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地震 第2輯
Vol. 42 (1989) No. 4 P 405-418



Shallow subsurface structure around Hino River in the south-east shore of Lake Biwa was investigated by means of seismic reflection survey. The target area on the western margin of a seismic quiescent zone between the lake and the Suzuka range includes a belt of steep gradient of Bouguer anomaly which inclines west.
Survey lines were designed, one on the east and another on the west of Hino River, in order to determine the subsurface structure beneath the steep gradient belt of gravity. Time sections were constructed by the Common Depth Point (CDP) method and followed by the frequency-wavenumber (F-K) migration to discuss the subsurface structure in detail.
In the time sections of two survey lines, a horizon “A”, which is smoothly continuous, and another horizon “B”, which is inclined to south-west are recognized. The “A” horizon is considered to be a reflective plane in the Biwako-group, Quaternary deposits, by referring to the deep drilling log available in the neighborhood of the survey line. The “B” horizon is inferred to be an uncomformity boundary between the Biwako-group and the basement rocks because no continuous reflector can be found thereunder. Since the “B” horizon exibits a buried valley in the center of the western survey line and an south-westward inclination through the eastern survey line, this structure may be a remnant channel which used to run SE to NW during the erosive period of this area.
It is noticeable that the “A” horizon shows onlap boundary relation against “B” horizon in the center of the eastern survey line. This suggests that this area turned into subsidence after the erosive period.
No active faults were found above the “B” horizon through this survey. Hence low seismicity around the target area is likely inherent in character.

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