琵琶湖西岸における過去の崩壊に関連した湖底地形調査(その2)

Abstract

A sub-bottom acoustic profiling in Lake Biwa and a soil survey with a drilling stickon the coastal zone have been carried out to obtain direct evidences on coastal orunderwater slope failure caused by past large earthquakes in a western coast of LakeBiwa. The results and findings are as follows.From the acoustic profiling, 1) a disturbed layer (Layer D) is found at a depth of1-2 m below the lake-floor spreading onshore in a flat and shallow area of water depthless than 15 m, 2) another disturbed layer is found in sedimentary layers in the lowerpart of underwater slope of water depth 15-50 m, and the layer is presumed to have beenformed as a result of underwater slope failure caused by a past large earthquake, and3) faults or other structures are presumed to exist beneath the steep underwater slope.Next, from the drilling survey, 4) a dark brownish, humic clayey layer (Layer C), underlaid by the recent shore sediments, is found at a depth of 1 m below the meanwater level of the lake in the western part of the surveyed zone; the same layer isdiscovered also at a point of farm area, which is located near to the shore area of thethickest Layer C, and 5) if Layer C is assumed to show a broken farm due to landsubsidence along the coastal zone accompanied by the large earthquake in 1662, it ispossible to interpret that the sediments in Layer D was brought by a slope failure at thetime.Further investigations are needed to check these findings and presumptions moreclearly.