Earth Planets Space, Vol. 58 (No. 12), pp. 1633-1636, 2006LETTER
Atsushi Watanabe, Satoshi Matsumoto, Takeshi Matsushima, Kenji Uehira, Norimichi Matsuwo, and Hiroshi Shimizu
Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, Japan
(Received October 15, 2005; Revised June 20, 2006; Accepted June 22, 2006; Online published February 2, 2007)
Crustal shear wave polarization anisotropy is caused by the alignment of vertical microcracks. Leading shear wave polarization directions (LSPDs) are presumed to be consistent with the maximum horizontal compressional axis in many cases. We analyzed shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake. Almost all of the LSPDs are oriented in the E-W direction, which is consistent with the maximum horizontal compressional axis inferred from the mechanism of the main shock. These E-to W-oriented LSPDs are caused by the alignment of stress-induced microcracks. Crack densities at most stations are estimated to be 0.02. Little spacial stress variation around focal region is suspected.
Key words: Shear wave polarization anisotropy, 2005 West off Fukuoka earthquake