LOCATING FAULT PLANE AND MAIN RUPTURE ONSET OF THE MARCH 20, 2005 FUKUOKA, JAPAN, EARTHQUAKE (Mw6.6)

TAKENAKA, H., YAMAMOTO, Y., NAKAMURA, T., and TOYOKUNI, G., Dept. Earth & Planet. Sci., Kyushu Univ., Fukuoka 812-8581, Japan, and Kawase, H., Dept. Human-Env. Studies, Kyushu Univ. Fukuoka 812-8581, Japan, takenaka@geo.kyushu-u.ac.jp.

The 2005 Fukuoka earthquake (Mjma7.0, Mw6.6) occurred on 20 March 2005 (local time = UT + 9 hours) in the northwestern offshore of Fukuoka Prefecture, Japan, which shook strongly Fukuoka city and surrounded area. In the Genkai Island near the focal area, especially heavy damage of houses and landslides were reported. Although no station was in the source region because this earthquake occurred in offshore, many strong-motion stations recorded the seismic motion near the source region. Near-source strong-motion records of this earthquake have two remarkable features. One is the presence of a relatively long period pulse with large amplitude on the fault-normal component of the velocity and displacement records, which is the result of the forward rupture directivity. The other is that 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"). We first determined the precise geometry of the fault plane of this earthquake by examining the horizontal particle motion of the main S-wave portion on the records, as follows: the strike is N304E, and the dip angle is 87 degrees. The dip direction of the fault plane is northeast, and the surface intersection of the fault plane passes by the eastern coast of the Genkai Island. We also obtained the relative location of the main rupture starting point with respect to the initial hypocenter, and the mean rupture velocity between them. The distance between them is 5.1 km, the onset of the main rupture is located southeast above the hypocenter. The rupture velocity is 1.4 km/s. It is found that the main rupture began 3.6 sec later from the origin time, at the central point between the hypocenter and the Genkai Island. Our results suggest that the Genkai Island directly suffered the strong effects of the forward rupture directivity during the earthquake.