating the loosened zones. The estimated loosened zones were compared with observed geomorphological features during field survey, and it is shown that ''average ruggedness of resistivity'' is a useful parameter for regional mapping of earthquake-induced deep-seated landslide susceptible slopes.

Shikoku Island has been threatened by great earthquakes occurred along the Nankai Trough and the Median Tectonic Line. Estimated magnitude of the largest earthquake along the Nankai Trough is estimated 9.0. Estimated magnitude of the Median Tectonic Line Earthquakes in northeastern part of Shikoku is 8.0 or more. The great earthquakes might trigger deep-seated landsides and the huge amount of landslide debris might paralyze transportation and isolate villages in mountainous areas. In order to mitigate the damage, estimating the hazard and preparation is necessary. In this study, index of susceptibility for earthquake-induced deep-seated catastrophic landslides is proposed geologically and geomorphologically. KeywordsResistivity Á Deep-seated catastrophic landslide Á Disaster management 120

Iwate volcano, northeast Japan is an active Quaternary volcano and is comprised of two parts: West-Iwate and East-Iwate. These bodies are underlain by early-middle Pleistocene volcanic rocks. In 1999, fumarolic areas were newly emerged along the ridge between Ubakura and Kurokura Mountains in West-Iwate and Iwate volcano was thought that an eruption was impending in 2000. However, the fumarolic activity has decreased since its peak in July 2001, and the disaster seems to have passed. In late 2000, a helicopter-borne EM and magnetic survey was conducted over Iwate volcano to better understand the subsurface structure of the volcano related to the ongoing volcanic activity. Recently we have conducted three-dimensional (3D) imaging of Iwate volcano to constrain its subsurface structure. Our model indicates that magnetization highs occupy the main edifice of East-Iwate, which reflects the surface and/or subsurface distribution of basaltic lavas. Meanwhile, magnetization lows are dominant inside the summit caldera of West-Iwate except for a magnetic high over the Onashiro lava flow. Magnetization highs are also distributed on the northern and southern slopes of West-Iwate but local magnetization lows lie on the heads of narrow valleys, corresponding to hydrothermal altered areas. These hydrothermal altered areas are also characterized by resistivity lows observed by the Airborne EM survey. Although the imaging improved our understanding of the surface and subsurface distribution of volcanic rocks in Iwate volcano, some limitations exist. No information about the magmas which should have intruded during the recent eruptive crisis was obtained by the imaging. The small magnetic contrast between the intruded magmas and their host rocks is the most probable reason.