Earth Planets Space, Vol. 58 (No. 1), pp. 81-86, 2006LETTER
Kazutoshi Imanishi1, Yasuto Kuwahara1, and Yoshikatsu Haryu2
1Geological Survey of Japan, AIST, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
2National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan
(Received August 10, 2005; Revised December 1, 2005; Accepted December 7, 2005; Online published January 27, 2006)
After the occurrence of the 20 March 2005 West Off Fukuoka Prefecture Earthquake (Mj 7.0), off-fault aftershocks occurred in and around Hakata bay adjacent to the main fault of the earthquake. The locations of the activity seem to coincide with the Ishido-Uminonakamichi fault (I-U fault), which is a structural boundary and not considered as an active fault. In order to elucidate relations between these off-fault aftershocks and the I-U fault, we determine the double-difference earthquake locations and focal mechanism solutions. Results of the present study show that the off-fault aftershocks are aligned in a sense of left-lateral faulting striking 120° from the north and in that of their conjugate right-lateral fault structures striking 30° from the north. Comparing the trends of these aligned structures with the orientation of the I-U fault, we conclude that earthquakes in and around the Hakata bay occurred along these aligned structures, not along a fault plane inferred from the surface trace of the I-U fault. The analysis of Coulomb failure stress due to the mainshock also supports this conclusion. Stress tensor inversion shows that the stress field in and around the Hakata bay is characterized by a strike-slip faulting. The angles between the fault trends and the maximum principal stress suggest that the aligned structures found in this study are favorably oriented, while the fault plane inferred from the surface trace of the I-U fault is unfavorably oriented.
Key words: 2005 West Off Fukuoka Prefecture Earthquake, off-fault aftershocks, Ishido-Uminonakamichi fault, focal mechanism solutions, double-difference location, stress field.