Detailed structure of fault planes and stress field of the 2004 mid-Niigata Earthquake (central Japan) from DD hypocenter relocation and focal mechanisms
Abstract
A large earthquake with a magnitude of 6.8 occurred in the central part (Chuetsu district) of Niigata Prefecture, central Japan, on October 23, 2004. The aftershock hypocenter distribution shows two parallel west-dipping alignments, which correspond to the mainshock and the largest aftershock (M6.5) faults, respectively, and an east-dipping alignment, which corresponds to the fault plane of a large aftershock (M6.1) that occurred 4 days after the mainshock (e.g. Okada et al., 2006). In the previous study, we determined focal mechanisms of aftershocks and stress field in the focal area using the stress inversion method (Hondo et al., 2006). Our results suggested that the direction of the maximum principal stress is WNW-ESE and the minimum principal stress is almost vertical, but the misfit value obtained by stress inversion indicates strong heterogeneity of the stress field in the focal area. In this study, we revealed more detailed structure of fault planes including smaller aftershocks by applying double-difference location method (Waldhauser and Ellsworth, 2000) to the travel time catalog and to the travel time differences measured by waveform cross-correlation. Earthquakes used in this study occurred during the period of temporary seismic observation (October 27 to November 22, 2004). These earthquakes were located by Okada et al. (2006) using double-difference tomography method (Zhang and Thurber, 2003). We used their result as initial hypocenter locations. We used data from the temporary aftershock observation network deployed by Tohoku University and the permanent observation stations of Tohoku University, the University of Tokyo, the Japan Meteorological Agency and Hi- net, within 60km of the epicenter. Since the aim of this study is to reveal the distribution of the fault planes smaller than a few kilometers, we divided the aftershock zone into several bins. This procedure allows us to use the event pairs which locates closely to each other and have highly correlated waveforms. Relocated aftershock distribution shows three major alignments of hypocenters which correspond to the mainshock, the largest aftershock and the M6.1 aftershock that occurred on October 27. Additionally we found some smaller aftershock alignments indicating the small-scale fault structure. Some events which have similar waveforms align parallel to the nodal planes of focal mechanisms and that enable us to find more complicated structure of fault planes from hypocenter distribution and focal mechanisms. Although most of the focal mechanisms are reverse fault, we can see other types of focal mechanisms having large misfit values by the stress tensor inversion near the cross points of aftershock alignments. North-western part of the aftershock region also has a lot of focal mechanisms other than reverse fault type. There are a lot of moderate sized aftershocks with a magnitude about 5 and hundreds of aftershocks are localized at shallow part (~5km depth) of this region. This localization of the aftershocks with different type of focal mechanisms indicate the small-scale heterogeneity of stress field.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T41F..07H
- Keywords:
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- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242);
- 8118 Dynamics and mechanics of faulting (8004);
- 8164 Stresses: crust and lithosphere