地震研究

云南地区背景噪声互相关函数中体波信号来源初探^*

(1.中国地震局地球物理研究所,北京 100081; 2.云南省地震局,云南昆明 650224; 3.中国科学院测量与地球物理研究所动力测量重点实验室,湖北武汉 430077)

Study on the Origin of the Body Wave Extracted from Ambient Seismic Noise Cross-correlation Function in Yunnan

WANG Wei-tao¹,YANG Run-hai²,ZHENG Ding-chang²,NI Si-dao³,WANG Bao-shan¹

(1.Institute of Geophysics, China Earthquake Administration, Beijing 100081, China)(2. Earthquake Administration of Yunnan Province,Kunming 650224,Yunnan,China)(3.Key Laboratory of Dynamic Geodesy, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077,Hubei,China)

Keywords：ambient noise; body wave signal; noise source; seasonal variation

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参考文献

利用云南地区43个宽频带地震台站记录的2008～2010年垂直分量数据,计算了台站对间的互相关函数并得到了5～40 s周期的瑞利面波信号。研究发现在5～10 s周期范围内,瑞利面波信号之前存在很强的前驱信号,该信号能量优势频段为0.1～0.2 Hz,其到时接近噪声互相关函数零点,视速度约为30 km/s。该信号到时随季节存在正负交替变化,进一步的质点分析表明该信号为出射角较小的P波信号。参考已有的研究,认为远场地脉动噪声源中的P波信号穿过地球深部到达云南地区,形成了噪声互相关函数中视速度较高的体波信号,并且相关的噪声源位置在冬季和夏季分别位于北太平洋和南印度洋,具有明显的季节性空间变化。

Using vertical component records observed at 43 broad-band seismic stations from 2008 to 2010 in Yunnan, we calculate the noise-cross correlation functions between each station-pair and obtain Raleigh surface wave in the period from 5s to 40s. We find that in the period from 5s to 10s, and before Rayleigh wave, there is a strong precursory wave whose dominant frequency band is between 0.1 Hz and 0.2 Hz and whose arrival time is near the zero of cross correlation functions. Moreover its negative arrival-time in Winter alternates with the positive ones in Summer. Through further analysis of particle-motion, we confirm that this wave is P-wave with smaller reflection angle whose apparent velocity is about 30km/s. Referring to the relevant research results, we conclude that originating from the ambient micro-tremor noise source at a teleseismic distance, P-wave travels through deep earth to Yunnan and reaches a higher apparent velocity. The ambient noise source, which locates in South Indian Ocean in Summer and North Pacific Ocean in Winter, varies seasonally and spatially.

引言
1 噪声互相关计算与结果
2 瑞利面波前驱信号分析
3 讨论与结论

图1 云南地区噪声互相关函数计算中使用台站的分布<br/>Fig.1 Distribution of the stations used in calculating ambient noise cross correlation in Yunnan

图1 云南地区噪声互相关函数计算中使用台站的分布
Fig.1 Distribution of the stations used in calculating ambient noise cross correlation in Yunnan

图2 部分路径上经过2008～2010年3年叠加得到的噪声互相关函数波形(a)周期为5～10 s的信号;(b)周期为10～40 s的信号<br/>Fig.2 Noise-cross correlation functions stacked for three years from 2008 to 2010 in partial path (a)Signals in 5～10 s period band; (b)Signals in 10～40 s period band

图2 部分路径上经过2008～2010年3年叠加得到的噪声互相关函数波形(a)周期为5～10 s的信号;(b)周期为10～40 s的信号
Fig.2 Noise-cross correlation functions stacked for three years from 2008 to 2010 in partial path (a)Signals in 5～10 s period band; (b)Signals in 10～40 s period band

图3 多条路径上前驱信号的归一化频谱图<br/>Fig.3 Normalized power spectrum of the precursors in various path

图3 多条路径上前驱信号的归一化频谱图
Fig.3 Normalized power spectrum of the precursors in various path

图4 2008～2010年前驱信号到时随月份的变化(a)JIG-ZAT路径;(b)LAC-YUM路径<br/>Fig.4 Variation of the precursor's arrival time from 2008 to 2010 with month(a)Path of JIG-ZAT;(b)Path of LAC-YUM

图4 2008～2010年前驱信号到时随月份的变化(a)JIG-ZAT路径;(b)LAC-YUM路径
Fig.4 Variation of the precursor's arrival time from 2008 to 2010 with month(a)Path of JIG-ZAT;(b)Path of LAC-YUM

图5 LAC-YUM路径上冬季和夏季三分量互相关函数波形及质点运动轨迹(a)5～10 s周期范围内的互相关函数波形;(b)5～10 s周期范围内前驱信号在冬季的质点运动轨迹;(c)5～10 s周期范围内前驱信号在夏季的质点运动轨迹;(d)10～20 s周期范围内的互相关函数波形;(e)10～20 s周期范围内的瑞利面波信号在冬季的质点运动轨迹;(f)10～20 s周期范围内的瑞利面波信号在夏季的质点运动轨迹
Fig.5 Waveforms of three components cross correlation functions for path LAC-YUM in winter and summer and the partial motions for precursors and Rayleigh waves respectively (a)Waveforms of cross correlation functions within 5～10 s periods;(b)Partial motions for precursors in winter within 5～10 s periods;(c)Partial motions for precursors in sunmer within 5～10 s periods;(d)Waveforms of cross correlation functions within 10～20 s periods;(e)Partial motions for Rayleigh wave in winter within 10～20 s periods;(f)Partial motions for Rayleigh wave in summer within 10～20 s periods

图6 P波信号的视速度与水平慢度分析(a)平面波以出射角θ出射地表的示意图(实线箭头表示射线路径,虚线表示平面波的波前); (b)以IASPEI91模型计算的不同类型的P波在不同角距离上的水平慢度<br/>Fig.6 Apparent velocity and horizontal slowness anylysis of different P wave (a)A plane wave emerges on a horizontal surface with the emergent angle θ.(arrow with solid line:the ray path,dashed line:the wavefront of the plane wave);(b)Horizontal slowness for different type P waves with respect to angular distance of source and receiver based on IASPEI91 model

图6 P波信号的视速度与水平慢度分析(a)平面波以出射角θ出射地表的示意图(实线箭头表示射线路径,虚线表示平面波的波前); (b)以IASPEI91模型计算的不同类型的P波在不同角距离上的水平慢度
Fig.6 Apparent velocity and horizontal slowness anylysis of different P wave (a)A plane wave emerges on a horizontal surface with the emergent angle θ.(arrow with solid line:the ray path,dashed line:the wavefront of the plane wave);(b)Horizontal slowness for different type P waves with respect to angular distance of source and receiver based on IASPEI91 model

图7 云南地区噪声相关函数中面波前驱信号的可能来源(黑色三角和倒三角为Landès等(2010)得到的5～10 s噪声源在夏季和冬季的可能位置; 黑色五角星表示泰国CMAR台阵的位置)<br/>Fig.7 Possible mechanics for the strong precursors observed in noise-cross correlation function in Yunnan region (Filled triangles and inverse triangles:the sources for 5～10 s microseisms for summer and winter respectively obtained by Landeès et al(2010); Filled star:the position of CMAR array in Thailand)

图7 云南地区噪声相关函数中面波前驱信号的可能来源(黑色三角和倒三角为Landès等(2010)得到的5～10 s噪声源在夏季和冬季的可能位置; 黑色五角星表示泰国CMAR台阵的位置)
Fig.7 Possible mechanics for the strong precursors observed in noise-cross correlation function in Yunnan region (Filled triangles and inverse triangles:the sources for 5～10 s microseisms for summer and winter respectively obtained by Landeès et al(2010); Filled star:the position of CMAR array in Thailand)