摘要
通过收集鄂尔多斯块体西缘固定地震台网2010年6月至2017年8月的近场地震资料,选择符合剪切波分裂分析的14个台站记录的共137个有效事件波形,得到了剪切波分裂参数,即快剪切波(简称快波)偏振方向和慢剪切波(简称慢波)时间延迟.结果表明,研究区的快波偏振方向和慢波时间延迟具有明显的分区特征,快波偏振方向主要与构造应力场方向或者断层走向大体一致.鄂尔多斯西缘紧邻块体边界的台站,快波偏振方向自北向南呈现NS、NNE、NE向的变化,与青藏高原东北缘主压应力方向变化基本一致.银川地堑东西两侧的快波偏振方向有差异,东侧区域主要受青藏高原NNE向挤压和黄河—灵武断裂共同影响,而西侧区域可能受到阿拉善块体与鄂尔多斯块体之间的NW方向的主张应力和阿拉善块体内部应力分布的影响;鄂尔多斯块体、阿拉善块体与青藏高原的交汇区快波优势偏振方向为NE向,与青藏高原东北缘主压应力方向一致;海原断裂带及以南区域快剪切波优势偏振方向为WNW向,与断裂走向基本一致,较好的说明了海原断裂带为活跃的活动断裂.构造与断裂分布都是控制快波偏振方向的主要因素,走滑断裂上的台站快波偏振方向与断裂走向一致,表明这些台站主要受到断裂的强烈影响;走滑断裂附近的个别台站快波偏振方向呈现与构造应力场一致的方向,表明几乎没有受到断裂的影响.鄂尔多斯、阿拉善与青藏高原的交汇区平均时间延迟高于其他地区,反映了青藏高原在NE向运动过程中,受到稳定的鄂尔多斯块体阻挡作用,导致了交汇区地壳介质各向异性程度增加.以海原断裂带到六盘山断裂带为界,其两侧区域的各向异性差异性明显,揭示了应力与介质特性的差异,暗示其邻近区域,特别在海原断裂带东端到六盘山断裂带与鄂尔多斯块体西缘交汇区域,可能有较高的强震危险背景.本研究还对该区域的地壳和上地幔的耦合问题进行了初步讨论.
Near-field earthquake data from June2010 to August 2017 recorded by seismic networks in the western edge of the Ordos block are collected, from which137 waveforms of seismic events at14.stations are chosen for shear-wave splitting analysis, yielding shear-wave splitting parameters, i.e. the polarizations of fast shear-waves (PFS) and the time delays of slow shear-waves. The results show that the PFS in the research area have a distinct zoning feature, largely consistent with the direction of the tectonic stress field or the fault strike. The PFS at stations in the western Ordos block, where are close to the block bordary, exhibits changes in NS, NNE and NE directions from north to south, which accords with the variation of the principal compressive stress in the northeastern Tibetan Plateau. The PFS is different in the eastern and western Yinchuan graben, the former is chiefly influenced by the NNE-directed extrusion from the Tibetan Plateau and the Huanghe-Linwu fault, and the latter is influenced by the principal compressive stress in NW direction between the Alax block and Ordos block and by internal stress distribution of the Alax block. The dominant direction of PFS in the intersection among the Ordos block, Alax block and Tibetan Plateau is in NE direction,consistent with the direction of the principal compressive stress of the northeastern margin of the Tibetan Plateau . The PFS dominant direction along the Haiyuan fault zone and its south is in WNW direction, which accords with the fault strike, suggesting the Haiyuan fault is active. Both tectonics and faults distribution are the main controlling factors to the PFS. The PFS of stations at the strike-slip faults are consistent with trends of these faults, which indicates that these stations are controlled by the faults, and the PFS of several other stations near the faults have the same direction with the tectonic stress, suggesting these stations are almst not affected by faults. The average delay time of slow shear-waves in the convergence zone of the Ordos block, Alax block and Tibetan Plateau are higher than other areas, suggesting that the degree of seismic anisotropy increases within this zone which is blocked by the Ordos block during the movement of the northeastern Tibetan Plateau towards northeast. Bounded by the Haiyuan fault zone to Liupanshan fault zone, the crustal seismic anisotropy on the both sides of the faults shows obvious differences, revealing the difference of stress and medium properties. It implies there may be risk of major earthquakes in adjacent areas, especially from the eastern end of the Haiyuan fault zone to the convergence zone between Liupanshan fault zone and the western Ordos block. In addition, the crust-upper mantle coupling in the research area is also preliminarily discussed in this paper.
作者
许英才
高原
石玉涛
王琼
陈安国
XU YingCai;GAO Yuan;SHI YuTao;WANG Qiong;CHEN AnGuo(Earthquake Administration of Ningxia Hui Autonomous Region,Yinchuan 750001,China;Institute of Earthquake Forecasting,China Earthquake Administration,Beijing 100036,China)
出处
《地球物理学报》
SCIE
EI
CAS
CSCD
北大核心
2019年第11期4239-4258,共20页
Chinese Journal of Geophysics
基金
国家自然科学基金项目(41474032)
宁夏自然科学基金项目(2019AAC03260)共同资助
关键词
鄂尔多斯块体西缘
剪切波分裂
地壳各向异性
主压应力
银川地堑
海原断裂带
West margin of Ordos block
Shear wave splitting
Crustal anisotropy
Principal compressive stress
Yinchuan graben
Haiyuan fault zone
