Submarine, across-arc normal fault system in the southwest Ryukyu Arc: trigger of the 1771 tsunami hazard?

Ryukyu Islands, an arcuate trench-arc-backarc system on the southeastern margin of the Asian continent, is characterised by N54W subduction of the Philippine Sea Plate at a rate of 6-8 cm/y. The most devastating geo-hazard in this area was the 1771 Yaeyama Earthquake Tsunami that was preceded by a M7.4 earthquake. In spite of many extensive onshore/offshore survey expeditions and numerical modelling studies, the tsunami source area and the seafloor deformation consistent with the observed inundation have not completely been identified so far. Bathymetric and seismic surveys by Hydrographic Department of Japan in 1970's show many submarine "active" normal faults crossing completely the Ryukyu arc. One of these is located off the eastern coast of Ishigaki Island. Since this fault can be the origin of the 1771 tsunami, a multibeam bathymetric survey was carried out to detect its activity, history and for future hazard assessment during the NT05-04 Expedition by R/V NATSUSHIMA in 2005. The survey figured the topographic feature of the whole fault, and identified the locations of its northwestern and southeastern termini, which revealed the total length to be 44km. The fault is composed roughly of five segments, not all of which can slip at once. A reconnaissance survey of three of these segments by the ROV Hyper-Dolphin during the same expedition revealed that the headwall scarp is an outcrop made up of the (Pleistocene) Ryukyu limestone and that the lower scarp and the graben are covered with talus fallen from the headwall. Two of these segments look fresher in fault activity rather than others. A submarine slump observed on the fault may also generate a tsunami of 1- 13m in height at its location (depending on the gradient of the slope underneath the failed mass). The fault, at least some of the segments, might cause the 1771 tsunami (and also past tsunamis with the similar magnitude).