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

Yusuke Yokota,Shin-Ichi Toyama,Shun-Ichi Watanabe,Masashi Mochizuki,Naoto Ujihara,Hiroaki Saito,Masayuki Fujita,Yoshihiro Matsumoto,Tetsuichiro Yabuki,Mariko Sato,Tadashi Ishikawa,Akira Asada

doi:10.1186/s40645-015-0040-y

Yusuke Yokota, Shin-Ichi Toyama + Show 10 more

Open Access

https://doi.org/10.1186/s40645-015-0040-y

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Abstract

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% to 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.

Highlights

Nankai trough subduction zone In southwestern Japan, the oceanic Philippine Sea plate is subducting beneath the continental Eurasian plate northwestward along the Nankai Trough
They pointed out the importance of seafloor geodetic observations for revealing the offshore region along the Nankai Trough
Observation results Our seafloor sites along the Nankai Trough are located on the offshore region that is predicted by the Central Disaster Management Council of the Japanese government (CDMC) to undergo the great Nankai Trough earthquake (CDMC 2013) (Figure 1)

Summary

Introduction

Nankai trough subduction zone In southwestern Japan, the oceanic Philippine Sea plate is subducting beneath the continental Eurasian plate northwestward along the Nankai Trough In this region, interplate magnitude-8-class earthquakes have been documented with an average recurrence time of about 100 years (e.g., Ando 1975) due to this plate motion. Yokota et al Progress in Earth and Planetary Science (2015) 2:10 data do not have the power to resolve the offshore region as indicated in Yoshioka and Matsuoka (2013) (Figure 1).

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