Solid Earth www.solid-earth.net 1869-9510 1869-9529 3 1 2012 10.5194/se-3-43-2012 http://www.solid-earth.net/3/43/2012/ http://www.solid-earth.net/3/43/2012/se-3-43-2012.html http://www.solid-earth.net/3/43/2012/se-3-43-2012.pdf 43 51 2012-02-14 Tomography of the 2011 Iwaki earthquake (M 7.0) and Fukushima nuclear power plant area P. Tong D. Zhao D. Yang Department of Geophysics, Tohoku University, Sendai 980-8578, Japan Department of Mathematical Sciences, Tsinghua University, Beijing, China High-resolution tomographic images of the crust and upper mantle in and around the area of the 2011 Iwaki earthquake (M 7.0) and the Fukushima nuclear power plant are determined by inverting a large number of high-quality arrival times with both the finite-frequency and ray tomography methods. The Iwaki earthquake and its aftershocks mainly occurred in a boundary zone with strong variations in seismic velocity and Poisson's ratio. Prominent low-velocity and high Poisson's ratio zones are revealed under the Iwaki source area and the Fukushima nuclear power plant, which may reflect fluids released from the dehydration of the subducting Pacific slab under Northeast Japan. The 2011 Tohoku-oki earthquake (Mw 9.0) caused static stress transfer in the overriding Okhotsk plate, resulting in the seismicity in the Iwaki source area that significantly increased immediately following the Tohoku-oki mainshock. Our results suggest that the Iwaki earthquake was triggered by the ascending fluids from the Pacific slab dehydration and the stress variation induced by the Tohoku-oki mainshock. The similar structures under the Iwaki source area and the Fukushima nuclear power plant suggest that the security of the nuclear power plant site should be strengthened to withstand potential large earthquakes in the future. Asano, Y., Saito, T., Ito, Y., Shiomi, K., Hirose, H., Matsumoto, T., Aoi, S., Hori, S., and Sekiguchi, S.: Spatial distribution and focal mechanisms of aftershocks of the 2011 off the Pacific coast Tohoku Earthquake, Earth Planet. Space, 63, 669–673, 2011. Cheng, B., Zhao, D., and Zhang, G.: Seismic tomography and anisotropy in the source area of the 2008 Iwate-Miyagi earthquake (M 7.2), Phys. Earth Planet. Inter., 184, 172–185, 2011. Gupta, S., Zhao, D., Ikeda, M., Ueki, S., and Rai, S.: Crustal tomography under the Median Tectonic Line in Southwest Japan using P and PmP data, J. Asian Earth Sci., 35, 377–390, 2009. Hasegawa, A. and Zhao, D.: Deep structure of island arc magmatic regions as inferred from seismic observations, Magmatic Systems (edited by M. Ryan), Academic Press, 179–195, 1994. Huang, Z., Zhao, D., and Wang, L.: Seismic heterogeneity and anisotropy of the Honshu arc from the Japan Trench to the Japan Sea, Geophys. J. Int., 184, 1428–1444, 2011. Imanishi, K., Ando, R., and Kuwahara, Y.: A generation mechanism of normal-faulting earthquakes in northeast Japan, activated after the 2011 Tohoku earthquake, Japan Geoscience Union 2011 annual meeting, MIS036-P108, 2011. Ishiyama, T., Sato, H., Sugito, N., Echigo, T., Ito, T., Kato, N., and Imaizumi, T.: Tectonic setting of coseismic surface rupture associated with the 2011 Iwaki earthquake, Japan Geoscience Union 2011 annual meeting, MIS036-P10, 2011. Iwamori, H. and Zhao, D.: Melting and seismic structure beneath the northeast Japan arc, Geophys. Res. Lett., 27, 425–428, 2000. Kato, A., Sakai, S., and Obara, K.: A normal-faulting seismic sequence triggered by the 2011 off the Pacific coast of Tohoku earthquake: Wholesale stress regime changes in the upper plate, Earth Planet. Space, 63, 745–748, 2011. Okada, T., Yoshida, K., Ueki, S., Nakajima, J., Uchida, N., Matsuzawa, T., Umino, N., Hasegawa, A., and Group for the aftershock observations of the 2011 off the Pacific coast of Tohoku Earthquake: Shallow inland earthquakes in NE Japan possibly triggered by the 2011 off the Pacific coast of Tohoku Earthquake, Earth Planet. Space, 63, 749–754, 2011. Padhy, S., Mishra, O. P., Zhao, D., and Wei, W.: Crustal heterogeneity in the 2007 Noto-Hanto earthquake area and its geodynamical implications, Tectonophysics, 509, 55–68, 2011. Salah, M. and Zhao, D.: 3-D seismic structure of Kii Peninsula in Southwest Japan: Evidence for slab dehydration in the forearc, Tectonophysics, 364, 191–213, 2003. Sugito, N., Ishiyama, T., Echigo, T., Sato, H., Kato, N., and Imaizumi, T.: Preliminary report on slip distribution of surface rupture associated with the 2011 Iwaki earthquake, Japan Geoscience Union 2011 annual meeting, MIS036-P1, 2011. Tong, P., Zhao, D., and Yang, D.: Tomography of the 1995 Kobe earthquake area: comparison of finite-frequency and ray approaches, Geophys. J. Int., 187, 278–302, 2011. Wang, Z. and Zhao, D.: Seismic images of the source area of the 2004 Mid-Niigata prefecture earthquake in Northeast Japan, Earth Planet. Sci. Lett., 244, 16–31, 2006a. Wang, Z. and Zhao, D.: Seismic evidence for the influence of fluids on the 2005 west off Fukuoka prefecture earthquake in southwest Japan, Phys. Earth Planet. Inter., 155, 313–324, 2006b. Wessel, P. and Smith, W.: New, improved version of the Generic Mapping Tools released, Eos Trans. AGU, 79(47), 579, http://dx.doi.org/10.1029/98EO00426doi:10.1029/98EO00426, 1998. Xia, S., Zhao, D., and Qiu, X.: The 2007 Niigata earthquake: Effect of arc magma and fluids, Phys. Earth Planet. Inter., 166, 153–166, 2008. Zhao, D., Hasegawa, A., and Horiuchi, S.: Tomographic imaging of P and S wave velocity structure beneath northeastern Japan, J. Geophys. Res., 97, 19909–19928, 1992. Zhao, D., Kanamori, H., Negishi, H., and Wiens, D.: Tomography of the source area of the 1995 Kobe earthquake: evidence for fluids at the hypocenter? Science, 274, 1891–1894, 1996. Zhao, D., Santosh, M., and Yamada, A.: Dissecting large earthquakes in Japan: Role of arc magma and fluids, Island Arc, 19, 4–16, 2010. Zhao, D., Huang, Z., Umino, N., Hasegawa, A., and Kanamori, H.: Structural heterogeneity in the megathrust zone and mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0), Geophys. Res. Lett., 38, L17308, http://dx.doi.org/10.1029/2011GL048408doi:10.1029/2011GL048408, 2011.