[H-DS07] Landslides and related phenomena

[HDS07-07] Geological features of landslides of tephra induced by earthquakes and rainstorms

*Masahiro Chigira¹ (1.Fukada Geological Institute)

Keywords:Landslides, Tephra, Earthquake, Rain

Landslides of tephra occurred in many countries with different backgrounds, which provide different characteristics for landslides. Young air-fall tephra, which forms the near surface beds, have slid during rainstorms as strong as 100 millimeters per hour in Japan in these decades; they might have been induced by water seepage in the beds above a less permeable beds and the landslide materials were commonly free from halloysite. In Italy, there occurred many landslides induced by much weaker rainstorms less than 200 mm in 3 days. Those beds, however, have never been the materials of sliding surface during earthquake-induced landslides. On the other hand, earthquake-induced landslides of tephra had sliding surfaces in deeper beds, in which halloysite was made by weathering. Our experience says that the formation of halloysite needs time and undrained condition. The water in the ground must be stagnant to react with tephra. It may need a time longer than 10 thousand years. Earthquake-induced landslides of tephra also show that the sliding surface materials were mostly made with association with pumice or scoria beds, which suggests that a hazard map for earthquake-induced landslides of tephra must be firstly based on the distribution map of pumice beds older than approximately 10 thousand years. On the other hand, a hazard map for rain-induced landslides of tephra need to include younger air-fall tephra. Rain-induced landslides might involve weathered Unwelded ignimbrites that include halloysite.