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Shear-wave anisotropy beneath the Ryukyu arc

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We have investigated regional shear-wave splitting beneath the Ryukyu arc in order to clarify the mantle flow beneath the Ryukyu arc and compare it with the extension of the Okinawa Trough. The splitting of regional shear waves (direct S) in the central and southern Ryukyu arc regions was analyzed in terms of fast-polarization direction and delay time using data from 11 short-period stations and two broadband seismic stations. The events occurred at depths ranging between 45 and 217 km, and the magnitude of the events ranged from 2.8 to 5.2. In general, the orientations of fast polarization were found to parallel to the strike of the Ryukyu Trench, with average delay times of 0.25–0.56 s. Specifically, the orientation of fast polarization was parallel to the direction of the extension in the south Ryukyu arc, suggesting that the direction of mantle flow is parallel to the extension in the south Okinawa Trough. However, the orientation of fast polarization was oblique to the direction of extension in the central Ryukyu arc, indicating that the mantle flow beneath the central Ryukyu arc is not parallel to the extension of the central Okinawa Trough. Based on these results, we suggest that the difference between the direction of the mantle flow and that of the lithosphere extension induces extensional strain with a simple shear component, which in turn causes oblique rifting in the central Okinawa Trough.


  1. Ando, M., Y. Ishikawa, and F. Yamazaki, Shear-wave polarization anisotropy in the upper mantle beneath Honshu, Japan, J. Geophys. Res., 88, 5850–5864, 1983.

  2. Audoine, E., M. K. Savage, and K. Gledhill, Seismic anisotropy from local earthquakes in the transition region from a subduction to a strike-slip plate boundary, New Zealand, J. Geophys. Res., 105, 8013–8033, 2000.

  3. Fouch, M. J. and K. M. Fischer, Mantle anisotropy beneath Northwest Pacific subduction zones, J. Geophys. Res., 101, 15987–16002, 1996.

  4. Fournier, M., O. Fabbri, J. Angelier, and J.-P. Cadet, Regional seismicity and on-land deformation in the Ryukyu arc: implications for the kinematics of opening of the Okinawa Trough, J. Geophys. Res., 106, 13751–13768, 2001.

  5. Jung, H. and S. Karato, Water induced fabric transitions in olivine, Science, 293, 1460–1463, 2001.

  6. Kimura, M., Y. Wang, and H. Yagi, Submarine geo-structure and tectonics in the Ryukyu arc, Chishitu News, 543, 24–38, 1999 (in Japanese).

  7. Kubo, A. and E. Fukuyama, Stress field along the Ryukyu Arc and the Okinawa Trough inferred from moment tensors of shallow earthquakes, Earth Planet. Sci. Lett., 210, 305–316, 2003.

  8. Letouzey, J. and M. Kimura, Okinawa Trough genesis: Structure and evolution of a back-arc basin developed in a continent, Mar. Pet. Geol., 2, 111–130, 1985.

  9. Liu, X., K. C. McNally, and Z.-K. Shen, Evidence for a role of the downgoing slab in earthquake slip partitioning at oblique subduction zones, J. Geophys. Res., 100, 15351–15372, 1995.

  10. Long, M. D. and R. D. van der Hilst, Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge, Phys. Earth Planet. Inter., 155, 300–312, 2006.

  11. Marson-Pidgeon, K., M. K. Savage, K. Gledhill, and G. Stuart, Seismic anisotropy beneath the lower half of the North Island, New Zealand, J. Geophys. Res., 104, 20277–20286, 1999.

  12. Nakamura, M., Crustal deformation in the central and southern Ryukyu Arc estimated from GPS data, Earth Planet. Sci. Lett., 217, 389–398, 2004.

  13. Nakamura, M. and S. Kaneshiro, Determination of subducted Philippine Sea plate in the Nansei islands deduced from hypocenter data, Bull. Sci., Univ. Ryukyus, 70, 73–82, 2000.

  14. Okada, T., T. Matsuzawa, and A. Hasegawa, Shear-wave polarization anisotropy beneath the north-eastern part of Honshu, Japan, Geophys. J. Int., 123, 781–797, 1995.

  15. Peyton, V., V. Levin, J. Park, M. Brandon, J. Lees, E. Gordeev, and A. Ozerov, Mantle flow at a slab edge: seismic anisotropy in the Kamchatka region, Geophys. Res. Lett., 28, 379–382, 2001.

  16. Seno, T., S. Stein, and A. E. Gripp, A model for the motion of the Phillipine Sea plate consistent with NUVEL-1 and geological data, J. Geophys. Res., 98, 17941–17948, 1993.

  17. Sibuet, J.-C., S.-K. Hsu, C.-T. Shyu, and C.-S. Liu, Structural and kinematic evolution of the Okinawa trough backarc basin, in Backarc Basins: Tectonics and Magmatism, edited by B. Taylor, pp. 343–378, Plenum, New York, 1995.

  18. Sibuet, J.-C., B. Deffontaines, S.-K. Hsu, N. Thareau, J.-P. Le Formal, C.-S. Liu, and ACT party, Okinawa trough backarc basin: Early tectonic magmatic evolution, J. Geophys. Res., 103, 30245–30267, 1998.

  19. Smith, G. P., D. A. Wiens, K. M. Fischer, L. M. Dorman, S. C. Webb, and J. A. Hildebrand, A complex pattern of mantle flow in the Lau backarc, Science, 292, 713–716, 2001.

  20. Zhang, S. and S. Karato, Lattice preferred orientation of olivine aggregates deformed in simple shear, Nature, 375, 774–777, 1995.

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Correspondence to Mamoru Nakamura.

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Nakamura, M., Yamamoto, A. Shear-wave anisotropy beneath the Ryukyu arc. Earth Planet Sp 61, 1197–1202 (2009) doi:10.1186/BF03352972

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Key words

  • S-waves
  • wave splitting
  • seismic anisotropy
  • Ryukyu arc
  • Okinawa Trough