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Secular Crustal Deformation and Interplate Coupling of the Japanese Islands As Deduced from Continuous GPS Array, 1996–2001 ⁎ Gamal El-Fiky A, , Teruyuki Kato B

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Secular Crustal Deformation and Interplate Coupling of the Japanese Islands As Deduced from Continuous GPS Array, 1996–2001 ⁎ Gamal El-Fiky A, , Teruyuki Kato B Tectonophysics 422 (2006) 1–22 www.elsevier.com/locate/tecto Secular crustal deformation and interplate coupling of the Japanese Islands as deduced from continuous GPS array, 1996–2001 ⁎ Gamal El-Fiky a, , Teruyuki Kato b a Construction Engineering and Utilities Department, Faculty of Eng., Zagazig Univ., El-Zagazig, Egypt b Earthquake Research Institute, University of Tokyo, Tokyo, Japan Received 23 September 2005; received in revised form 31 March 2006; accepted 25 April 2006 Available online 10 July 2006 Abstract Data from the nation-wide GPS continuous tracking network that has been operated by the Geographical Survey Institute of Japan since April 1996 were used to study crustal deformation in the Japanese Islands. We first extracted site coordinate from daily SINEX files for the period from April 1, 1996 to February 24, 2001. Since raw time series of station coordinates include coseismic and postseismic displacements as well as seasonal variation, we model each time series as a combination of linear and trigonometric functions and jumps for episodic events. Estimated velocities were converted into a kinematic reference frame [Heki, K., 1996. Horizontal and vertical crustal movements from three-dimensional very long baseline interferometry kinematic reference frame: implication for reversal timescale revision. J. Geophys. Res., 101: 3187–3198.] to discuss the crustal deformation relative to the stable interior of the Eurasian plate. A Least-Squares Prediction technique has been used to segregate the signal and noise in horizontal as well as vertical velocities. Estimated horizontal signals (horizontal displacement rates) were then differentiated in space to calculate principal components of strain. Dilatations, maximum shear strains, and principal axes of strain clearly portray tectonic environments of the Japanese Islands. On the other hand, the interseismic vertical deformation field of the Japanese islands is derived for the same GPS data interval. The GPS vertical velocities are combined with 31 year tide gage records to estimate absolute vertical velocity. The results of vertical deformation show that (1) the existence of clear uplift of about 6 mm/yr in Shikoku and Kii Peninsula, whereas pattern of subsidence is observed in the coast of Kyushu district. This might reflect strong coupling between the Philippine Sea plate and overriding plate at the Nankai Trough and weak coupling off Kyushu, (2) no clear vertical deformation pattern exists along the Pacific coast of northeastern Japan. This might be due to the long distance between the plate boundary (Japan trench) and overriding plate where GPS sites are located, (3) significant uplift is observed in the southwestern part of Hokkaido and in northeastern Tohoku along the Japan Sea coast. This is possibly due to the viscoelastic rebound of the 1983 Japan Sea (Mw 7.7) and the 1993 Hokkaido–Nansei–Oki (Mw 7.8) earthquakes and/or associated with distributed compression of incipient subduction there. We then estimate the elastic deformation of the Japanese Islands caused by interseismic loading of the Pacific and Philippine Sea subduction plates. The elastic models account for most of the observed horizontal velocity field if the subduction movement of the Philippine Sea Plate is 100% locked and if that of the Pacific Plate is 70% locked. However, the best fit for vertical velocity ranges from 80% to 100% coupling factor in southwestern Japan and only 50% in northeastern Japan. Since horizontal data does not permit the separation of rigid plate motion and interplate coupling because horizontal velocities include ⁎ Corresponding author. E-mail address: [email protected] (G. El-Fiky). 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.04.021 2 G. El-Fiky, T. Kato / Tectonophysics 422 (2006) 1–22 both contributions, we used the vertical velocities to discriminate between them. So, we can say there is strong interplate coupling (80%–100%) over the Nankaido subduction zone, whereas it is about 50% only over the Kurile–Japan trench. © 2006 Elsevier B.V. All rights reserved. Keywords: Crustal deformation; Interplate coupling; Continuous GPS array; Japanese Islands 1. Introduction the Sagami Trough. Relative motion among these plates accumulates tectonic stress in the lithosphere and causes The Japanese Islands are located at a complex plate observable crustal deformation. The occurrence of large boundary. The convergence of four plates including the earthquake along plate boundaries and crustal faults re- Eurasian (EUR) or Amurian (AMR), the Pacific (PAC), leases such tectonic stress. Consequently, study of the Philippine Sea (PHS) plate, and North American crustal deformation is a key to understand the physical (NAM) or the Okhotsk (OKH) predominates (Fig. 1). process in the crust as well as to forecast crustal activity. The oceanic PAC is descending beneath the continental Dense arrays of continuous GPS tracking networks NAM (or OKH) at the Kurile–Japan trench, and oceanic provide us with an ideal tool for monitoring crustal PHS plate is descending beneath the EUR at the Suruga– deformation. In Japan, the Geographical Survey Institute Nankai Trough and beneath NAM (or OKH) plate along (GSI) started to establish the dense GPS array of Fig. 1. Major plate boundaries in and near the Japanese Islands. EUR or AMR: Eurasian or Amurain plate, NAM or OHK: North American or Okhotsk plate, PHS: Philippine Sea plate, and PAC: Pacific plate. Source regions of recent conspicuous earthquakes which might affect on the GPS data are shown. Arrows indicate the relative plate motion directions. G. El-Fiky, T. Kato / Tectonophysics 422 (2006) 1–22 3 Fig. 2. An example of the time series at site number 950155 (39.93°E, 40.58°N). (a) Uncorrected time series of E–W component. (b) Annual change of E–W component. (c) Difference between uncorrected time series and annual change of E–W component. (d), (e), and (f) are same but for vertical component. continuous tracking network in the Kanto–Tokai region horizontal signals (horizontal displacement rates) were in 1992 and has further expanded it to cover the entire differentiated in space to calculate principal components nation. The GSI has been operating in 610 sites, for of strain. continuous monitoring of the daily site coordinates since One problem in employing only the horizontal velocity April 1996. The number of sites increased to 1200 by components as data for crustal deformations studies is that 2004, with average site intervals of about 20 km (e.g., it is difficult to segregate rigid plate motions from inter- Miyazaki et al., 1998; Sagiya et al., 2000). The array is plate coupling strain. This is because horizontal velocities now considered one of the most fundamental infra- include both effects (Aoki and Scholz, 2003). So, rigid structures for monitoring crustal activity in Japan. plate motions could be mapped into the interplate coupling. Details of the operation and analysis of this GPS array This problem can be solved by using vertical velocities to are described in Tada et al. (1997), Miyazaki et al. separate between the rigid plate motions and interplate (1997), and Sagiya (2004a). coupling. On the other hand, the repeatability of horizontal In the present study, we try to delineate the horizontal components of GPS coordinates is highly precise com- crustal strains as well as the interseismic vertical defor- pared with that of their vertical components. So, vertical mation field of the Japanese Islands, using data for about component has rarely been used in scientific discussions. five years from the GPS array. For this purpose, we first Here, we try to estimate the interseismic vertical velocity extracted site coordinate from daily SINEX files for the field for Japanese Islands using GSI's GPS array data period from April 1, 1996 to February, 24, 2001, and between 1996 and 2001. The estimated GPS vertical vel- estimated site velocities by modelling each time series as a cities are combined with 31 year tide gage records to obtain combination of step functions, trigonometric functions, absolute vertical velocity. and a linear function. A Least-Squares Prediction (LSP) Finally, we investigate the origin of the GPS defor- technique has been used to segregate the signal and noise mation in the Japanese Islands by comparing velocity in horizontal as well as vertical velocities. Estimated fields determined from GPS data with those calculated velocities are converted to a kinematic reference frame from the elastic dislocation models involving interplate (Heki, 1996) to discuss the crustal deformation relative to motion at the subduction zones in northeastern and the stable interior of the Eurasian plate. Finally, estimated southwestern Japan. We then show that most of this GPS 4 G. El-Fiky, T. Kato / Tectonophysics 422 (2006) 1–22 velocity field can indeed be accounted for by full elastic sites. (c) and (f) are E–W and vertical components of coupling along the Philippine thrust zones and only 50% corrected time series (difference between uncorrected coupling for the Pacific locked zone. We compare the time series and annual variation), respectively. Finally, GPS-derived strain with those obtained from seismo- the estimated velocities are converted into a kinematic logical and geological data and show that they are qua- reference frame (Heki, 1996) to discuss the crustal litatively similar but that the GPS deformation is much deformation relative to the stable interior of the Eurasian larger that the other two. plate. Fig. 3 shows the horizontal component of the velocity vectors relative to the stable part of the Eurasian 2. Time series and horizontal velocity field plate with 95% confidence error ellipses. Coseismic crustal deformation associated with large earthquakes Processing of the data of GSI's GPS array with the IGS which occurred during the observation period, were precise orbit yields daily coordinates of all stations. We estimated as formulated in Eq. (1), e.g., the 1996 Hyu- used daily GPS data from April 1, 1996 to February 24, ganada earthquakes (MJMA 6.6 on October 19 and MJMA 2001.
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