Ambient noise Rayleigh wave tomography in the northeastern Tibetan Plateau
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摘要: 利用青海、甘肃和宁夏3个区域地震台网两年的波形数据，通过地动噪声层析成像方法给出了青藏高原东北缘8—40 s的瑞雷面波二维群速度结构和三维地壳上地幔顶部的剪切波速度结构。研究结果显示，相比传统的面波层析成像方法，噪声层析成像给出的短周期面波信息能够较好地约束地壳和上地幔顶部结构。8 s和10 s周期的群速度图像与沉积层和基底结晶深度有关，具体而言：祁连山和西秦岭造山带表现为高速体特征，柴达木盆地、河西走廊和鄂尔多斯西缘表现为低速特征；周期为15 s和20 s的瑞雷面波群速度图像反映的是中地壳结构，15 s周期群速度图像上造山带下方高速体向北和向东方向扩展，20 s周期群速度图像呈现大范围低速体，推测为中下地壳低速层影响所致；30 s和40 s周期的群速度图像反映的是莫霍面深度附近的速度结构，具体表现为青藏高原大范围的低速体，向北和向东逐渐表现为高速体，说明青藏高原的莫霍面深度较深，且向北和向东逐渐减薄。另外，三维剪切波速度结构显示祁连地块和甘孜地块中地壳存在大范围低速层，且由柴达木盆地东侧深部的低速层连接，该低速层可能是青藏高原物质北移的一个通道。Abstract: Two years of seismic ambient noise observed by three provincial networks are used to estimate 2D Rayleigh wave group velocity and 3D shear wave velocity structure of northeastern Tibetan Plateau. Compared with traditional surface wave tomography, ambient noise tomography may provide refined structure of the crust and uppermost mantle. Low group velocities of 8 s and 10 s periods coincide with sedimentary layer and crystalline basement, while the high velocity zones denote igneous cores of the major mountain ranges. Qilian mountain and Qinling mountain show high velocity, but Qaidam basin, Hexi corridor of Gansu Province and western Ordos block show low velocity. 15 s and 20 s period group velocities represent the structure of middle crust, and the high velocity zones expand to north and east in 15 s period group velocity map. Whereas in the 20 s period group velocity map most areas show low velocity anomaly, which may correspond to the low velocity crustal layer revealed by previous studies. 30 s and 40 s period group velocity maps represent the velocity structure around the depth of Moho. The velocity maps show low velocity zones in Tibetan Plateau and high velocity towards the north and the east, suggesting that the crust is thick in Tibetan Plateau and thinned in the northern and the eastern studied areas. In addition, 3D shear velocity structure shows low velocity zone beneath Qilian orogen and Garze terrain. The two zones are connected by a lower crustal low velocity zone to the east of Qaidam basin, which may be a channel of northward material movement.
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图 1 青藏高原东北缘地震台站分布及区域构造简图
F1：青铜峡—固原断裂；F2：六盘山断裂；F3：海原断裂；F4：门源断裂；F5：西秦岭北缘断裂；F6：东昆仑断裂；F7：主峰断裂；F8：龙门山断裂带
Figure 1. Distribution of seismic stations and regional tectonic settings in northeastern Tibetan Plateau
F1：Qingtongxia-Guyuan fault；F2：Liupanshan fault；F3：Haiyuan fault；F4：Menyuan fault；F5：North edge of west Qinling fault；F6： Eastern Kunlun fault；F7：Zhufeng fault；F8：Longmenshan fault zone
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图 2 玉树台与其它台站噪声互相关结果展示图
Figure 2. Demonstration of cross correlations of long time waveforms between the station YUS and other ones
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图 3 不同周期T上台站对的互相关信噪比统计图
Figure 3. Histogram of cross correlation signal-noise-ratio between station pairs at different periods T
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图 4 青海德令哈台与青海花土沟台经验格林函数（上）及其时频分析图像（下）
Figure 4. Empirical Green’s function （top） between the stations DLH and HTG and their frequency-time analysis diagram （bottom）
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图 5 8—40 s周期群速度成像所使用的射线路径分布
Figure 5. Distribution of ray paths used in the group velocity tomography for periods from 8 s to 40 s
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图 6 周期为8，10，15，20，30 和40 s的瑞雷面波群成像检测板结果
Figure 6. Checkerboard test of Rayleigh wave group velocity tomography at periods 8，10，15，20，30 and 40 s
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图 7 周期为8 s （a），10 s （b），15 s （c），20 s （d），30 s （e）和40 s （f）的瑞雷面波群速度图
Figure 7. Rayleigh wave group velocity map at periods 8 s （a），10 s （b），15 s （c），20 s （d），30 s （e） and 40 s （f）
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图 8 青藏东北缘地区的地壳厚度分布图
黑点表示计算接收函数所用台站
Figure 8. Distribution of crustal thickness in the northeastern Tibetan Plateau
Black dots are seismic stations used for calculating receiver functions
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图 9 5—55 km深度范围内的剪切波速度结构
Figure 9. Shear wave velocity in the depth range of 5—55 km
（a） 5—10 km；（b） 10—15 km；（c） 20—25 km；（d） 30—35 km；（e） 40—45 km；（f） 50—55 km
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