Phase velocity maps of the Taiwan region from the ambient noise tomography of the cross terms in Green’s function tensors
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摘要: 基于我国台湾地区24个宽频带地震台记录到的长达18个月的三分量连续波形数据，采用交叉项互相关方法提取了瑞雷波的经验格林函数张量，进而反演获得了台湾地区6—22 s周期的瑞雷波相速度分布图像，较好地刻画了该地区的地壳速度结构。结果显示：短周期图像上的滨海平原、屏东平原等呈低速特征，西部丘陵、中央山脉和海岸山脉呈高速特征；低速区域随相速度周期的增大而逐渐东移；中长周期图像上，中央山脉南北部均呈低速且南侧的速度较北侧低，表明欧亚板块和菲律宾海板块碰撞后的物质从东北和西南两个方向被侧向挤出时南部可能较北部活跃；台中—南投地区在对应深度附近的高速异常，表明新生代时期的澎湖地台在南海北缘的拉张与碰撞演化过程中，保留了其相对稳定的性质。Abstract: Based on the three-component continuous waveform data recorded by the 24 broadband seismic stations in the Broadband Array in Taiwan for Seismology （BATS） from January 2016 to June 2017, the empirical Green’s function tensors of surface waves are extracted by the cross-term correlation method. Rayleigh wave phase velocity maps from the period 6 s to 22 s are subsequently derived from cross-terms of the Green’s function tensors, which made good description of the crustal velocity structure in Taiwan region. Our results show that the Coastal plain and the Pingtung basin exhibit low-velocity characteristics, while the Western Foothills, Central Range and Coastal Range exhibit high-velocity characteristics in the short-period band. The low velocity zone shifts eastward gradually with period increasing. On the medium and long periods, velocities in southern and northern segments of Central Range are significantly low, indicating the northward and southward material extrusion related to the lithospheric collision between the Eurasian Plate and the Philippine Sea Plate. Besides, the velocities in the southern segment of the Central Rang are slightly lower than that of the northern segment, which may imply the more active southward extrusion. In addition, the high-velocity anomaly near the related depth in Taichung-Nantou area may indicate that the Penghu platform retains its relatively stable state since the Cenozoic Era during the extension and collision evolution of the northern margin of the South China Sea.
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图 1 台湾地区的构造背景和基本构造单元
Figure 1. The tectonic settings and basic tectonic units in Taiwan region
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图 2 垂向（a）和交叉项（b）的互相关函数图像对比
Figure 2. The comparison of cross-correlation functions from vertical-vertical （a） and cross （b） terms
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图 3 垂向互相关CZZ与交叉项互相关CRZ的信噪比SNR对比
Figure 3. SNR comparison between the vertical-vertical correlation CZZ and the cross-term correlation CRZ
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图 4 基于垂向互相关CZZ （红色圆点）与交叉项互相关CRZ （绿色圆点）提取的瑞雷波频散曲线对比
Figure 4. The comparison of Rayleigh wave dispersion curves extracted from vertical-vertical correlation （red dots） and cross-term correlation （green dots）
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图 5 不同周期上的射线分布（三角形表示台站）
Figure 5. Distribution of ray paths for different periods （Triangles represent stations）
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图 6 交叉项互相关在不同周期上的相速度分辨率图像（a−h）和垂向互相关在12 s周期上的分辨率图像（i）
Figure 6. The phase velocity resolution maps of different periods based on cross-term correlation （a−h） and the resolution map based on vertical-vertical correlation for the period 12 s （i）
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