2018年胆振東部地震による降下火砕物の崩壊 : 特に火砕物の風化状況について

Abstract

2018 Eastern Iburi Earthquake induced over 8000 landslides in an area of 400 km2 with the seismic intensities of 7 and 6+. Most of the landslides were of pyroclastic fall deposits and highly mobile. Those deposits were of Ta-d pumice of 9 ka and En-a pumice of 20 ka, and their sliding surfaces were made within specific beds, which were the bottom of Ta-d, reworked Ta-d in its base or reworked En-a beneath Ta-d, or volcanic soil with pumice grains beneath the Ta-d or volcanic soil beneath En-a. The materials that accommodated the sliding surfaces were heavily weathered to be rich in halloysite, which mineral has been found from the sliding surface materials of previous earthquake-induced landslides of pyroclastic fall deposits and is supposed to be very weak to earthquake shaking. The depths of the landslides varied from 2 to 3 m. Most of the beds that slid were undercut at the lower portion of the slopes, because of man-made cutting or convex slope breaks of which origin is in dispute.