地震研究

基于密集台阵的背景噪声成像是目前研究地壳结构的重要技术手段,背景噪声的噪声源演化规律的分析可加深对噪声互相关中信号的认识。对密集台阵开展聚束分析可对其所记录到的噪声类型、来源方位和季节变化等做出分析。利用2013—2016年ChinArray二期台阵南部322个宽频带地震台的垂直分量数据,计算各台站对的噪声互相关函数(NCF),通过基于NCF的台阵聚束方法对4～30 s的噪声信号做慢度分析,得到了4～8、8～12、12～20、20～30 s 4个不同频段范围的聚束输出图像。结果表明:①在不同的周期频段,噪声信号成分并不相同:在周期较长的频段,信号主要为面波,在周期较短的频段,除了面波信号,还存在一定的体波信号; ②在聚束输出图像上可以直观地观察到不同频段信号成分的变化; ③在不同的周期频段,噪声信号的优势来源方向各不相同,且具有明显的季节变化,其中来源方向对应于北半球海域的噪声能量呈现出冬季较强、夏季较弱的特征,而对应于南半球海域的噪声能量变化趋势与之相反,与全球海洋活动相符。

Seismic ambient noise tomography based on dense seismic array is currently an important method for the investigation of the crustal structure.Analyzing the changes in ambient noise source plays an important role in improving our understanding of the signal of NCF.The NCF-based slowness beamforming technique can study the composition of microseisms,azimuthal anisotropy and seasonal variation.In this study,we calculate the NCF from 2013 to 2016 using the continuous vertical records of 322 broadband seismic stations which are evenly distributed in a square in southern part of the ChinArray Phase II.Using the NCF-based slowness beamforming technique,we carry out direct slowness analysis in the 4-30 s period and obtain the beamforming output in the period bands 4-8 s,8-12 s,12-20 s and 20-30 s respectively.The result shows that the components of ambient noise sources vary in different period bands.In higher-frequency bands the body wave is observed while in lower-frequency bands the body wave is weaker and surface wave signal becomes dominant.In different period bands dominant ambient noise comes from different directions,and varies along with the season.The ambient noise coming from the northern Hemisphere features stronger in Winter,and weaker in Summer.Meanwhile,the ambient noise coming from the southern Hemisphere features weaker in Winter,and stronger in Summer.This seasonality and frequency dependence of noise sources distribution is well correlated with variations of the ocean activity.

引言
1 方法与数据
2 研究结果分析
3 结论

图1 ChinaArray二期台站分布<br/>Fig.1 Distribution of the substations of the ChinArray Phase II

图1 ChinaArray二期台站分布
Fig.1 Distribution of the substations of the ChinArray Phase II

图2 未滤波的NCF<br/>Fig.2 Records of the unfiltered NCF

图2 未滤波的NCF
Fig.2 Records of the unfiltered NCF

图3 单频率聚束输出结果<br/>Fig.3 Slowness beamforming images for single frequency

图3 单频率聚束输出结果
Fig.3 Slowness beamforming images for single frequency

图4 NCF在4个频段的噪声信号整体慢度聚束输出结果(a),及面波(b)和体波(c)信号的能量分布<br/>Fig.4 Slowness beamforming images from NCF in four frequency bands(a),and energy distributions of surface wave(b)and body wave(c)singnals

图4 NCF在4个频段的噪声信号整体慢度聚束输出结果(a),及面波(b)和体波(c)信号的能量分布
Fig.4 Slowness beamforming images from NCF in four frequency bands(a),and energy distributions of surface wave(b)and body wave(c)singnals

图5 4～8 s(a)、8～12 s(b)、12～20 s(c)和20～30 s(d)4个频段月叠加的聚束成像结果<br/>Fig.5 Slowness beamforming images from seasonal stacked NCF in the frequency bands 4-8 s(a),8-12 s(b),12-20 s(c)and 20-30 s(d)

图5 4～8 s(a)、8～12 s(b)、12～20 s(c)和20～30 s(d)4个频段月叠加的聚束成像结果
Fig.5 Slowness beamforming images from seasonal stacked NCF in the frequency bands 4-8 s(a),8-12 s(b),12-20 s(c)and 20-30 s(d)

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备注

引言

1 方法与数据

2 研究结果分析

3 结论

期刊信息