A mega-splay fault system and tsunami hazard in the southern Ryukyu subduction zone

In April 1771, a subduction earthquake generated a great tsunami that struck the south Ryukyu islands and killed similar to 12,000 people, whereas its mechanism is still enigmatic (Nakata and Kawana, 1995; Nakamura, 2006; Matsumoto et al., 2009). In this paper, we show its probable source on a mega-splay fault system existing along the southern Ryukyu forearc. Analyses of deep multi-channel seismic reflection profiles indicate that the mega-splay fault system is rising from the summit of a similar to 1 km high ridge situated at a similar to 5 degrees landward dipping plate interface. An outer ridge marks the seafloor outcrop of the splay fault system and separates the landward inner wedge and the oceanward outer wedge. The inner wedge is uplifting and exhibits widespread normal faulting while the outer wedge shows folded structures. The mega-splay fault system is parallel to the Ryukyu Trench east of 125.5 degrees E and is estimated to be similar to 450 km long. The origin of this south Ryukyu mega-splay fault system is ascribed to a resistant subduction of the elevated transverse ridges associated with the subducting portion of the trench-parallel Luzon-Okinawa Fracture Zone. In contrast, no similar splay fault is found west of 125.5 degrees E where the oblique subduction has produced large shear zones along the south Ryukyu forearc. We infer that a thrust earthquake linked to the mega-splay fault system is responsible for the south Ryukyu tsunami. However, another possible scenario of generating a large tsunami affecting the south Ryukyu islands is that the subducted ridge in the western end of the mega-splay fault system nucleated a large earthquake and simultaneously triggered the similar to 100 km long E-W trending strike-slip fault west of 125.5 degrees E and induced a southward-dipping tsunami-genic subsidence. In any case, after a quiescence of similar to 241 yr, a large earthquake and tsunami is anticipated in the south Ryukyu forearc in the near future. (C) 2012 Elsevier B.V. All rights reserved.