JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B10, 2470, doi:10.1029/2003JB002441, 2003 Interseismic deformation at the Nankai subduction zone and the Median Tectonic Line, southwest Japan Yosuke Aoki and Christopher H. Scholz1 Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA Received 10 February 2003; revised 9 June 2003; accepted 7 July 2003; published 11 October 2003. [1] GPS velocities in the vicinity of the Nankai Trough, southwest Japan, were inverted into three components: interplate loading due to Nankai Trough subduction, deformation associated with deep slip on the Median Tectonic Line (MTL), and the residual rigid plate motion. The results show a gradual decrease of interplate seismic coupling between 25 and 40 km depth on the Nankai Trough interface. This is consistent with the deep slip model, in which the fault extends as a ductile shear zone, with a depth-variable interplate strain accumulation rate. The top 20 km of the interface may not be fully coupled, although the resolution is poor there. The low coupling at shallow depths is consistent with the accretionary prism detected by seismic surveys. The MTL slip rate is estimated to be between 0.00 and 5.50 mm/yr if we assume a vertical fault and between 0.00 and 3.88 mm/yr if we assume a north dipping fault. Combining our results with a geological estimate (4–9 mm/yr) and a geodetic estimate with a denser network (5 mm/yr) suggests that the MTL slip rate may be near the upper bound of our geodetic estimate, that is, 4–5 mm/yr. The rigid plate motion with respect to the stable Eurasian craton was estimated to be very small, indicating that southwestern Japan is on the Eurasian plate rather than a separate plate. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 1243 Geodesy and Gravity: Space geodetic surveys; 8158 Tectonophysics: Plate motions—present and recent (3040); 8159 Tectonophysics: Rheology—crust and lithosphere; KEYWORDS: southwest Japan, Nankai Trough, Median Tectonic Line, brittle-plastic transition zone, three-dimensional deformation Citation: Aoki, Y., and C. H. Scholz, Interseismic deformation at the Nankai subduction zone and the Median Tectonic Line, southwest Japan, J. Geophys. Res., 108(B10), 2470, doi:10.1029/2003JB002441, 2003. 1. Introduction strains by differentiation of velocities avoids this problem [Mazzotti et al., 2000], but the spatial resolution of the [2] Interseismic crustal deformation in southwest Japan is interplate coupling is then poor because the spatial differ- dominated by the interplate coupling between the Philippine entiation of displacement data results in the introduction of Sea Plate (PHS) and the overriding southwest Japan (SWJ) additional noise. with a relative velocity of 50 mm/yr [Seno et al., 1993] [4] These previous studies focused on seismic potential (Figure 1). Accumulated stress is released as periodic M8 and did not discuss much the details of the strain accumu- thrust earthquakes, such as the 1946 Nankaido, 1854 Ansei, lation at the subduction zone. The Nankaido region is an and 1707 Genroku earthquakes [Ando, 1975]. ideal place to explore this because of the dense geodetic [3] The recent development of a continuous Global network, the proximity of the trench to the geodetic surveys, Positioning System (GPS) network in the Japanese islands and a shallow-dipping subduction interface, which allow allows us to obtain the spatial variation of interplate better resolution of the depth variation of the interplate coupling between the PHS and SWJ [Ito et al.,1999; coupling than a vertical fault such as the San Andreas fault. Mazzotti et al., 2000; Miyazaki and Heki, 2001]. Aoki and Here we will discuss the interplate coupling with emphasis Scholz [2003] pointed out, however, that horizontal veloc- on the mechanism of interseismic strain accumulation at a ities contain both rigid plate motion and the interplate fault, using three-dimensional GPS data. coupling effect, so that rigid plate motion could be mapped [5] About 230 km north of the Nankai Trough, there into the interplate coupling effect. Employing volumetric exists the contact of a pair of metamorphic belts separated by a dextral fault called the Median Tectonic Line (MTL; 1 Figure 1), south of which is the low T/P (temperature-to- Also at Department of Earth and Environmental Sciences, Columbia pressure ratio) Sanbagawa belt and north of which is the University, New York, USA. high T/P Ryoke belt [Scholz, 1980]. Geological and trilat- Copyright 2003 by the American Geophysical Union. eration data show that the MTL slip rate is 4–9 mm/yr 0148-0227/03/2003JB002441$09.00 [Tsutsumi and Okada, 1996] and 5 ± 3 mm/yr [Hashimoto ETG 5 - 1 ETG 5 - 2 AOKI AND SCHOLZ: DEFORMATION IN SW JAPAN and Jackson, 1993], respectively. Because of the lack of 130 135 140 145 historical large earthquakes for at least past 400 years [Tsutsumi and Okada, 1996], it is important for seismic 45 (a) 45 hazard assessment to estimate the slip rate of the MTL precisely. This has been difficult, however, because defor- mation due to the MTL is much smaller than the interplate 40 NA 40 EUR coupling effect at the Nankai Trough, so that the deforma- tion due to the MTL is masked by the interplate coupling 35 effect. Miyazaki and Heki [2001] estimated the slip rate at 35 MTL qualitatively to be 2–3 mm/yr, but they also acknowl- PAC MTL PHS edged that more GPS sites are needed to qualitatively assess 30 50mm/yr 30 the MTL slip rate. Tabei et al. [2002] recently estimated the MTL slip rate to be 5 mm/yr from densely occupied 130 135 140 145 campaign GPS measurements. 133 134 135 36 36 2. Data Set (b) X [6] We obtained three-dimensional velocities from contin- 7 uous GPS data operated by the Geographical Survey Institute of Japan (GSI) between 1996 and 1999. The data in 2000 and 6 later were omitted to avoid the coseismic and postseismic effects of the 2000 Tottori earthquake (Mw = 6.6). Vertical 35 35 velocities of each station were obtained by the method 5 described by Aoki and Scholz [2003] and horizontal veloc- ities were obtained in the new reference frame for the stable 4 Eurasian craton of Steblov et al. [2003], who constructed the new reference frame of the Eurasian and North American 40 plates by analyzing GPS data across Russia. Typical uncer- 3 34 34 tainties for velocities are 1 mm/yr for both horizontal and vertical velocities. Table 1 shows a list of GPS sites used in 30 2 this study with their velocities and uncertainties. 30 [7] The velocity field in Figure 1 is a superposition 1 of (1) interplate coupling between the PHS and SWJ, (2) deformation due to the deep slip at the MTL, and 20 (3) the SWJ rigid plate motion. It is appropriate to assume 33 33 0 20 that horizontal velocities contain all three, while vertical velocities contain only 1. -1 10 3. Model Setting 10 -2 [8] We employed a two-dimensional model for interplate 32 Y 32 coupling (Figure 2) in which the interplate coupling coef- km -3 30mm/yr Vertical velocity ficient is a function of depth only. Some attempts have been made to image both the lateral and depth variation of the 0 50 (mm/yr) interplate coupling along the Nankai Trough [e.g., Ito et al., 1999; Miyazaki and Heki, 2001], but we used a two- 133 134 135 dimensional model to gain better resolution of the depth variation by simplifying the problem. [9] The three-dimensional velocity of each GPS site was Figure 1. (a) Tectonic setting around the studied region rotated into vertical, MTL-normal, and MTL-parallel com- indicated by a square. The Median Tectonic Line (MTL) is ponents (see Figure 1). Note that the vertical velocities are also shown by a solid line. EUR, NA, PAC, and PHS stand due to the downdip component of interplate coupling, the for the Eurasian, North American, Pacific, and Philippine MTL-normal velocity is a superposition of the downdip Sea Plates. (b) GPS velocity field in the studied area with component of the interplate coupling and the MTL-normal the depth contour of the plate interface between PHS and component of rigid plate motion, and the MTL-parallel EUR. Vertical velocities are obtained by the method of Aoki velocity is a superposition of the arc-parallel component and Scholz [2003], and horizontal velocities are according of the interplate coupling, rigid plate motion, and MTL deep to a new reference frame by Steblov et al. [2003] with slip. Although the Nankai Trough is not precisely parallel to respect to the stable Eurasian craton. Also appended are the MTL, we assumed that the MTL-normal direction is the lines along X and perpendicular to Y for which the downdip direction of the Nankai Trough to simplify the interplate coupling and the MTL slip rate are estimated. problem. This assumption should be valid because the strike of the Nankai Trough differs only 15° from the MTL. AOKI AND SCHOLZ: DEFORMATION IN SW JAPAN ETG 5 - 3 [10] We employed the back-slip model of Savage [1983] (Figure 2) in which the overriding plate is down dragged at Table 1. Coordinates, Velocities, and Their Standard Deviations the same velocity as the subducting plate when the interplate of GPS Sites Used in This Studya coupling is full and the overriding plate does not deform at Latitude, Longitude, East, mm/yr North, mm/yr Up, mm/yr all when no interplate coupling occurs. The shape of the Code °N °E Ve se Vn sn Vu su plate interface is taken from Sagiya and Thatcher [1999], 940072 35.587 134.331 À12.54 1.39 À14.04 1.67 À2.14 0.63 who compiled it from the distribution of microseismicity.