A seismic refraction experiment was conducted in the northern part of Lake Biwa, Kinki District, Central Japan, in October, 1978. The purpose of the experiment was to know the thickness of the sedimentary layer beneath Lake Biwa, preparing for the boring project on paleolimnological study.
Four shots were detonated on land and fifteen seismometers were set at the bottom of the lake, along a profile of 34km long. By the analyses of the clear P wave onsets, the underground structure of Lake Biwa was obtained. The P wave velocity of the sedimentary layer varies from 1.5 to 2.0km/s, and it is assumed to be 1.8km/s in this study. The thickness of the sedimentary layer is 0.8km in the western part of the lake, decreasing towards northeast part with an inclination of 6-7 degrees. The basement layer has a velocity gradient, which has the P wave velocity of 5.0-5.2km/s near the upper boundary and the velocity gradient of 0.25km/s per 1km. Comparing this velocity increace with that from laboratory experiment on rocks, this velocity gradient in the basement layer is considered to show the in situ pressure effect.
