Aseismic deep subduction of the Philippine Sea plate and slab window Zhouchuan Huang a,⇑ , Dapeng Zhao a,⇑ , Akira Hasegawa a , Norihito Umino a , Jung-Ho Park b , Ik-Bum Kang b a Department of Geophysics, Tohoku University, Sendai 980-8578, Japan b Earthquake Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea article info Article history: Received 6 March 2013 Received in revised form 24 June 2013 Accepted 4 July 2013 Available online 16 July 2013 Keywords: Subduction zones Pacific plate Philippine Sea plate Slab window Mantle wedge Seismic tomography Korean Peninsula Japan Islands abstract We have made great efforts to collect and combine a large number of high-quality data from local earth- quakes and teleseismic events recorded by the dense seismic networks in both South Korea and West Japan. This is the first time that a large number of Korean and Japanese seismic data sets are analyzed jointly. As a result, a high-resolution 3-D P-wave velocity model down to 700-km depth is determined, which clearly shows that the Philippine Sea (PHS) plate has subducted aseismically down to 460 km depth under the Japan Sea, Tsushima Strait and East China Sea. The aseismic PHS slab is visible in two areas: one is under the Japan Sea off western Honshu, and the other is under East China Sea off western Kyushu. However, the aseismic PHS slab is not visible between the two areas, where a slab window has formed. The slab window is located beneath the center of the present study region where many telese- ismic rays crisscross. Detailed synthetic tests were conducted, which indicate that both the aseismic PHS slab and the slab window are robust features. Using the teleseismic data recorded by the Japanese sta- tions alone, the aseismic PHS slab and the slab window were also revealed (Zhao et al., 2012), though the ray paths in the Japanese data set crisscross less well offshore. The slab window may be caused by the subducted Kyushu-Palau Ridge and Kinan Seamount Chain where the PHS slab may be segmented. Hot mantle upwelling is revealed in the big mantle wedge above the Pacific slab under the present study region, which may have facilitated the formation of the PHS slab window. These novel findings may shed new light on the subduction history of the PHS plate and the dynamic evolution of the Japan subduction zone. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The Philippine Sea (PHS) plate is one of the marginal sea com- plexes in the western Pacific and it started to subduct northwest- wards about 40 Ma ago when the Pacific plate changed its direction of motion from NNW to WNW (Karig, 1971; Seno and Maruyama, 1984). Along the Nankai Trough off Southwest Japan, the PHS plate is composed of several blocks with ages increasing from the east to west, which are the Izu-Bonin arc and back-arc (0–2 Ma), Shikoku Basin (15–30 Ma), Kyushu-Palau Ridge, and Amami Plateau (40–49 Ma) (Hilde and Lee, 1984; Hall et al., 1995). In the present study region (Fig. 1), the PHS plate is ac- tively subducting beneath Southwest Japan and the Ryukyu arc from the Nankai Trough and Ryukyu Trench at a rate of 3–5 cm/ year (e.g., Bird, 2003; DeMets et al., 2010). The Korean Peninsula and western Japan belong to the Eurasian (or Amur) plate. Accord- ing to the recent regional and global tomographic studies (e.g., Hasegawa et al., 2009; Zhao, 2004; Wei et al., 2012; Zhao et al., 2012, 2013), the Pacific plate is subducting beneath the Japan Islands and the PHS plate, and then it becomes stagnant in the mantle transition zone (MTZ). Hence the stagnant Pacific slab ex- ists beneath the present study region (see Fig. 12 on the geometry of the Pacific slab). Analyses of teleseismic data show that the Pacific slab is 85–100 km thick under the Japan Islands and the Ja- pan Sea (e.g., Zhao et al., 1994, 2012; Abdelwahed and Zhao, 2007; Jiang et al., 2008; Jiang and Zhao, 2011). The strong interactions of these lithospheric plates on the Earth’s surface and at depth have lead to complex structures in the crust and upper mantle as well as intensive seismic and volcanic activities in the study region. In addition to many active arc volcanoes on the Japan Islands, a few active intraplate volcanoes exist in and around the Korean Penin- sula, such as Changbai, Jeju and Ullung (Siebert and Simkin, 2002; Kim et al., 2008; Zhao et al., 2009; Wei et al., 2012)(Fig. 1). It is very important to study the detailed three-dimensional (3-D) structure of the crust and upper mantle so as to improve our understanding of seismotectonics, volcanism and geodynamics of the study region. So far the 3-D seismic velocity structure of the crust and upper mantle under the Japan Islands has been studied extensively by using various seismological methods (see Hasegawa et al., 2009 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.07.002 ⇑ Corresponding authors. Tel.: +81 22 225 1950 (D. Zhao). E-mail addresses: zhouchuan.huang@gmail.com (Z. Huang), zhao@aob.gp.tohoku. ac.jp (D. Zhao). Journal of Asian Earth Sciences 75 (2013) 82–94 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes