Abstract
Single- and multi-channel seismic reflection analyses allow us to better understand the seismic stratigraphy of the forearc region in the eastern Nankai Trough and provide a crucial clue for the reconstruction of the tectonic framework. The lowest Kodaiba Group is composed of the middle to upper Miocene forearc basin fill. Since deposition of the Kodaiba Group the forearc basin had uplifted and then widely eroded over the study area. The main part of the outer arc high is likely to uplift during the deposition of the late Pliocene to early Pleistocene Atsumi-oki Group. In addition the central uplift zone has started to develop particularly in the accumulation stage of the uppermost of the group. During the unconformity between the underlying Atsumi-oki and the overlying Hamamatsu-oki Groups, numerous minor conjugate normal faults had developed and the tilting of the upper forearc slope toward southeast had been proceeded. Normal faults, on the other hand, to run parallel to the Nankai Trough axis with 70-80º dipping toward north were activated on the shelf to the upper shelf slope area off Hamamatsu. The normal faults have been active since onset of the deposition of the Hamamatsu-oki Group because some displaced the shelf surface floor to be made by the sea level falling in the last glacial maximum. An average of maximum vertical displace component of the normal faults may reach up to several ten cm/1000 years. Considering the time and space, a development of the normal faulting was synchronized with the tilting of the upper forearc slope, probably caused by ridge subduction to progress along the Nankai decollemant fault in the study area.
