The Relation between Weathering of Pyroclastic Fall Deposits and the Collapses Caused by The 1968 Tokachi-oki Earthquake

Abstract

 The collapses occurred in south eastern part of Aomori prefecture caused by The 1968 Tokachi-oki earthquake. This study mention the geological background of the collapses that investigated a stratum of pyroclastic fall deposits, physical property, clay minerals and coefficient of permeability. In south eastern part of Aomori prefecture, pyroclastic fall deposits of from Towada-Hachinohe tephra distributed. A sliding surface of the collapses had been formed at volcanic ash mixed with pumice that below yellowish volcanic ash in pyroclastic fall deposits. Loss of ignition, contents of 10Å halloysite clay minerals increase and N_C value by portable penetration test decrease from volcanic ash mixed with pumice. The change of these values contributed to the collapses. Because of volcanic ash mixed with pumice that had low coefficient of permeability distributed in the collapses area.