High-Resolution Imaging of Rupture Process of The 2005 Fukuoka Earthquake (Mw6.6)

The 2005 West Off Fukuoka Prefecture earthquake (hereafter called Fukuoka earthquake) occurred on 20 March 2005 (JST) in the northwestern offshore of Fukuoka Prefecture, Japan, which shook strongly Fukuoka City and surrounded area. Local strong-motion records of this earthquake have a remarkable feature: the records show several seconds of small but increasing amplitude arrival ("initial rupture phase") followed by the onset of the main energy release ("main rupture phase"). In this study we get the high resolution image of the rupture process of this earthquake from these records. To investigate source rupture process, waveform inversions of teleseismic and/or nearfield strong-motion records are now popular, but these source inversion methods do not have so high resolution. Recently another approach with high resolution, the back-projection method (Ishii et al., 2005) was exploited, which images earthquake rupture directly from dense network data. The source-scanning algorithm (Kao and Shan, 2004) is also based on a similar idea to this approach. The back-projection method is useful to teleseismic array data of an earthquake with very large dimension such as the 2004 Sumatra-Andaman earthquake (Mw9.3), as mentioned by Ishii et al. (2005). Here we apply a source imaging technique to near-source strong-motion records of the Fukuoka earthquake (Mw 6.6), which is similar to the back-projection method, but is useful to middle to large earthquakes. Our imaging technique is based on a master-event travel-time calculation and Nth-root stacking, so that this method has a high spatial resolution and emphasizes high-coherent image events of the rupture process generating high-frequency radiated energy. We used P-wave portion of UD motion records and derived the following rupture history: The initial rupture plane and the main rupture plane are different. The rupture first propagated toward the direction of strike (opposite direction of Fukuoka City) of the initial rupture plane for about one second and changed to the main rupture plane, where this changing point is the rupture starting position on the main rupture plane and can be considered as the "second hypocenter" of this earthquake. The rupture advanced toward the direction of Fukuoka City along the main rupture plane. The seismic energy release then started to increase to 2.7 seconds after the origin time, and the main rupture (breaking of the asperity) began at a position of 5.1 km apart and southeast above the original hypocenter at 3.7 seconds after the origin time. Our results may suggest that applying the conventional source inversion method to this earthquake the "second hypocenter" should be used instead of the hypocenter.