Earth Planets Space, Vol. 58 (No. 1), pp. 87-92, 2006LETTER
Active Fault Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8567 Japan
(Received August 10, 2005; Revised December 3, 2005; Accepted December 7, 2005; Online published January 27, 2006)
The 2005 West off Fukuoka Prefecture, Japan, earthquake (MS 6.7) is a moderate-size crustal event. The rupture process of this earthquake was inferred from strong motion data. It is observed regardless of epicentral distance and azimuth that the initial P-wave with a small amplitude continued for 2.5-4 s, suggesting prominent initial rupture. The duration of the initial rupture was estimated to be 3.3 s from travel time analysis. Inversion analysis shows that the initial rupture continued for 3.5 s and the overall rupture finished within 10 s. An area of large slip with two peaks of slip amount appeared to the southeast of the hypocenter, and the location of the large-slip area is consistent with the origin of the main rupture inferred from travel time analysis. Small amount of slip was found at and around the hypocenter, and the rupture velocity was slow there. The seismic moment of this earthquake was estimated to be 5.7 × 1018 Nm (Mw 6.4). The maximum stress drop calculated from the derived slip distribution exceeded 10 MPa. An area of negative stress drop expanded, corresponding to the distribution of small slip. Forward modeling of the observed waveforms suggests that the negative stress drop is not fully attributable to poor resolution of inversion analysis for small amount of slip. Since dynamic stress drop should be positive around the hypocenter so that rupture propagates over the entire fault, it is plausible that the negative stress drop appeared after healing.
Key words: 2005 Fukuoka earthquake, strong motion data, nucleation phase, dynamic stress drop, negative stress drop.