Fine fault structure of Xinfengjiang water reservoir area from high-frequency ambient noise tomography
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摘要:
广东河源新丰江水库位于华南典型断层上，自1959年水库蓄水以来，库区地震活动性明显增强，并发生了6.1级触发地震.然而我们对其地下结构的了解还远远不够，对库区地震的研究也因此存在很多不确定性.为了研究库区浅层地下结构，我们在2015年初在新丰江库区人字石断裂周边布设了42个短周期地震仪的密集地震台网.台间距大约为100~500 m，时间跨度为1个月.我们对获得的每个台站的连续信号分段进行处理，并对任意台站对的信号进行互相关和叠加处理，恢复所有台站对间的经验格林函数（EGF）.然后利用多次滤波分析方法测量瑞利波群速度频散曲线，并反演该区域的群速度分布和三维剪切波速度结构.我们的结果表明，对于这样比较密集的台站间距，高频（0.2~1.2 s）频散信号基本可以得到很好的恢复，且该频段信号对浅层地下结构（< 1 km）较为敏感.剪切波速度分布表明该区域人字石断裂位置出现明显的低速分布，并且低速区在断裂南端向两侧延伸，有可能代表与人字石断裂交错的小断裂区域.同时，人字石断裂把该区域划分成了东西两部分，两侧整体上呈现高速异常，与地表山丘相吻合.我们的结果得到了新丰江人字石断裂附近的近地浅层的小尺度精细速度结构，为进一步理解该区域的构造以及地震发生提供了重要依据.
Abstract:The Xinfengjiang (XFJ) water reservoir in Guangdong, China locates in Yanshanian granites. After the impoundment of the reservoir in 1959, numerous earthquakes occurred, including the M6.1 one in 1962. XFJ water reservoir is one of the most active seismic zones in Guangdong. However, the underground structure of this area is not yet well understood, and there are uncertainties about the mechanism of reservoir earthquakes. To investigate the shallow crustal structure near XFJ water reservoir area, we deployed 42 short period stations near Renzishi Fault (RZSF), with inter-station distance between 100~500 m in year 2015. We retrieved the empirical Green's function (EGF) of all possible station pairs, and obtained their Rayleigh wave group velocity dispersion. Finally, we inverted Rayleigh group velocity and 3D shear wave velocity beneath RZSF area. Our results show that for such dense spacing stations, high frequency (0.2~1.2 s) dispersion signals can be well retrieved, which are important to resolve shallow crustal structure (< 1 km). Shear wave velocity maps show obvious low velocity anomalies beneath RZSF, and this slow anomaly extend to both sides of the fault in the south. The two sides of the RZSF show relatively high velocities, which may correlate with surficial geology. Our results show fine structures of the distribution and geometry of fractures near the RZSF, and provide important constraints in understanding the geological tectonics and seismicity in XFJ water reservoir area.
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Key words:
- Xinfengjiang water reservoir /
- Dense array /
- Ambient noise /
- Tomography
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图 1
(a) 新丰江库区地理位置示意图.其中红色实线分别代表河源断裂(HYF)、人字石断裂(RZSF)、大坪-岩前断裂(DYF)、石角-新港-白田断裂(SXBF)，红色虚线表示北东东向深部断裂, 灰色圆圈表示自1970年至今该地区发生的3级以上地震及2010—2015年0级以上地震.左下角红色方框中显示了图 1a的位置.靠近新丰江水库的黑色方形即为图 1b的位置；(b)人字石断裂及密集地震台网的分布情况.黑色三角形表示台站位置，红色实线表示人字石断裂.蓝色实线为图 2中台站26和34的射线路径
Figure 1.
(a) The geological setting of XFJ water reservoir area. Red lines in the map are the NNE and NNW faults, which include Heyuan Fault (HYF), Renzishi fault (RZSF), Daping-Yanqian fault (DYF) and Shijiao-Xingang-Baitian faults (SXBF). The NEE fault is represented by red dashed line, since it is an inferred deep fault. The grey circles indicate earthquakes with magnitude greater than 3 since 1970 and earthquakes with magnitude greater than zero from 2010 to 2015. The red rectangular in the lower left-hand shows the region of Fig. 1a. The black rectangular in Fig. 1a next to XFJ reservoir shows the location of Fig. 1b. (b) Distribution of the dense seismic array and the RZSF in XFJ area. Stations and the fault are denoted by black triangles and red line, respectively. The blue line represent the ray path between the station pair 26 and 34 in Fig. 2
图 2
台站26和34之间一个月的瑞利波互相关波形，其在不同滤波频段的信号也同时展示，表现出明显的频散现象
Figure 2.
Example of a one-month cross-correlation between the station pair 26 and 34. The raw cross-correlation (bottom) is filtered by 4 different frequency bands, and are shown in the same figure as comparison. The dispersion of the Rayleigh waves is clearly observed
图 3
台站34与其他所有台站之间的经验格林函数，滤波范围分别为(a) 0.5~10 Hz、(b) 0.5~3 Hz、(c) 1~5 Hz.虚线为零时间线，灰色斜线表示该频段内瑞利波在不同距离的近似到时, 其斜率为区域的平均慢度
Figure 3.
Example of a one-month cross-correlation record-sections between station 34 and all the other stations. These seismograms are bandpass filtered from (a) 0.5 to 10 Hz, (b) 0.5 to 3 Hz and (c) 1 to 5 Hz, respectively. The dashed line is the zero time and thick grey line indicates the approximate arrival times of Rayleigh waves at different distances, whose slope can be approximated as averaged slowness of this area
图 4
不同周期射线路径覆盖图
Figure 4.
Ray path coverage of Rayleigh wave group velocity at four different periods
图 8
100、300、450、650 m深度的剪切波速度图像
Figure 8.
Maps of shear wave velocity at depth of 100 m, 300 m, 450 m and 650 m
图 9
0.2~1.0 s瑞利波群速度对剪切波速度的深度敏感核
Figure 9.
Depth sensitivity kernels for 0.2~1.0 s Rayleigh wave group velocity on the shear wave velocity
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