The Median Tectonic Line (MTL) is one of the most significant tectonic lines in Japan, spreading up to 1,000 km through the southwest part of Japan. Especially MTL stretching from the western Shikoku Island to the western Kii Peninsula includes many faults active in the late Quaternary. The marine seismic reflection survey using Uniboom and Water gun as energy sources has been carried out in Tomogashima strait, where is a part of the MTL spreading between the Shikoku Island and the Kii Peninsula. The survey revealed the characteristics of faults in this area, which are similar to the fault distribution pattern pointed by the previous study (Maritime Safety Agency, 1978). These faults, however, contain different sense of displacement and also seem to have different activity age, namely some show the deformation of the Holocene layer, while others are limited in the estimated middle Pleistocene layer in their activities. It is estimated that MTL in this area is not the single continuous active fault, but is the combination of faults showing different type in its activity and age.
For the detection of plate motions and deformations of oceanic crust, one approach is to construct the seabottom geodetic stations(SGS). In this paper, we conducted a pilot field experiment for the assessment of the possibility of the precise positioning of SGS. The method of positioning is the combination of kinematic GPS positioning of a boat and the acoustic ranging from the boat to the sea bottom. The depth of SGS is about 100 m, and the baseline length from the reference point of kinematic GPS to the boat is about 1.5 km. The results of the data processing show us the possibility of the SGS positioning with the precision of 10 cm.