Abstract
Lake Biwa, the largest and oldest lake in Japan, has existed in the Ohmi Basin since 5 Ma B. P.. (lower Pliocene time). Many active faults are densely distributed in and around this lake, and the destructive earthquakes in 1662, 1819 and 1909 were generated from these faults. It is inferred that the development of the basin of Lake Biwa has been closely connected with tectonic movement.
The purposes of this paper are to reveal the distribution and characteristics of sublacustrine active faults and to explain the relationship between the development of the lake basin and the active tectonics of the bottom of Lake Biwa, on the basis of the interpretation of Uniboom records, air gun records and bathymetric charts of the bottom of the lake.
The results obtained are as follows:
(1) Western, Southern and Eastern sublacustrine faults extend along the margins of North and Central basins. The froms of the lake basins are determined by these faults. They are reverse faults, and are conjugate sets controlled by E-W compressional stress in the upper crust. North and Central basins, therefore, are regarded as ramp valleys that bounded by reverse faults.
(2) Western sublacustrine fault extends about 45 km trending N-S, and the fault planes dip westward. This has Class A activity, and the fault has been formed as a boundary on which Lake Biwa Block underthrusts beneath the Hira and Tamba Mountains Block.
(3) Central Flexure and Drilling Point faults are situated in the central part of the lake bottom. The bulge dividing North basin from Central basin is determined its eastern margin by Central Flexure. The mean rates of flexuring and faulting have increased many times since 0.5 Ma B. P.. Judging from the acceleration of faulting not only in this lake but in Tamba Highland and the central part of the Kinki district, it could be inferred that the rate of E-W horizontal stress in the upper crust has increased during the last 0.5 Ma B. P..
(4) Sublacustrine topography and deposition of the lake bottom have been strongly affected by westward tilting that is known as Ohmi tilting movement. It could be inferred that the tilting move-ment started at 1 Ma B. P. and that the rate has accelerated during the last 0.4 Ma B. P..
(5) Based on the air gun records and geologic data of the Kobiwako Group (Plio-Pleistocene series), the development of the present lake basin is divided into three stages: (1) birth stage (almost 2 Ma B. P.), (2) shallow lake stage (0.41.8 Ma B. P.), and (3) deep lake stage (0.4 Ma B. P. to present).
