Earth Planets Space, Vol. 58 (No. 12), pp. 1621-1626, 2006LETTER
M. Hori1, S. Matsumoto1, K. Uehira1, T. Okada2, T. Yamada3, Y. Iio4, M. Shinohara3, H. Miyamachi5, H. Takahashi6, K. Nakahigashi3, A. Watanabe1, T. Matsushima1, N. Matsuwo1, T. Kanazawa3, and H. Shimizu1
1Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, Japan
2Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Japan
3Earthquake Research Institute, University of Tokyo, Japan
4Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Japan
5Faculty of Science, Kagoshima University, Kagoshima, Japan
6Institute of Seismology and Volcanology, Hokkaido University, Japan
(Received November 14, 2005; Revised October 8, 2006; Accepted October 20, 2006; Online published February 2, 2007)
On March 20, 2005 theWest off Fukuoka Prefecture earthquake (magnitude of 7.0 on the JMA scale) occurred in southeastern Japan. The earthquake fault was a left-lateral strike-slip having a nearly vertical fault plane and a strike in the WNW-ESE direction. The largest aftershock with a magnitude of 5.8 (JMA) followed 1 month later. To gain more detailed aftershock data, several teams from different Japanese universities jointly installed a number of temporary seismic stations and positioned Ocean Bottom Seismometers (OBSs) immediately above the focal area. Double-difference tomography was used to estimate the three-dimensional (3D) (Zhang and Thurber, 2003) velocity structures in and around the focal area based on the travel time data collected during seismic observations. The high-velocity regions estimated by the inversion are located on the edge of the aftershock area and on the shallow part of asperity, as inferred from the slip distribution. Conversely, the Vp/Vs ratio is not always as high as that found at the location of the asperity. This finding suggests that the construction of the medium is not uniformly elastic but complex, with different relations between elastic constants and strength.
Key words: Double-difference tomography, west off Futuoka prefecture earthquake, 3D velocity structures.