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    Heterogeneous interplate coupling along the Nankai Trough, Japan, detected by GPS-acoustic seafloor geodetic observation

    Yokota Y, Ishikawa T, Sato M, Watanabe S, Saito H, Ujihara N, Matsumoto Y, Toyama S, Fujita M, Yabuki T, Mochizuki M, Asada A

    Seafloor geodetic observation, Nankai Trough, Interplate coupling, Seamount subduction

    Seafloor velocity vectors are shown by red arrows. Light blue arrows indicate the seafloor geodetic observations by Nagoya University (Tadokoro et al. 2012). The onshore velocity vectors are calculated for the period from March 2006 to February 2011 using GEONET stations (black arrows). Yellow arrows indicate the convergence rate of the Philippine Sea plate under the Eurasian plate (DeMets et al. 1994). The purple and orange regions show the maximum source model of the Nankai Trough great earthquake and the source model of the future Tokai earthquake assumed by the CDMC (2013). The blue and green contour lines are coseismic slip distributions associated with the 1944 Tonankai and 1946 Nankaido earthquakes, respectively (Baba and Cummins 2005).

    The recurring devastating earthquake that occurs in the Nankai Trough subduction zone between the Philippine Sea plate and the Eurasian plate has the potential to cause an extremely dangerous natural disaster in the foreseeable future. Many previous studies have assumed interplate coupling ratios for this region along the trench axis using onshore geodetic data in order to understand this recursive event. However, the offshore region that has the potential to drive a devastating tsunami cannot be resolved sufficiently because the observation network is biased to the land area. Therefore, the Hydrographic and Oceanographic Department of Japan constructed a geodetic observation network on the seafloor along the Nankai Trough using a GPS-acoustic combination technique and has used it to observe seafloor crustal movements directly above the Nankai Trough subduction zone. We have set six seafloor sites and cumulated enough data to determine the displacement rate from 2006 to January 2011. Our seafloor geodetic observations at these sites revealed a heterogeneous interplate coupling that has three particular features. The fast displacement rates observed in the easternmost area indicate strong interplate coupling (> 75 %) around not only the future Tokai earthquake source region, but also the Paleo-Zenisu ridge. The slow displacement rates near the trench axis in the Kumano-nada Sea, a shallow part of the 1944 Tonankai earthquake source region show a lower coupling ratio (50–75 %). The slow displacement rate observed in the area shallower than the 1946 Nankaido earthquake source region off Cape Muroto-zaki reflects weakening interplate coupling (about 50 %) probably due to a subducting seamount. Our observations above the subducting ridge and seamount indicate that the effect of a subducting seamount on an interplate coupling region depends on various conditions such as the geometry of the seamount and the friction parameters on the plate boundary.