Timing of Large-scale Ridge-moving Landslides around Aigawa, Northern Nagano Prefecture, Central Japan, and Their Tectonic Implications

Abstract

This study documents the nature of large-scale ridge-moving landslides around Aigawa, in the northern part of Nagano Prefecture, central Japan, to determine the history of the northern segment of the active fault system of the Itoigawa-Shizuoka Tectonic Line. Blocky fragments of the Omachi APm Tephra Beds deformed by landslide movement were observed in a small depression of a landslide mound underlain by the well-fractured Omine Welded Tuff, and overlain by the mantlebedding layer of the Tateyama-D Tephra. This stratigraphy and the eruption ages of these tephras indicate that the large-scale ridge-moving landslides occurred within the range 0.30-0.40 to 0.12Ma. The timing of the large-scale ridge-moving landslides suggests that the northern segment of the active fault system of the Itoigawa-Shizuoka Tectonic Line was tectonically active then, and this would account for the relative height of the fault scarp which generated the landslides.