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Existence of low-velocity zones under the source areas of the 2004 Chuetsu and 2007 Chuetsu-oki earthquakes inferred from travel-time tomography
Earth, Planets and Space volume 60, pages1127–1130(2008)
Travel-time tomography reveals the presence of highly heterogeneous seismic velocity structures around the source areas of the 2004 Chuetsu and 2007 Chuetsu-oki earthquakes. A prominent low-velocity zone present below the Moho under the source areas of the two events is probably formed by fluids being conveyed through a small-scale upwelling in the mantle wedge. A low-velocity zone distributed in the lower crust below each source area is attributable to fluids supplied from the uppermost mantle to the source area. Since this region with these two source areas is exceptional in that the low-velocity zone is distributed in the uppermost mantle but no Quaternary volcanoes are formed at the surface, a relatively large amount of fluids may be stored below the source areas. These localized fluids may have reduced the strength in the lower crust as well as in the fault zones in the upper crust and, consequently, have promoted a brittle failure in the seismogenic layer. Relocated hypocenters of aftershocks of the two events become several kilometers shallower than the original locations determined by the Japan Meteorological Agency, suggesting the importance of using the three-dimensional velocity model in locating the hypocenter.
Hasegawa, A. and J. Nakajima, Geophysical constraints on slab subduction and arc magmatism, in The State of the Planet: Frontiers and Challenges in Geophysics, Geophys. Monogr. Ser., 150, 81–94, AGU, Washington, D.C., 2004.
Hasegawa, A., J. Nakajima, N. Umino, and S. Miura, Deep structure of the northeastern Japan arc and its implications for crustal deformation and shallow seismic activity, Tectonophys., 403, 59–75, 2005.
Kato, A., E. Kurashimo, N. Hirata, T. Iwasaki, and T. Kanazawa, Imaging the source region of the 2004 Mid-Niigata prefecture earthquake and the evolution of the seismogenic thrust-related fold, Geophys. Res. Lett., 32, L07307, doi:10.1029/2005GL022366, 2005.
Nakajima, J. and A. Hasegawa, Anomalous low-velocity zone and linear alignment of seismicity along it in the subducted Pacific slab beneath Kanto, Japan: Reactivation of subducted fracture zone?, Geophys. Res. Lett., 33, L16309, doi:10.1029/2006GL026773, 2006.
Nakajima, J. and A. Hasegawa, Deep crustal structure along the Niigata-Kobe Tectonic Zone, Japan: Its origin and segmentation, Earth Planets Space, 59, e5–e8, 2007a.
Nakajima, J. and A. Hasegawa, Subduction of the Philippine Sea slab beneath southwestern Japan: Slab geometry and its relationship to arc magmatism, J. Geophys. Res., 112, B08306, doi:10. 1029/2006JB004770, 2007b.
Nakajima, J., T. Matsuzawa, A. Hasegawa, and D. Zhao, Three-dimensional structure of Vp, Vs and Vp/Vs beneath the northeastern Japan arc: Implications for arc magmatism and fluids, J. Geophys. Res., 106, 21843–21857, 2001.
Nishimoto, S., M. Ishikawa, M. Arima, T. Yoshida, and J. Nakajima, Simultaneous high P-T measurements of ultrasonic compressional and shear wave velocities in Ichino-megata mafic xenoliths: Their bearing on seismic perturbations in lower crust of northeast Japan arc, J. Geo-phys. Res., 2008 (submitted).
Okada, T. et al., Aftershock distribution and 3D seismic velocity structure in and around the focal area of the 2004 mid Niigata prefecture earthquake obtained by applying double-difference tomography to dense temporary seismic network data, Earth Planets Space, 57, 435–440, 2005.
Okada, T., T. Yaginuma, N. Umino, T. Matsuzawa, A. Hasegawa, H. Zhang, and C. H. Thurber, Detailed imaging of the fault planes of the 2004 Niigata-Chuetsu, central Japan, earthquake sequence by double-difference tomography, Earth Planet. Sci. Lett., 244, 32–43, 2006.
Sagiya, T., S. Miyazaki, and T. Tada, Continuous GPS array and present-day crustal deformation of Japan, Pageoph, 157, 2303–2322, 2000.
Sano, Y. and H. Wakita, Geographical distribution of 3He/4He ratios in Japan: Implications for arc tectonics and incipient magmatism, J. Geo-phys. Res., 90, 8729–8741, 1985.
Sato, H., The relationship between late Cenozoic tectonic events and stress field and basin development in northeast Japan, J. Geophys. Res., 99, 22261–22274, 1994.
Tanaka, A., M. Yamano, Y. Yano, and M. Sasada, Geothermal gradient and heat flow data in and around Japan (I): Appraisal of heat flow from geothermal gradient data, Earth Planets Space, 56, 1191–1194, 2004.
Ueno, H., S. Hatakeyama, T. Aketagawa, J. Funasaki, and N. Hamada, Improvement of hypocenter determination procedures in the Japan Meteorological Agency, Quart. J. Seismol., 65, 123–134, 2002 (in Japanese).
Wang, Z. and D. Zhao, Seismic images of the source area of the 2004 Mid-Niigata prefecture earthquake in Northeast Japan, Earth Planet. Sci. Lett., 244, 16–31, 2006.
Zhao, D., A. Hasegawa, and S. Horiuchi, Tomographic imaging of P and S wave velocity structure beneath northeastern Japan, J. Geophys. Res., 97, 19909–19928, 1992a.
Zhao, D., S. Horiuchi, and A. Hasegawa, Seismic velocity structure of the crust beneath the Japan islands, Tectonophys., 212, 289–301, 1992b.
Zhao, D., H. Kanamori, H. Negishi, and D. Wiens, Tomography of the source area of the 1995 Kobe earthquake: Evidence for fluids at the hypocenter?, Science, 274, 1891–1894, 1996.
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Nakajima, J., Hasegawa, A. Existence of low-velocity zones under the source areas of the 2004 Chuetsu and 2007 Chuetsu-oki earthquakes inferred from travel-time tomography. Earth Planet Sp 60, 1127–1130 (2008) doi:10.1186/BF03353148
- Seismic tomography
- Chuetsu earthquake
- Chuetsu-oki earthquake
- low-velocity zone
- mantle upwelling