Tectonic landform and geological structure surveys in the sea area near Itoigawa.

The theory that North-East Japan is on the North American Plate was suggested by scientists after the Japan Sea Earthquake (M = 7. 7) of May 26th, 1983. According to this theory, the boundary between the North American and Eurasian Plates runs from the eastern border of the Japan Sea, through Fossa Magna in the middle of Japan, to Sagami Bay. The Hydrographic Department of Japan conducted the surveys in order to investigate this theory and to contribute to the earthquake prediction program. The surveys were conducted in two periods. The first period was from 1985 to 1987, and the second was the next two years.In the first period, the area off Itoigawa was surveyed and reported by Asada et al. (1989) and Kato et al. (1990) . The land area was surveyed by other government offices. If we survey the middle area, we should be able to determine the features of the entire lineation possibly considered to be plate boundary. In the second period, we used the narrow-multi beam swath survey system (Hydrochart II) to investigate the bottom topography, and the seismic profiler system by electrical discharge to investigate the geological structure in the sea area near Itoigawa. From the data gathered by these surveys, we have determined some geological features in the area. The results are as follows: 1 . In the sea area near Itoigawa, there are some active folds tending from northeast to southwest. They intimate northwest-southeast pressure in the crust. 2 . There are some thrust type faults on the north-south line from the Toyama Trough to Itoigawa. They run obliquely on the folds. The line is not necessarily the plate boundary because of irregularities along the faults line. The lowered or depressed side of the fault changes along the fault line. However it must be a geological "weak line’.