Earth Planets Space, Vol. 58 (No. 1), pp. 75-80, 2006LETTER
Hiroshi Takenaka1, Takeshi Nakamura1, Yosuke Yamamoto1, Genti Toyokuni1, and Hiroshi Kawase2
1Department of Earth and Planetary Sciences, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, Japan
2Department of Architecture and Urban Design, Faculty of Human-Environment Studies, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, Japan
(Received August 11, 2005; Revised November 2, 2005; Accepted November 4, 2005; Online published January 27, 2006)
Near-source strong-motion records of the 2005 West Off Fukuoka Prefecture 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 N304°E, and the dip angle is 87°. The dip direction of the fault plane is northeast, and the surface intersection of the fault plane passes by the eastern coast of Genkai Island. We also obtained the relative location of the onset of the main rupture with respect to the 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 mean rupture velocity along the straight path 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 Genkai Island. Our results suggest that Genkai Island directly suffered the strong effects of the forward rupture directivity during the earthquake.
Key words: 2005 Fukuoka earthquake, fault plane, initial rupture, strong motion.