Heterogeneous structure in and around 2005 West off Fukuoka Earthquake inferred from dense seismic observation

The 2005 West Off Fukuoka Prefecture earthquake (Fukuoka EQ.) occurred on March 20, 2005, with magnitude of 7.0 in JMA scale in the offshore region of Fukuoka Prefecture, Kyushu, Japan. This earthquake damaged many buildings in the surrounding area, especially on Genkai Island as well as in and around Fukuoka City. 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. In order to obtain both detail seismic activity and inhomogeneous structure, we performed dense seismic observation in and around the aftershock area. The velocity structure was inferred from P and S arrival time data by the Double Difference tomography method, signifying high-velocity regions found on the edge of the aftershock area and on the area with large co-seismic slip. We achieved attenuation structure in the aftershock area from spectral data. Regions with high attenuation were found at the southeastern edge and below the earthquake fault. The scatterer distribution inferred from the seismic array observation also showed existence of strong heterogeneity at the edge. Hypocenter distribution relocated using three dimensional velocity structure implied that the alignment of the aftershock distribution in shallower part is slightly different from the deeper ones. This shows the difference has similar tendency to difference between focal mechanisms by CMT solution and that from polarities of P wave first motions. These results showed the rupture at the beginning of the earthquake had different behavior from the major slip generating large seismic wave. An active fault called Kego fault lies at southeastern of the fault of the Fukuoka EQ. The direction of the Kego fault is not similar to that of Fukuoka EQ in strike direction, which means that a segment boundary in the fault system exists at the part. The rupture of the mainshock stopped at the boundary. In addition, we revealed that strong heterogeneity found at the boundary by velocity, attenuation and scatterer distribution. These results suggested that heterogeneity could control the faulting of Fukuoka EQ., and gave us insights in terms of the stress accumulation process for the forthcoming earthquake at the Kego fault.