HR: 1340h
AN: S43A-1045    [Abstracts]
TI: Initial Rupture of the 2005 Fukuoka, Japan, Earthquake (Mw6.6): 3D Rupture Focusing Effects?
AU: * Takenaka, H
EM: takenaka@geo.kyushu-u.ac.jp
AF: Dept. Earth & Planet. Sci., Kyushu Univ., Hakozaki 6-10-1, Fukuoka, 812-8581 Japan
AU: Yamamoto, Y
EM: yosuke@geo.kyushu-u.ac.jp
AF: Dept. Earth & Planet. Sci., Kyushu Univ., Hakozaki 6-10-1, Fukuoka, 812-8581 Japan
AU: Nakamura, T
EM: nakamura@geo.kyushu-u.ac.jp
AF: Dept. Earth & Planet. Sci., Kyushu Univ., Hakozaki 6-10-1, Fukuoka, 812-8581 Japan
AU: Toyokuni, G
EM: toyokuni@geo.kyushui-u.ac.jp
AF: Dept. Earth & Planet. Sci., Kyushu Univ., Hakozaki 6-10-1, Fukuoka, 812-8581 Japan
AU: Kawase, H
EM: kawase@arch.kyushu-u.ac.jp
AF: Dept. Human-Env. Studies, Kyushu Univ., Hakozaki 6-10-1, Fukuoka, 812-8581 Japan
AB: The 2005 Fukuoka earthquake (Mw6.6) occurred on 20 March 2005 (JT) in the northwestern offshore of Fukuoka Prefecture, Japan, which shook strongly Fukuoka city and surrounded area. 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. We then obtained the relative location of the main rupture onset 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 mean rupture velocity is 1.4 km/s. It is found that the main rupture began 3.6 sec later from the origin time. The main rupture onset point is located around bottom boundary of the asperity area derived from a source inversion by Asano and Iwata (2005). It suggests that the onset point may correspond to the rupture starting point of the asperity. We estimated the moment release from the initial rupture event and found it corresponds to that of an event of Mw5.6. We further found that these results can be successfully interpreted by using a rupture nucleation theory of Shibazaki and Matsu'ura (1998) and 3D effects of rupture focusing based on a pre-slip model which was studied in numerical simulation by Fukuyama and Madariaga (2000). We obtained the weak-zone size and the critical nucleation-zone size of this earthquake to be 10.5 km and 4.2 km, respectively.
DE: 7200 SEISMOLOGY
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: Fall Meeting 2005