2016年熊本地震により発生した阿蘇カルデラ西部における斜面崩壊の地形・地質的特徴

Abstract

The 2016 Kumamoto Earthquake, which occurred on 16th April 2016 with a moment magnitude of 7.0, induced many landslides in the western part of the Aso caldera. The bedrocks of the survey area consists of lava flows and pyroclastic rocks with a wide range of chemistry from basalt to rhyolite. They are thickly covered by tephra, of which volcanic soil can be classified from color tone; we classified the volcanic soil into brown volcanic soil, black volcanic soil, and blackish brown volcanic soil. One of the most devastating types of landslides induced by this earthquake was of tephra slopes, because they occurred on rather gentle slopes and ran out long distances. The landslides had sliding surfaces in three types of tephra; these were Kusasenrigahama pumice layer 30, 000 years ago (Kpfa), black or blackish brown volcanic soil layer and brown volcanic soil layers that contained felsic lava blocks. These layers that accommodated sliding surfaces had high water content and rich in a clay mineral halloysite, which was formed by the weathering of the buried tephra. Many of the landslides of the tephra layers occurred on slopes where the sliding surface horizon had been undercut beforehand.