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Interplate Coupling in Southwest Japan Deduced from Inversion Analysis of GPS Data

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Interplate Coupling in Southwest Japan Deduced from Inversion Analysis of GPS Data Physics of the Earth and Planetary Interiors 115Ž. 1999 17±34 www.elsevier.comrlocaterpepi Interplate coupling in southwest Japan deduced from inversion analysis of GPS data Takeo Ito a,), Shoichi Yoshioka a, Shin'ichi Miyazaki b a Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu UniÕersity, Hakozaki 6-10-1, Higashi ward, Fukuoka 812-8581, Japan b Satellite Geodetic DiÕision, Geodetic ObserÕation Center, Geographical SurÕey Institute, Kitasato 1, Tsukuba, Ibaraki 305-0811, Japan Received 5 August 1998; received in revised form 16 April 1999; accepted 16 April 1999 Abstract Recently, the Geographical Survey Institute of Japan completed the installation of a GPS continuous observation network in Japan, which has enabled us to investigate real-time crustal movements. In this study, we attempt to obtain spatial distribution of interplate coupling and relative plate motion between subducting and overriding plates in southwest Japan, using horizontal and vertical deformation rates, which were observed at 247 GPS observation stations during the period from April 6, 1996 to March 20, 1998. For this purpose, we carried out an inversion analysis of geodetic data, incorporating Akaike's Bayesian Information CriterionŽ. ABIC . As a result, strong interplate coupling was found off Shikoku and Kumanonada regions, which corresponds well with the fault regions of the 1946 NankaiŽ. M 8.1 and the 1944 Tonankai Ž M 8.0. earthquakes, respectively. We also found that interplate coupling becomes weak at depths deeper than about 30 to 40 km beneath the Shikoku and Kii peninsula. The recurrence time of great trench-type earthquakes was roughly estimated as 107 years, which is consistent with previous research. The direction of relative plate motion is oriented N538W, which is close to the direction predicted from the plate motion model. On the other hand, a large forward slip was found in the Hyuganada region off southeast of Kyushu. Since the coseismic displacements associated with the two 1996 Hyuganada earthquakesŽ. M 6.6, M 6.6 are removed from the GPS data, this suggests that after-slip occurred near the source region andror that Kyushu moves southeastward stationarily due to other tectonic forces. q 1999 Elsevier Science B.V. All rights reserved. Keywords: GPS; Back slip; Interplate coupling; Relative plate motion; Forward slip 1. Introduction the North AmericanŽ. NA plate . interact with each otherŽ. Fig. 1 . The plate motion in southwest Japan, Southwest Japan is the region where the Amurian especially the eastward motion of the AM plate has Ž.AM plate Ž or the Eurasia Ž. EU plate . , the Philip- been debated by many researchersŽ e.g., Zonenshain pine SeaŽ. PH plate, and the Okhotsk Ž OK . plate Ž or and Savostin, 1981; Kimura et al., 1986; Tsukuda, 1992; Ishibashi, 1995. There are two theories con- ) Corresponding author. Tel.: q81-92-642-2647; fax: q81-92- cerning the location of the southern boundary of the 642-2684; E-mail: [email protected] AM plate: one places it along the Median Tectonic 0031-9201r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0031-9201Ž. 99 00063-1 18 T. Ito et al.rPhysics of the Earth and Planetary Interiors 115() 1999 17±34 Fig. 1. Map showing horizontal displacement rates relative to the stationary part of the Eurasian plate with confidence ellipses of 1s at 247 GPS stations in southwest Japan during the period from April 6, 1996 to March 20, 1998. Coseismic crustal deformations associated with the 1996 Hyuganada earthquakesŽ October Ž M 6.6 . , December Ž M 6.6 .. and the 1997 Kagoshima±Hokuseibu earthquakes Ž March Ž M 6.3 . , MayŽ.. M 6.2 , which occurred during the observation period, were removed. The epicenters of the four events are shown with star symbols. The inset shows four plates in and around the Japanese islands. AMsAmurian PlateŽ. or EUsEurasia plate ; OKsOkhotsk plate Ž or NAsNorth American plate. ; PAsPacific plate; PHsPhilippine Sea Plate. LineŽ. MTL , and the other places it along the Nankai ated with the subduction of the PH plate along the trough. Geophysical exploration of underground Nankai troughŽ e.g., Thatcher, 1984; Savage and structure beneath the MTLŽ Yoshikawa et al., 1992; Thatcher, 1992; Tabei et al., 1996. Yuki et al., 1992; Ito et al., 1996. and fault simula- Great interplate earthquakes have occurred repeat- tion using strain data obtained by Geographical Sur- edly along the Nankai trough, with recurrence inter- vey InstituteŽ. GSI of Japan Ž Hashimoto and Jack- val of about 90 to 150 yearsŽ e.g., Shimazaki and son, 1993. have been conducted. However, we have Nakata, 1980; Thatcher, 1984. The most recent not arrived at a conclusion to determine a preferred events were the 1944 TonankaiŽ. M 7.9 and the 1946 theory. Moreover, the spatial pattern of tectonic NankaiŽ. M 8.0 earthquakes. It is believed that these crustal movement in southwest Japan is complicated events released accumulated stress in association with due to elastic strain accumulation and release associ- the subduction of the PH plate. Many studies have T. Ito et al.rPhysics of the Earth and Planetary Interiors 115() 1999 17±34 19 been done to determine the coseismic slip distribu- interplate coupling and the direction of relative plate tion of the two earthquakes, using geodetic data, motion were estimated more objectively through in- seismic waves and tsunami dataŽ e.g., Fitch and version analysis of leveling and trilateration data in Scholz, 1971; Kanamori, 1972; Ando, 1975, 1982; the Kanto±Tokai districts and southwest Japan Yoshioka et al., 1989; Yabuki and Matsu'ura, 1992; Ž.Yoshioka et al., 1993, 1994; Sagiya, 1995 . During Satake, 1993; Sagiya and Thatcher, 1999. the last several years, the Geographical Survey Insti- Some studies have also attempted to obtain inter- tute of JapanŽ. GSI has installed and maintained GPS seismic interplate coupling using geodetic data. In continuous observation networks throughout the southwest Japan, YoshiokaŽ. 1991 investigated spa- country. Recently, Nishimura et al.Ž. 1998 estimated tial distribution of the strength of interplate coupling interplate coupling in southwest Japan, using hori- along the Nankai trough, based on leveling, tide zontal displacement rates of GPS data, on the basis gauge, and trilateration data, using a three-dimension of the least squares method. However, since their finite element method. However, the results were analysis is based on forward modeling, the obtained obtained using forward modeling, and the model was results cannot be evaluated objectively as well. not satisfactory to evaluate the spatial distribution of In this study, we attempt to obtain interplate interplate coupling objectively. Later, the strength of coupling, using an inversion analysis for continuous Fig. 2. An example of the time series obtained at the CHIYODA stationŽ. latitude 34.6778N, longitude 140.0888E . Vertical and horizontal axes represent displacementŽ. mm and time Ž year . , respectively. Ž. a Uncorrected time series of north±south component. Ž. b Annual change of north±south component.Ž. c Difference between uncorrected time series and annual change of north±south component.Ž. d Uncorrected time series of vertical component.Ž. e Annual change of vertical component. Ž. f Difference between uncorrected time series and annual change of vertical component. 20 T. Ito et al.rPhysics of the Earth and Planetary Interiors 115() 1999 17±34 GPS data obtained in southwest Japan. The real-time 2. GPS data and their correction observations have enabled us to reveal detailed crustal movement in southwestern Japan, elucidating east- We employed Bernese version 4 software for ward motion of the AM plate. We used data of 247 analysis of GPS data. We used International GPS horizontal and 237 vertical displacement rates from Service for GeodynamicsŽ. IGS final orbits for satel- April 6, 1996 to March 20, 1998. The purpose of this lite information and International Earth Rotation Ser- study is to obtain the direction of relative plate viceŽ. IERS bulletin B for Earth rotation parameters. motion and the spatial distribution of the strength of Only the TSKB station, which is one of the IGS interplate coupling on the plate boundary between global sites in the GSI campus in Tsukuba, is used the subducting PH plate and the overriding continen- for the tie with IGS global site because we adopted a tal plate through inversion analysis, using Akaike's distributed strategy. We resolved ambiguities by the Bayesian Information CriterionŽ.Ž ABIC Yabuki and sigma dependent strategyŽ e.g., Rothacher and Mer- Matsu'ura, 1992. vart, 1996. The reference frame we used is ITRF94 Fig. 3. Horizontal displacement rates which were obtained correcting the eastward motion of the Amurian plate. The area in the north of the boundary along the Median Tectonic Line, Arima±Takatsuki Tectonic Line, and the western part of Lake Biwa is regarded as the AM plate. We corrected the movement in this area as the motion of rigid body based on Euler vectorŽ. 218S, 1088E, vsy0.0928rMyr by Heki et al. Ž.1998 . T. Ito et al.rPhysics of the Earth and Planetary Interiors 115() 1999 17±34 21 Ž.ITRF International Terrestrial Reference Frame . the stationary part of the EU plate. According to Following Heki et al.Ž. 1998 , we obtained the crustal HekiŽ. 1996 , the horizontal movement of the TSKB velocity field relative to the EU plate by subtracting station relative to the stationary part of the EU plate its absolute motion from ITRF velocities for each is 2.7 mmryr to the north and 20.5 mmryr to the site because the kinematic part of ITRF94 is nnr- west. The result was obtained so as to minimize the NUVEL1a plate motion modelŽ Argus and Gordon, difference in the horizontal displacement rates be- 1991. tween the Very Long Baseline InterferometryŽ.
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