HR: 0800h
AN: S31A-0208    [Abstracts]
TI: High-Resolution Source Imaging of the 2007 Niigataken Chuetsu-oki Earthquake From Dense Strong-Motion Networks: Focus on the First 5 Seconds of the Rupture Process
AU: * Takenaka, H
EM: takenaka@geo.kyushu-u.ac.jp
AF: Dept. of Earth Planet. Sci., Kyushu Univ., Hakozaki 6-10-1, Fukuoka, 812-8581, Japan
AU: Yamamoto, Y
EM: yosuke@geor.or.jp
AF: Geo-Research Institute, Itachibori 4-3-2, Osaka, 550-0012, Japan
AU: Yamasaki, H
EM: yamasaki@geo.kyushu-u.ac.jp
AF: Dept. of Earth Planet. Sci., Kyushu Univ., Hakozaki 6-10-1, Fukuoka, 812-8581, Japan
AB: The 2007 Niigataken Chuetsu-oki earthquake (Mw6.6) occurred on 16 July 2007 (JST) in the western offshore of Niigata Prefecture, Japan, which strongly shook the near-source area and was recorded with dense strong- motion observation networks. 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. The local strong- motion records of this earthquake have a remarkable feature: 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"). In this study we get from these records the high resolution image of the source process during the five seconds after the rupture initiation of this earthquake. To investigate source rupture process, waveform inversions are now popular, but these source inversion methods do not have so high resolution. Recently another approach with high resolution, the back-projection method (e.g., Ishii et al., 2005; Yamamoto and Takenaka, 2006) was exploited, which images earthquake rupture directly from dense network data. The back- projection method is useful to teleseismic array data of an earthquake with very large dimension such as the 2004 Sumatra-Andaman earthquake (Mw9.3), as mentioned by Ishii et al. (2005, Nature). The back-projection method was also adapted for analyzing local strong-motion records of a middle earthquake, the 2005 Fukuoka earthquake (Mw 6.6), by Yamamoto and Takenaka (2006, AGU Fall Meeting). Their imaging technique is useful to middle to large earthquakes. Here we applied their technique to near-source strong-motion records of the 2007 Chuetsu-oki earthquake. We used P-wave portion of UD motion records and derived the following detailed process during the five seconds after the rupture initiation: The rupture plane is a NW-dipping one which is one of the P-nodal planes in the focal mechanism solution. The rupture mainly propagated toward the direction of Kashiwazaki City (around N200E). The main rupture (breaking of the asperity) began at a position of 4 km apart and southwest above the original hypocenter at 2.1 seconds after the origin time. The asperity distributed toward the direction of Kashiwazaki City on the fault plane. This spatial configuration suggests that the rupture front then swept the asperity upward and to the southwest, so that Kashiwazaki City directly suffered the strong effects of the forward rupture directivity ("killer pulse"). (Acknowledgements) We used the strong-motion records supplied by the National Institute for Earth Science and Disaster Prevention (NIED; K-NET, KiK-net, F-net), and the Japan Meteorological Agency (JMA). This study are patially supported by "Special Project for Earthquake Disaster Mitigation in Metropolitan Tokyo Area".
DE: 7200 SEISMOLOGY
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: 2007 Fall Meeting