GRAVITY SURVEY AROUND LAKE BIWA, SOUTHWEST JAPAN

A precise gravity survey was carried out around Lake Biwa in Shiga Prefecture, southwest Japan. The gravity data reported so far are also compiled in this work. The gravity stations were situated roughly in 1 or 2 km mesh on the land and in 5 or 10 km mesh at the lake bottom. The large-scale regional trend decreasing from south to north is observed in the southern part of Lake Biwa. This trend terminates around the latitude of 35°10’N. As the residual distribution of the gravity anomalies in the southern part of the Ohmi basin is obtained after subtracting the N-S regional trend, small-scale negative anomaly centers are observed and this distribution gives information about the sedimentary basins in which the Kobiwako sediments were deposited. Besides this trend, large negative anomalies are observed in the center of the northern Lake Biwa basin. These negative anomalies cannot be explained by the distribution of thick soft sediments in the lake. Excess negative anomalies can be explained if we assume the depression of the Conrad surface or the existence of low-density materials caused by faulting beneath the northern part of the Ohmi basin. © 1990, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.