Submarine active normal faults completely crossing the southwest Ryukyu Arc

Abstract Several identified ‘active faults’ located in the southwest Ryukyu area may be an origin of M5-class earthquakes taking place frequently every year, sometimes of M7-class earthquakes which cause a severe geohazard in this area. ‘The East Ishigaki Fault’, one of the ‘officially identified’ active faults, was studied precisely by multibeam echo sounding and the in-situ ROV observation. The survey expedition figured the topographic feature of the fault clearly and more precisely than that mapped by the Hydrographic Department of Japan. The survey also identified the locations of northwestern and southeastern termini of the fault, which revealed the total length to be 44 km: corresponding to a M7.4 earthquake. The new topographic map shows that the fault is composed of four or five segments with maximum vertical offset of ∼ 80 m in the southernmost segment; not all of the segment can slip at once. A reconnaissance survey of three of these segments by the ROV Hyper-Dolphin dives during the same expedition revealed that the headwall scarp is an outcrop made up of the Ryukyu Limestone of the Middle Pleistocene and that the lower scarp and the trough are covered with mostly talus apparently from the headwall. At least two of these segments look fresher in fault activity rather than the others, though all the observations show very recent activities along the fault. Based on these results, the East Ishigaki Fault, at least some of the segments, might cause the 1771 tsunami and also future tsunamis with the similar magnitude.