Did the Submarine, Across-Arc Normal Fault System in the Southwest Ryukyu Arc Trigger the 1771 Tsunami? Field Evidence from Multibeam Survey and In-Situ Observation

Recent numerical simulation of tsunami propagation proposed a new hypothesis about the origin of the 1771 tsunami that devastated the southwest Ryukyu district of Japan; a slip of the East Ishigaki Fault, a 44kmlong fault lying 50km off the east coast of Ishigaki Island, might be the cause of the 1771 tsunami. The present study is to test this hypothesis through visual observation by means of the precise seafloor image collected by the Hyper-Dolphin remotely operated vehicle. The hypothesis may be proved if definite evidence of a slip along the whole fault is obtained. Investigating the fault was accomplished by a reconnaissance survey at three representing fault segments: southern, central and northern. The result of the survey at the southern segment shows that the main fault scarp is covered by many large boulders. On the escarpment, 6m sections with a gradient of almost 90 degrees were observed. The result of the survey at the central segment shows similar characteristics as that at the southern segment. The northern segment was characterized by wide exposure of limestone outcrop with many cracks and fissures on the outcrop which represents nascent faulting. These facts suggest the northward propagation of the faulting along the main scarp. The result demonstrates that the amount of displacement at the fault segments is not uniform. This does not support the assumption taken into the numerical simulation; thus, it is unlikely that the slip at the fault generated the 1771 tsunami, even though simultaneous rupture at multiple fault segments are taken into account.