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Precise aftershock distribution and crustal structure in and around the northern focal area of the 2008 Iwate-Miyagi Nairiku Earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 12 November 2010
Accepted: 7 April 2011
Published: 29 December 2011
The 2008 Iwate-Miyagi Nairiku Earthquake (Mj 7.2) occurred on June 14, 2008, at the eastern flank of the Ou backbone range, in the central part of northern Honshu, Japan. In the northern part of the focal area, seismic reflection/refraction experiments conducted in 2006 and 2007 revealed a series of west-dipping faults. To investigate the relation between these faults and aftershocks, we conducted a high-density seismic array observation across the northern focal area. The arrival times of earthquakes were used in a joint inversion for earthquake locations and velocity structure. The V p structure shows tilted blocks and the block boundaries coincide with the revealed west-dipping faults. These faults were interpreted to have formed during the Miocene extension. The aftershock distribution shows a concentration on a plane dipping westward approximately 40° beneath the eastern margin of the Ou backbone range. The zone of aftershock concentration can be correlated to the known range-bounding fault inferred from seismic profiling and does not coincide with the known active reverse fault (Dedana fault). These results indicate the significance of the potential source fault located along the boundary of the Ou backbone range.
2. Seismic Array Observation
We conducted a high-density, 40-km-long seismic array observation across the northern focal area of the 2008 Iwate-Miyagi Nairiku Earthquake over an eight-day period beginning at 18:00 on July 4 (Kurashimo et al., 2011 in press). 277 seismic stations, approximately 150 m apart, were installed on a survey line in the east-west direction (Fig. 1). 257 stations had a vertical-component velocity seismometer with a natural frequency of 4.5 Hz and the remaining 20 stations had three-component velocity seismometers with a natural frequency of 2.0 Hz or three-component accelerometers. Three-component seismic stations, approximately 1 km apart, were principally installed at the western part of the survey line. To improve the accuracy of hypocenter locations, we additionally deployed 5 three-component seismic stations around the survey line. We used two types of off-line recorders: one was LS8200SD (Kurashimo et al., 2006), and the other was a JGI’s MS2000. Both off-line recorders include a global positioning system (GPS) receiver to maintain the accuracy of the internal clock. Seismic waveform data were continuously recorded at sampling rates of 125 Hz (LS8200SD) and 250 Hz (MS2000), respectively.
We combined our seismic array data with telemetered seismic data obtained by the Earthquake Research Institute, the University of Tokyo, the National Research Institute for Earth Science and Disaster Prevention, and the JMA. We used seven telemetered seismic stations in the present study (Fig. 1). The continuously recorded data were divided into event files, starting from an origin time determined by the JMA. During the seismic array observation, the JMA located 353 earthquakes in a latitude range of 38.9–39.3°N and a longitude range of 140.6–141.3°E (Fig. 1). We selected 135 earthquakes occurring within 7 km distance from the center of the seismic array and having high signal-to-noise ratios. We picked P-wave, and S-wave, arrivals of 135 local earthquake events at 289 seismic stations. 34,027 P-wave and 2,860 S-wave arrival times were obtained and used in an inversion analysis.
4. Travel Time Inversion
5. Results and Discussion
In spite of the clear topographical features, the RBF was not identified as an active fault. The Ou backbone range has been an arc parallel to a major uplifted zone since the Pliocene and is commonly bounded by active reverse faults. There is not much difference in the summit altitudes of the backbone range, whether bounded by active reverse faults or not, suggesting potential hidden active faults beneath the mountain flank.
We conducted a high-density seismic array observation across the northern focal area of the 2008 Iwate-Miyagi Nairiku Earthquake for eight days in order to investigate the aftershock distribution and the crustal structure. We obtained P-wave and S-wave arrival time data from 135 events and used 34,027 P-wave and 2,860 S-wave arrival times for the inversion analysis. The P-wave velocity structure shows westward-dipping block structures. The aftershock plane coincides with the deeper extension of the range-bounding fault estimated by Abe et al. (2008). GPS observation revealed that the upper edge of the coseismic fault was located several kilometers west of the surface trace of the Dedana fault, which is the southern part of the western marginal faults of the Kitakami lowland (Ohta et al., 2008). These results indicate that, in the northern part of the focal area, the 2008 Iwate-Miyagi Nairiku Earthquake occurred along the range-bounding fault, which is located at the eastern margin of the Ou backbone range.
We are grateful to Mamoru Saka, Masaru Noda, Masato Serizawa, Fumiaki Sato, Fumiko Watahiki, Taro Mogi, and Rui Hanada, for data acquisition and wish to express our gratitude to Shinichi Sakai, Aitaro Kato, and Toshihiko Kanazawa for their help with preparations for the aftershock observation. We also wish to thank the National Research Institute for Earth Science and Disaster Prevention: the Japan Meteorological Agency: and the Earthquake Observation Center of the Earthquake Research Institute, the University of Tokyo, for allowing us to use their waveform data. We are very grateful to the reviewers for their useful comments regarding our manuscript. This study was partly supported by the Special Coordination Funds for the Promotion of Science and Technology offered by the Ministry of Education, Culture, Sports, Science and Technology of Japan, (MEXT), titled as the integrated research on the Iwate-Miyagi Nairiku Earthquake in 2008.
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