Surface wave tomography of Guangdong and its adjacent areas from ambient seismic noise
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摘要: 通过收集广东及其邻域104个固定地震台站近10个月的垂直分量连续波形数据资料， 使用地震背景噪声互相关格林函数方法， 获得了大部分台站对的背景噪声互相关曲线. 基于这些对称叠加的互相关曲线， 利用时频分析方法， 进一步提取了该地区周期为5—40 s的基阶瑞雷波群速度频散曲线. 其噪声来源分析结果显示: 广东及其邻域的噪声场来源有很强的方向性， 短周期(5—10 s)噪声主要来自东南方向， 范围基本与海岸线分布一致， 可能是由于近海水陆相互作用产生的；较长周期 (15—30 s) 噪声主要来自三大洋的方位. 以这些提取的噪声面波资料为基础， 采用噪声面波层析成像方法反演得到了该地区周期为5—28 s的瑞雷波群速度层析成像图， 从该图可以看出， 广东及其邻域地下结构的横向变化总体较小， 沉积层厚度较薄， 地壳中可能普遍存在一个低速层；从研究区历史地震的分布及其表层地质构造的发育特征来看， 地震主要分布在高、低速过渡带附近， 表明面波群速度与地震之间具有较强的耦合关系；从群速度的低速异常特征来看， 广东及其邻域普遍分布的温泉和高地热主要受深部构造的控制和影响.Abstract: We obtain the ambient noise cross-correlation curves for most of the station-pairs using 10 months of continuous data from the 104 permanent seismic stations of the Guangdong seismic network and its surrounding regions. Based on these symmetrically superimposed cross-correlation curves, we complete Rayleigh wave group velocity dispersion measurements for the periods from 40 s down to about 5 s using time-frequency analysis method. Ambient noise source analyses show that in Guangdong and its neighbor areas, sources of noise field has a strong directionality. Short-period noise (5—10 s) is mainly from the southeast, and its distribution is consistent with the scope of the coastline, so it may be caused by interaction of coastal land and water. Longer-period noise (15—30 s) mainly comes from the orientation of the Pacific, Atlantic, Indian Oceans. Furthermore， we obtain the Rayleigh wave group velocity distribution maps in the whole studied area for the periods from 5 s to 28 s using the ambient surface wave tomography algorithm. The tomographic results revealed that in Guangdong and its adjacent areas the crustal lateral variation is small, and the surface sediment thickness is thinner, indicating that there may be a widespread low-velocity layer in the crust. According to the characteristics of history earthquakes and surface geological structure, it can be obtained that earthquakes are mainly distributed in the vicinity of high- and low-velocity transitional zone, suggesting there is a strong coupling relationship between surface wave velocity and earthquakes. From low group velocity anomaly, it is inferred that the high geothermal and hot springs widely distributed in Guangdong and its neighbor areas is mainly under the control and influence of the deep structure.
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图 1 用于噪声互相关提取的台站分布图细黑线为省界，粗黑线为断层分布(邓起东，2007)，白色粗虚线为从CHZ台到SCD台的大圆路径
Figure 1. Distribution of seismic stations used for ambient noise correlation Thin dark lines denote provincial boundaries，thick dark lines denote faults(Deng，2007)，the thick white dashed line from the station CHZ to SCD indicates the selected great circle path for Fig.2
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图 2 瑞雷波经验格林函数、噪声强度方位分布及噪声互相关频散曲线测量图 (a)MEZ台与其它台站之间的噪声互相关曲线；(b)CHZ-SCD台站对之间的经验格林函数在不同周期段的带通滤波；(c)不同周期段的噪声源方位分布；(d)CHZ-SCD台站对之间的瑞雷波群速度频散曲线的测量．白色实线表示实测群速度值，黑色虚线表示基于华南地壳速度结构模型(郑圻森等，2003)的理论群速度值
Figure 2. Example of Rayleigh wave empirical Green’s function，orientation distribution of noise intensity and dispersion measurements obtained from ambient seismic noise correlations (a)Ambient noise correlations between the station MEZ and other stations;(b)Empirical Green’s function filtered in different frequency b and s for the CHZ-SCD path;(c)Orientation distribution of noise intensities in different frequency b and s；(d)Rayleigh wave group velocity dispersion curve for the CHZ-SCD path．The white solid line represents the measured value，the black dashed curve is the theoretical value from the South China crustal velocity model(Zheng et al，2003)
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图 3 不同周期测量得到的频散路径数分布(a)和0.25°×0.25°网格条件下周期为10 s时的频散路径密度分布(b)．在周期为5—28 s范围内，频散路径的密度分布在空间上类似
Figure 3. (a)Distribution of the number of dispersion paths for different periods;(b)Ray density map for the period of 10 s，the ray density is the number of rays passing through a 0.25 degree by 0.25 degree cell．In the period range of 5—28 s the ray coverage is similar in space
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图 4 1°×1°分辨率的检测板(a)和华南地壳速度结构模型及其群速度随深度敏感核(b)
Figure 4. >(a)The checkboard for 1°×1° resolution;(b)The sensitive kernel of group velocity with depth based on the South China crustal velocity model(left panel)
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图 5 周期为5，8，15 s和28 s的检测板层析成像结果
Figure 5. The tomography results of checkboard test at periods of 5，8，15 and 28 s
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