| 剪切波分裂揭示的青藏高原上地壳地震各向异性基本特征 |
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| 引用本文： | 郭铁龙,高原.剪切波分裂揭示的青藏高原上地壳地震各向异性基本特征[J].地球物理学报,2020,63(3):1085-1103. |
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| 作者姓名： | 郭铁龙 高原 |
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| 作者单位： | 1. 中国地震局地震预测研究所(地震预测重点实验室), 北京 100036;2. 中国地震台网中心, 北京 100045 |
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| 基金项目： | 国家重点研发计划（2016YFC0600302）与国家自然科学基金重点项目（41730212）资助. |
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| 摘 要： | 青藏高原整体隆升,构造运动与介质变形强烈,然而由于地震观测数据不足,青藏高原内部上地壳各向异性研究一直是一个空白.本研究使用西藏地区的地震台网(2009年5月—2017年5月)的观测资料,利用剪切波分裂研究青藏高原上地壳地震各向异性特征.由于青藏高原固定地震台站分布稀疏,可用于进行剪切波分裂研究的近场地震事件记录稀少,本研究采用地震事件的单台定位技术,对公开的地震目录里没有震源深度数据的地震事件进行震源位置约束,并引入微震模板匹配定位方法,对连续地震波形进行检索,识别出地震目录里遗漏的新的微震(小地震)事件波形.微震识别获得的新地震事件记录是地震目录里报告的地震事件记录的大约6倍,用于补充研究区的剪切波分裂数据分析.通过数据分析,对比快波偏振方向,证实微震识别获得的数据极大地增加了有效数据的数量,提高了结果的可靠性.研究结果表明,雅鲁藏布江缝合带与班公—怒江缝合带之间的拉萨地块东部地区,台站的快剪切波(快波)偏振方向主要受区域应力场影响,快波偏振方向主要是NS或NNE方向,表明了区域最大主压应力方向;但个别地震台站(当雄台)快波偏振方向受原地主压应力影响,其快波偏振方向既不平行于断裂走向也不平行于区域主压应力方向,揭示出地壳介质的局部变形导致的局部应力方向不同于青藏块体里的其他地区.研究区西部的改则、普兰和研究区北部的双湖,快波偏振方向显示与断裂等构造走向一致的特点.研究区东部的昌都和察隅,快波偏振方向除了与断裂走向(或构造线)一致,还与地表运动的方向相同,揭示了青藏块体东部的深部物质可能的运移方向.这个现象虽然还需更多的研究证实,但这个发现的重要启示是,地震各向异性结合地表变形可用于探讨地壳深部物质的运动.
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| 关 键 词： | 青藏高原 地震各向异性 上地壳 剪切波分裂 微震识别 地震单台定位 |
| 收稿时间： | 2019-05-13 |
Seismic anisotropy in the upper crust within Tibetan Plateau revealed by shear-wave splitting |
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| GUO TieLong,GAO Yuan.Seismic anisotropy in the upper crust within Tibetan Plateau revealed by shear-wave splitting[J].Chinese Journal of Geophysics,2020,63(3):1085-1103. |
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| Authors: | GUO TieLong GAO Yuan |
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| Institution: | 1. Institute of Earthquake Forecasting, China Earthquake Administration, Beijing 100036, China;2. China Earthquake Networks Center, Beijing 100045, China |
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| Abstract: | The Tibetan Plateau generally uplifts with strong tectonic movement and medium deformation. Due to insufficient seismic observation data, the study of seismic anisotropy in the upper crust in the Tibetan Plateau has been keeping a blank. In this study, we use seismic waveform data from May 2009 to May 2017 recorded by Xizang (Tibetan) seismic networks to investigate seismic anisotropy in the upper crust in the Tibetan Plateau by shear-wave splitting. Due to sparse distribution of seismic stations in the Tibetan Plateau, the near-field seismic events applicable to shear-wave splitting are rare. This study uses single-station location technology to constrain hypocentral location on those events without focal depth information in the published earthquake catalog. In addition, we apply Match and Locate (M&L) method to retrieve the continuous seismic waveform and identify the new microseismic events (small earthquakes) missed in the catalog. The number of new microseismic events obtained by microseismic identification is about six times more than those in the earthquake catalogue, and new data are adopted to analyze shear wave splitting in the study area. Comparing the polarization directions of fast shear-waves (PFS), it is confirmed that new data obtained by microseismic identification greatly increase the number of effective data and improve the reliability of the results. The results show that the PFS in the eastern part of the Lhasa block between the Yarlung Zangbo suture zone and the Bangong-Nujiang suture zone are mainly affected by regional stress field, and the PFS are mainly in NS or NNE direction, indicating the direction of the maximum regional principal compressive stress. However, the PFS of the individual seismic station (Damxung station) is affected by the in situ principal compressive stress, and the PFS is neither parallel to the fault strike nor parallel to the direction of the regional principal compressive stress, revealing the local deformation of the crustal medium. In Gaize and Pulan in the western part of the study area, and in Shuanghu in the northern part of the study area, the PFS are consistent with the strikes of the faults. In Changdu and Zayu in the eastern part of the study area, the PFS are consistent not only with the strikes of the faults (or tectonic lines), but also with directions of the surface movements, revealing the possible migration direction of the deep matter in the eastern part of the Tibetan Plateau. Although this phenomenon still needs verification by more research, it suggests that seismic anisotropy in the upper crust combining with surface deformation data possibly are applicable to explore the movement of deep crustal material. |
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| Keywords: | Tibetan Plateau Seismic anisotropy Upper crust Shear-wave splitting Identification on microseismic event Single-station location of earthquake |
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