Imaging heterogeneous velocity structures and complex aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake by a dense seismic observation

Abstract

The velocity structure and accurate aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake (thrust type) are obtained by inverting the arrival times from 848 aftershocks observed by a dense seismic network deployed immediately after the mainshock (8 h later). Both the detailed velocity structure and the accurate aftershock distribution show lateral heterogeneity along the fault strike. In the northeast area, aftershocks are aligned along both the NW-and SE-dipping planes. These planes are conjugate to each other. The mainshock hypocenter is located close to the bottom of an approximately 50• NW-dipping plane, which indicates that the mainshock rupture could have initiated on the NW-dipping plane. The high-V p body beneath this aftershock alignment shows a convex upward shape. In contrast, from the center to the southwest area, most of the aftershocks are aligned along SE-dipping planes. The high-V p body beneath this aftershock alignment shows a convex downward shape. Based on these results, we suggest that the crustal structure in the source region is divided into two segments by a boundary zone situated between the northeast and southwest areas. It should be noted that this segment boundary zone is coincident with the complex aftershock zone where numerous conjugate fault planes exist. We propose that the mainshock rupture initiated near the bottom of the NW-dipping fault plane and ran to the southwest, then transferred at the segment boundary zone which has numerous conjugate fault planes to the SE-dipping plane.

Rupture process at the beginning of the 2007 Chuetsu-oki, Niigata, Japan, earthquake

“…Similar patterns of aftershocks were also observed by Sakai et al (2007), Shinohara et al (2008), and Yukutake et al (2008). Based on the configuration of the mainshock hypocenter and the aftershock distribution, Kato et al (2008) speculated that rupture of the mainshock may have initiated on the NW-dipping plane and then transferred to the conjugated fault plane (i.e., the SE-dipping plane).…”

“…However, the aftershock distribution may not be so simple that a single fault plane would be identified. Kato et al (2008) inverted the arrival times from aftershocks observed by a dense seismic network deployed immediately after the mainshock and Copyright c 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.…”

Stress field in the source region after the 2007 Mw 6.6 Niigataken Chuetsu-oki earthquake deduced from aftershock focal mechanisms: Implication for a pre-mainshock stress field

“…In particular, the former two groups of researchers found that aftershocks in the northern region were distributed along both the northwestwardand southeastward-dipping planes. Furthermore, Kato et al (2008) revealed a complex aftershock distribution with numerous conjugated fault planes in the central aftershock region, which corresponds to the F-F cross-section in Fig. 3.…”

“…Yukutake et al (2008) estimated aftershock distributions of the 2007 earthquake by applying a double-difference method (Waldhauser and Ellsworth, 2000) to both manually picked and waveform cross-correlation data which were observed at permanent online stations. Kato et al (2008) determined the velocity structure and aftershock distributions of the 2007 earthquake by applying a double-difference tomography method (Zhang and Thurber, 2003) to arrival times observed by a dense seismic network deployed immediately after the mainshock. The aftershock distributions determined in the above two studies present a sharper image than we obtained in our present study and reveal several discrete fault segments.…”

“…The focal mechanisms of the mainshock and the largest aftershock were reported by several institutes (e.g., Japan Meteorological Agency, Harvard University, and U.S. Geological Survey) and described as a pure reverse faulting with a strike in the NNE-SSW direction. The relocated aftershocks suggest that aftershocks were mainly distributed along the southeast dipping planes and that a northwest dipping plane might also exist near the hypocenter of the mainshock (Kato et al, 2008;Yukutake et al, 2008).…”