Rayleigh wave azimuthal anisotropy in western Sichuan region
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摘要: 本文使用川西密集地震台阵记录的面波资料，利用程函方程面波成像方法获得了周期为14—60 s的瑞雷波相速度及方位各向异性分布。结果显示：川滇菱形地块的川西北地块内部的低速异常明显，其下地壳各向异性快波方向以NS向为主，松潘—甘孜地块内部的低速异常稍弱，下地壳各向异性快波方向以NW−SESE向为主，表明川西北地块可能存在下地壳通道流，松潘—甘孜地块内部存在的通道流相对较弱；龙门山断裂带和丽江—小金河断裂两侧的速度结构和方位各向异性均有明显差异，可推测青藏高原内部的地壳流在东部和南部分别受高速、高强度的四川盆地和滇中地块阻挡，沿高原边界带发生了侧向流动；周期大于25 s的面波方位各向异性方向为NW−SE；与SKS分裂优势方向相近，说明四川盆地的剪切波各向异性可能主要源于上地幔；而龙门山断裂带附近壳幔各向异性较为复杂，面波方位各向异性与SKS分裂的NW−SE向弱各向异性存在差异，表明该处的剪切波各向异性可能来自地幔更深处，有待进一步研究。Abstract: Based on the observation data of dense seismic array in western Sichuan, we obtain Rayleigh wave phase velocity and azimuthal anisotropy distribution images at 14−60 s period by eikonal surface wave tomography method. The results show that the low velocity of the northwestern block of the Sichuan-Yunnan diamond block is obvious and anisotropic fast wave direction of the lower crust is dominated by NS. The low velocity anomaly inside the Songpan-Garze block is slightly weaker and the anisotropic fast wave direction of the lower crust is dominated by NW-SE, indicating that there may be lower crust channel flow in the northwestern Sichuan-Yunnan diamond block and there is a relatively weak channel flow inside the Songpan-Garze block. The velocity structure and azimuthal anisotropy are significantly different on both sides of the Longmenshan fault zone and the Lijiang-Xiaojinhe fault zone. We speculate that the crustal flow inside the Tibetan Plateau is blocked by two high-velocity and high-intensity blocks in the Sichuan basin and the Central Yunnan block in the east and south, and lateral flows occur along the boundary zone. The azimuthal anisotropy of the surface wave over the period 25 s by NW-SE is similar to that of SKS splitting. It is concluded that the shear wave anisotropy in Sichuan basin may be mainly from the upper mantle. The anisotropy of the crust in the vicinity of the Longmenshan fault zone is complex and the azimuthal anisotropy of the surface wave is different from the weak anisotropy of SKS splitting by NW-SE, indicating that the shear wave anisotropy may be from the deeper mantle and needs further research.
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图 1 川西地区构造简图（引自邓起东等，2002）
F1：理塘断裂；F2：鲜水河断裂；F3：大凉山断裂；F4：龙门山断裂带；F5：龙泉山断裂；F6：华蓥山断裂；F7：丽江—小金河断裂；F8：安宁河断裂；F9：小江断裂
Figure 1. Tectonic settings of western Sichuan region （after Deng et al，2002）
F1：Litang fault；F2：Xianshuihe fault；F3：Daliangshan fault；F4：Longmenshan fault zone；F5：Longquanshan fault；F6：Huayingshan fault；F7：Lijiang-Xiaojinhe fault； F8： Anninghe fault；F9：Xiaojiang fault
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图 3 川西台阵记录到的地震波形垂直向分量
Figure 3. Vertical component records of western Sichuan seismic array
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图 6 台站KYB06和XYY03附近18 s （a）和50 s （b）周期的方位各向异性拟合实例
Figure 6. Examples of azimuthal anisotropy near the stations KYB06 and XYY03 at the periods of 18 s （a） and 50 s （b）
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图 7 周期为14—60 s的面波相速度和方位各向异性分布
Figure 7. The Rayleigh wave phase velocity and azimuthal anisotropy at period of 14−60 s
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图 8 基阶瑞雷波相速度相对于S波速度随深度变化的敏感核曲线
Figure 8. Depth sensitivity kernels curves fundamental Rayleigh wave phase velocity relative to the shear wave velocity
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图 9 研究区地壳厚度分布图（引自Wang et al，2017）
Figure 9. The distributions of the crustal thickness （after Wang et al，2017）
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图 10 周期为18 s时面波相速度和方位各向异性标准差分布
Figure 10. Standard deviation of surface wave phase velocity and azimuthal anisotropy at the period 18 s
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图 11 周期为18 s （a）和50 s （b）的面波方位各向异性与接收函数各向异性和SKS各向异性的对比
图中接收函数各向异性结果来自Zheng等（2018）；SKS各项异性结果来自Flesch 等（2005），常利军等（2008）和Wang 等（2008）
Figure 11. Comparison of Rayleigh wave azimuthal anisotropy at 18 s （a） and 50 s （b） periods with receiver function anisotropy and SKS anisotropy
The receiver function anisotropy refers from Zheng et al （2018），and SKS anisotropy are from Flesch et al （2005），Chang et al （2008） and Wang et al （2008）
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