HR: 17:45h
AN: S34A-08    [Abstracts]
TI: Estimation of Broadband Ground Motion at Ocean-bottom Strong-motion Stations for the 2003 Tokachi-oki Earthquake
AU: * Yamamoto, Y
EM: yosuke@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka, 812-8581 Japan
AU: Takenaka, H
EM: takenaka@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka, 812-8581 Japan
AU: Hirata, K
EM: hiratak@jamstec.go.jp
AF: Program for Deep Sea Research,JAMSTEC, 2-15 Natsuhima-cho, Yokosuka, 237-0061 Japan
AU: Watanabe, T
EM: tomwat@jamstec.go.jp
AF: Program for Deep Sea Research,JAMSTEC, 2-15 Natsuhima-cho, Yokosuka, 237-0061 Japan
AB: The 2003 Tokachi-oki earthquake ($M_{JMA}8.0$) occurred on September 25, 2003 (UT). In this study, we reproduce the broadband ground motion from the earthquake using near-field strong-motion records (accelerograms) at three ocean-bottom stations (KOB1, KOB2 and KOB3) on the sea floor off Kushiro, Hokkaido, installed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The distance and direction from the epicenter to KOB1, KOB2 and KOB3 are 28 km, east-southeast and 83 km,east and 80 km, east-northeast, respectively. Three components (x, y, z) strong motion observation system, enclosed within a cylindrical pressure housing, can record ground motion in broadband frequency range up to DC. The x component is parallel to the axis of the cylinder which is almost horizontal. Since it is suspected that the strong-motion observation systems themselves had moved during the main shock, a simple time-integration of the original acceleration results in wrong velocity and displacement ground motion. So we apply the following processing to the data: We assume that the motion of each strong-motion seismometer can be represented by (1) rotation around the cylinder axis (i.e., roll), (2) tilting of the cylinder (i.e., pitch), and (3) parallel motion. To estimate rotation and tilting, we first use a median-filter for the original records. After the compensation of these movements, the rotated records are integrated into velocity ones. Next, we follow the base-line correction method of Boore (2001) and obtain the ground motion using the amount of submarine upheaval estimated from the two seabed tsunami sensors near KOB1 and KOB3 by Hirata and Baba (2004). By this approach we have successfully obtained broadband velocity and displacement ground motion including DC components. The maximum horizontal (vector resultant) and vertical velocities at KOB1 and KOB3 are estimated to be approximately 160 cm/s, 40 cm/s and 130 cm/s, 20 cm/s, while the corresponding maximum accelerations are approximately 790 cm/s$^2$, 130 cm/s$^2$ at KOB1 and 880 cm/s$^2$, 120 cm/s$^2$ at KOB3, respectively. As for KOB2, since there is no seabed tsunami sensor near it, we assume from consideration of plausible fault models of the main shock that the vertical static displacement is nearly zero, and estimate the maximum horizontal and vertical velocities to be approximately 70 cm/s and 20 cm/s. The corresponding maximum horizontal and vertical accelerations are then approximately 590 cm/s$^2$, 70 cm/s$^2$, respectively. In this presentation, we will show particle velocity, and displacement at the three stations. We used the strong-motion data of JAMSTEC, which is opened through its homepage. (http://www.jamstec.go.jp)
DE: 7294 Instruments and techniques
DE: 7200 SEISMOLOGY
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting