Is the Ryukyu subduction zone in Japan coupled or decoupled? —The necessity of seafloor crustal deformation observation

Abstract

The 2004 Sumatra-Andaman earthquake of M w 9.3 occurred in a region where a giant earthquake seemed unlikely from the point of view of tectonics. This clearly implies that our current understanding of strain accumulation processes of large earthquakes at subduction zones needs to be reexamined. The Ryukyu subduction zone is one such zone since no large earthquake has been anticipated there for reasons similar those pertaining to the Sumatra-Andaman arc. Based on our analysis of historical earthquakes, plate motion, back-arc spreading, and GPS observation along the Ryukyu trench, we highly recommend monitoring seafloor crustal deformation along this trench to clarify whether a large earthquake (M w > 8) could potentially occur there in the future.

Holocene Antarctic melting and lithospheric uplift history of the southern Okinawa trough inferred from mid- to late-Holocene sea level in Iriomote Island, Ryukyu, Japan

“…Global positioning system (GPS) measurements indicate northward movement of the PSP at up to 8 cm/year (Gripp and Gordon, 2002). The status of plate coupling (strong or weak) in the region is not well understood, which has implications for the likelihood of future large thrustrelated earthquakes (Mw > 8) (Kao et al, 1998;Wei and Seno, 1998;Ando et al, 2009). In 1771 AD, a tsunami in the southern Ryukyus caused approximately 12,000 casualties, but no other devastating tsunami of similar size are recorded in historical documents (Watanabe, 1985).…”

Could a Sumatra-like megathrust earthquake occur in the south Ryukyu subduction zone?

“…In April 1771, a subduction earthquake generated a very large tsunami that struck the south Ryukyu Islands and killed approximately 12,000 people (Ando et al 2009;Matsumoto et al 2009;Nakamura 2006Nakamura , 2009. Reef boulders of building size were transported by the tsunami to beaches along the east and southeast coasts of Ishigaki Island (Goto et al 2010), which suggests that the source of the tsunami was located east and southeast of Ishigaki Island (Figure 2d).…”

“…This westward rupture movement could be a possible tsunami source similar to the 2004 Sumatra earthquake. Moreover, based on seismic profile interpretations, Hsu et al (2013) demonstrated the presence of a splay fault system within the trench-parallel high gravity anomaly area of the Ryukyu subduction zone, where a highly resistant subduction due to the integration of significantly developed fracture zones or strong plate coupling is expected (Ando et al 2009). Similar splay faults have been reported along the Sumatra subduction zone (Sibuet et al 2007).…”

High-density surveys conducted to reveal active deformations of the upper forearc slope along the Ryukyu Trench, western Pacific, Japan

“…The seriousness of these events underscores the need for further detailed investigations of the active geological structures in and around the source areas of these earthquakes and tsunamis, particularly at the plate convergent margins. The Ryukyu subduction zone is one such region that requires further study (Ando et al 2009) because of the numerous tsunami boulders that can be found widely scattered along the coasts of the southern Ryukyu Arc (e.g., Araoka et al 2013).…”