Undersea interplate coupling off the western Japan revealed by GPS-A observation

Abstract:

The recurring devastating earthquakes in the Nankai and Sagami Troughs subduction zone between the Philippine Sea plate and the continental plates (Eurasian and North American plates) have a potential to cause an extremely dangerous natural disaster in not-so-distant future. Many studies assumed the interplate coupling ratios using the onshore geodetic data only. However, the offshore region that has a potential to derive a devastating tsunami cannot be resolved enough due to the observation network that is biased to land area.

The Hydrographic and Oceanographic Department of Japan (JHOD) have constructed the seafloor geodetic observation network on the seafloor surface using a GPS-acoustic combination technique (GPS-A) [e.g., Sato et al., 2011, Science] and observed seafloor crustal movements directly above the Nankai and Sagami Troughs subduction zones. We have set 17 sites off the western Japan to lead an undersea interplate coupling distribution.

The observations around the Sagami Trough are consistent with the block motions presented by Nishimura [2011, Tectonophysics]. On the other hand, our seafloor geodetic observation network along the Nankai Trough revealed heterogeneity and complexity interplate coupling off the western Japan. The fast displacement rates observed in the eastern area indicate a strong interplate coupling around not only the future Tokai earthquake source region but also the Paleo-Zenisu ridge. Our observations in the westernmost area also suggest that the area off the Cape Ashizuri has a high-coupling ratio. The slow displacement rate observed in the shallower area than the 1946 Nankaido earthquake source region off the Cape Muroto-zaki reflects a weakening interplate coupling probably due to a subducting seamount.

The observation in the westernmost area differs depending on the depth. The north deep area below Bungo-channel has a high-coupling ratio. The south shallow area has a low-coupling ratio. In these deep and shallow areas, VLFEs, long-term slow slip events, and tremors are activated broadly [e.g., Hirose et al., 2010, Science; Yamashita et al., 2015, Science]. A difference of interplate coupling observed by GPS-A might be related with the activation of these slow events spatially.