Shear-wave splitting in the source region of the 2004 M6.8 Niigata-ken Chuetsu earthquake, central Japan
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. A temporary seismic network composed of 54 stations was installed by Tohoku University two days after the main shock. All stations have 3 components of seismographs with a sampling rate of 100 Hz. The observation lasted for about a month and many small aftershocks were detected. We analyze shear wave splitting using the waveform data obtained by the aftershock observation and investigate anisotropy around the focal area of the main- and after-shocks. We also focus on the difference between the anisotropy within an earthquake fault zone and that in the surrounding area. We use M>1.5 earthquakes that have incident angle of less than 35 degrees. The leading shear wave polarization direction (fast direction) and the lag time of two quasi shear waves (DT) are obtained by applying the cross-correlation method [Ando et al., 1983]. Seismograms are band-pass filtered at 2-8 Hz at the cross-correlation computation. We search for the optimizing pair of fast direction and DT from the two horizontal component of each seismogram using a grid search, in the range of 0-180 degree with increments of 5 degrees for fast direction, and in the range of 0-1s with increments of 0.01s for DT. We set the time windows of the cross-correlation computation to be the first one cycle of the shear wave for each seismogram. We observed shear wave splitting at 37 stations, and the obtained results showed different pattern of shear wave splitting between the northeastern and southwestern parts of the study area. Fast directions are E-W or ESE-WNW at many stations in the southwestern part of the study area. This direction is consistent with the predominant P-axis of the focal mechanisms of microearthquakes [Kosuga, 1999], which suggests that the regional stress field control the anisotropy. However, NE-SW fast directions are dominant at stations in the northern part of the study area inconsistent with the regional stress field. We compared the present results with the seismic velocity structure of the study area determined by Okada et al. [2005]. General feature of the velocity structure is that the western part of the focal area shows lower velocity compared with the eastern part of the focal area. So the low-velocity area corresponds to the area that fast directions are E-W and the high-velocity area corresponds to the area that fast directions are NE-SW. Many aftershocks occurred in the northeast of the main shock and the strikes of major active faults are NE-SW there. Our results with NE-SW fast directions observed in the northeastern part of the study area may be related to local fault alignment. The results obtained in this study suggest the small-scale (20-30km) variation in shear-wave splitting around the focal area of M6.8 Niigata-ken Chuetsu earthquake.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.S51A0973H
- Keywords:
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- 7200 SEISMOLOGY