| Title | Spatiotemporal variation of crustal anisotropy in the source area of the 2004 Niigata, Japan earthquake |
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| Author | Cao, L; Kao, H ;
Wang, K; Chen, C; Mori, J; Ohmi, S; Gao, Y |
| Source | Bulletin of the Seismological Society of America 2019 p. 1-12, https://doi.org/10.1785/0120180195 |
| Image |  |
| Year | 2019 |
| Alt Series | Natural Resources Canada, Contribution Series 20180384 |
| Publisher | Seismological Society of America (SSA) |
| Document | serial |
| Lang. | English |
| Media | paper; on-line; digital |
| File format | pdf (Adobe® Reader®); html |
| Area | Niigata; Sea of Japan; Japan |
| Lat/Long WENS | 138.3000 139.5000 37.8000 36.9000 |
| Subjects | geophysics; tectonics; structural geology; seismology; crustal studies; anisotropy; earthquakes; source areas; tectonic elements; deformation; seismic waves; s waves; structural controls; bedrock
geology; structural features; faults; folds; stress analyses; seismological network; pore fluids; pore pressures; 2004 Niigata Earthquake; Niigata-Kobe Tectonic Zone; Okhotsk Plate; Eurasian Pplate; Philippine Sea Plate; Pacific Plate |
| Illustrations | geoscientific sketch maps; location maps; profiles; tables; plots; rose diagrams; focal mechanisms; time series; schematic representations |
| Program | Public Safety Geoscience Assessing Earthquake Geohazards |
| Released | 2019 05 28 |
| Abstract | We investigate the spatiotemporal pattern of crustal anisotropy in the source area of the 2004 Niigata earthquake (M6.8) that occurred in the northern segment of the Niigata-Kobe tectonic zone, central
Japan, by measuring shear-wave splitting parameters from waveform data of local earthquakes. Our results show that the fast polarization directions in the upper crust have spatial variations across the region of the earthquake that are likely caused
by both structural and stress field effects. The northwest-southeast direction near the northeastern end of the source zone (beneath station N.NGOH) and the east-west direction to the southwest (beneath station N.KWNH) are consistent with the spatial
variation of the orientation of the maximum compression of the local stress field. Fast polarization directions at other stations tend to align in the directions of active faults and folds and thus are considered to be structure induced. These
spatial patterns were unaffected by the earthquake. However, at two stations (N.NGOH and N.KWNH) we observe an increase in both the average and scatter of the normalized delay times (delta-t) during the aftershock period. In addition, two stations
(HIROKA and N.YNTH) that are located in the strike-normal direction east of the source area show an increase in the average of the normalized delta-t and a rotation of up to 90° of the fast direction immediately after the mainshock. We also notice
that stations located very close to the source fault (DP.YMK and DP.OJK) show larger average delay times compared with stations farther away (HIROKA and N.YNTH) during the postseismic stage. To explain the temporal changes in the strength of the
anisotropy, we speculate that spatiotemporal variations in microcrack development in and around the source area could be caused by static stress changes due to tectonic deformation and the earthquake rupture. |
| Summary | (Plain Language Summary, not published)
This study focus on the analysis of crustal anisotropy in the source area of the 2004 Niigata earthquake (M6.8) that occurred in the northern segment of
the Niigata-Kobe Tectonic Zone (NKTZ), central Japan. Systematic spatiotemporal pattern is observed through the analysis of the anisotropic property of the upper crust using shear wave splitting measurements. The observed pattern can be explained as
the growth of stress-aligned fluid-saturated microcracks. We propose that microcracks in and around a big earthquake's source area can develop characteristic patterns in response to the change of stress field during the seismic cycle of a major
thrust event. |
| GEOSCAN ID | 313581 |
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