Features of Initial Rupture for the 2005 West off Fukuoka Prefecture Earthquake

P-phase with smaller amplitude (P1) that precedes that with larger amplitude (P2) has often been observed for major seismic events (e.g., Umeda, 1990), which indicates a small-scale rupture precedes a large earthquake about a few seconds in the vicinity of the rupture initiation point. It is shown experientially that the relation between the precedence time of the former small seismogram and the magnitude of the latter event is given as log (P2-P1)=0.5*M - 3 (Umeda, 1990). Hirata et al. (2002) showed that the hypocenter estimated from the latter large phase was about several km away from the rupture initiation point (hypocenter of former small phase) for 2000 Western Tottori Earthquake. The same kind of two phases were observed in seismograms of the 2005 West off Fukuoka Prefecture Earthquake that occurred on March 20, 2005 (Mw = 6.6). We investigated the former small amplitude phase to know the initial rupture process. At first, we check whether these two phases were generated from the different but adjacent ruptures by means of the master event hypocenter relocation technique. We used seismograms recorded by Digital Strong-Motion Seismograph Network that deployed by NIED, and detected P1, P2, and S2 (S-phase that corresponds to P2) using AIC of AR model fitting. The distance and interval of these two ruptures are 6-10 km and 3.5-4.0 s, respectively. An estimated average rupture propagation speed of the former rupture was comparable to the optimal first time window propagation speed of the main shock (2.1 km/s) by Asano and Iwata (2005). The moment magnitude of the initial rupture is estimated to be around 6, and its apparent stress is less than 0.1 MPa.