Rayleigh wave group velocity tomography beneath Dabie-Sulu and its adjacent areas from ambient seismic noise
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摘要: 大别-苏鲁造山带是中国大陆东部地区最重要的构造带之一. 为了研究该地区上中下地壳的速度结构， 选用国家数字地震台网和中国区域地震数据台网5省(山东、 安徽、 江苏、 河南和湖北)连续两年(2009年5月—2011年5月)的垂直向地震记录， 进行背景噪声互相关处理， 叠加得到了台站对间的面波经验格林函数. 采用多重滤波法提取了近4000条频散曲线， 并反演得到了研究区10—25 s的瑞雷波群速度分布结果. 通过分析大别-苏鲁及其邻区的瑞雷波群速度结构图像， 发现不同构造块体具有不同的瑞雷波群速度结构： ① 研究区内的郯庐断裂带及其周边地区包括鲁西地块和胶北地块上中下地壳均表现出明显的高速异常， 可能是在拉张环境下岩浆岩上涌， 导致高速、 高密度的变质岩在地壳富集而形成； ② 苏鲁高压变质带的瑞雷波速度在10—25 s周期内明显高于其它地区， 其上中下地壳均表现出较高的群速度结构特征， 认为苏鲁高压变质带至少延伸到下地壳， 而大别造山带在10 s时表现出高速特征， 但在15—25 s没有明显的高速特征， 故无法从其结果中判断大别高压变质带的垂向延伸范围； ③ 华北板块上中下地壳均表现为低速特征， 体现了研究区内华北板块的大陆地壳减薄特征.Abstract: The Dabie-Sulu orogenic belt is one of the major structural belts in eastern China. In order to obtain the S-wave velocity structure beneath the study area, we obtain Rayleigh wave empirical Green’s functions after cross-correlation and stacking between all possible station pairs using continuous vertical components seismograms recorded by Shandong, Anhui, Jiangsu, Henan and Hubei seismograph networks and China Broadband Digital Seismograph Network during May 2009 to May 2011. Then we extract nearly 4000 Rayleigh wave group velocity dispersion curves by using the multiple filtering method, and group velocity maps at 10—25 s are reconstructed by inversion. The ambient noise group velocity maps reveal that there are evident Rayleigh wave group velocity differences among different tectonic regions. ① Tanlu fault and its surrounding areas including Luxi block and Jiaobei block exhibit higher velocity anomaly in the crust, which may be caused by the enrichment of the higher velocities and higher density metamorphite rocks in the crust in the extensional environment. ② Rayleigh wave group velocity is apparently higher beneath the Sulu high-pressure metamorphic belt at the period of 10—25 s. Upper, middle and lower crust presents higher group velocity structure, which implies that the metamorphic belt extends probably down to the lower crust. On the other hand, it is difficult to define the range of the Dabie high-pressure metamorphic belt, because high velocity appears only at the period of 10—15 s. ③ Lower velocity characteristic of upper, middle and lower crust beneath North China block indicates that the crust there is thinning.
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Keywords:
- Dabie-Sulu orogenic belt /
- ambient noise /
- Rayleigh tomography /
- crustal structure
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图 1 研究区主要构造背景和台站(红色三角形)分布图(构造线根据杨文采和余长青(2001简化得到) Ⅰ . 大别造山带；Ⅱ . 华北板块；Ⅲ . 苏鲁造山带；Ⅳ . 扬子板块．A. 鲁西地块；B. 胶北地块；C. 苏鲁高压变质带；D. 大别高压变质带
Figure 1. Regional tectonic setting of the study area and location of stations (red triangles) where the tectonic lines are simplified from Yang and Yu (2001 Ⅰ . Dabie orogenic belt; Ⅱ . North China block; Ⅲ . Sulu orogenic belt; Ⅳ . Yangtze block; A. Luxi block; B. Jiaobei block; C. Sulu high-pressure metamorphic belt; D. Dabie high-pressure metamorphic belt
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图 2 以JYU台为中心的互相关波形图(经5—50 s滤波)
Figure 2. Cross-correlation functions between the station JYU and other stations (bandpass filtered between 5—50 s)
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图 3 瑞雷波经验格林函数及其频散曲线示意图 (a) JYU-GAY台站对的大圆弧路径；(b) 由多重滤波法提取的瑞雷波群速度频散曲线；(c) 沿图(a)的经验格林函数不同频带滤波后的结果
Figure 3. Schematic diagram of Rayleigh wave empirical Green’s functions and dispersion curves (a) Great circle ray path between the station-pair JYU and GAY; (b) Rayleigh wave group velocity dispersion extracted by multiple filtering method; (c) Empirical Green’s functions filtered in different frequency bands along the path in Fig.(a)
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图 4 各周期射线密度分布(左)和横向分辨率(右)
Figure 4. Ray density maps (left) and lateral resolution (right) calculated for different periods
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