Abstract
Submarine geological and geophysical surveys in and around the Kii Strait were carried out by two different methods. A single channel watergun method was executed to clarify the fault structures on the shelf in the northern part of the Kii Strait, and a multichannel airgun method was performed to reveal the basement structures of the shelf slope, the Muroto Trough, the outer ridge, the inner trench slope, the Nankai Trough and the Shikoku Basin. The results show that the single channel watergun system is available for the geophysical survey on the shelf in the open sea and the multichannel airgun system is effective for the investigation of deep sea geological structure.
Many small faults were detected on the shelf off Tanabe city. They are distributed widely in the shelf area, cannot be recognized as strike-slip faults and are not active in the latest Pleistocene and Recent. Upheaval and subsidence axes trending N-S direction and other upheaval axis trending ENE-WSW direction were recognized in the surveyed area. The last axis was generated later than those trending N-S direction. The provident scarps observed in the northern margin of the Muroto Trough and at the submarine canyons' sides have been proved to be caused not by faulting but by submarine landslides. The outline of the geological structure in the southernmost part of the outer zone of southwest Japan across the outer ridge, the Nankai Trough and the Shikoku Basin has been revealed by this survey.
