Characteristics of ambient seismic noise of broadband seismic array in the southeastern Gansu region
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摘要: 基于甘肃东南地区150个宽频带流动台站2010年的垂直分量连续波形记录，通过计算台站对之间背景噪声的互相关函数并叠加得到5—10s和10—20s两个周期的瑞雷面波信号，并通过信噪比和归一化背景能量流两种方法研究了该地区背景噪声源的时空演化特征.研究结果表明，甘东南地区5—10s和10—20s周期的背景噪声源具有明显的季节变化特征和各自的优势方位.5—10s周期的背景噪声在夏季的能量优势方位为170°—240°，噪声源主要位于印度洋，而冬季为100°—150°，主要位于北太平洋；10—20s周期的背景噪声源则比较复杂，其优势方位受多个大洋的交替影响，夏季噪声源能量优势方位为170°—210°，噪声源主要位于印度洋，冬季为90°—150°和310°—355°，噪声源分别位于北太平洋和北大西洋.由于这两个周期的背景噪声源在甘东南地区存在明显的季节变化，因此在利用背景噪声方法研究该地区介质速度结构时需充分考虑噪声源的非均匀性所产生的影响.Abstract: Using the vertical component records of 150 broadband seismic stations from January to December 2010 in the southeastern Gansu region, this paper calculated the ambient noise cross-correlation functions between each station-pair and stacked them to obtain Rayleigh surface wave signals in the periods of 5-10 s and 10-20 s. And then the method of signal-to-noise ratio (SNR) and normalized background energy flow (NBEF) measurement are adopted to study the characteristics of ambient noise source in southeastern Gansu region based on the asymmetrical features of cross-correlation functions. The results show that the seasonal variations of ambient noise source for both periods of 5-10 s and 10-20 s have their respective azimuthal distribution of noise energy. The azimuth of ambient noise source in the period of 5-10 s is 170°-240° in summer, which means the energy of ambient noise source is mainly affected by the Indian Ocean, and it moves to 100°-150° in winter, which means the energy of ambient noise source is mainly affected by the North Pacific Ocean. In contrast, the characteristics of ambient noise source in the period of 10-20 s is more complex, because the azimuth of ambient noise source is controlled by several oceans. The energy of ambient noise source focuses on the azimuth of 170°-210° in summer, which means it is mainly affected by the Indian Ocean, then it moves to 90°-150° and 310°-355° in winter, which means it is mainly affected by the North Pacific Ocean and the North Atlantic Ocean. Because the ambient noise source of southeastern Gansu region has obvious seasonal variation in both periods of 5-10 s and 10-20 s, inhomogeneous ambient noise source should be taken into consideration when the method of ambient noise is used to study the seismic velocity structure in the region.
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图 1 甘东南地区流动台阵(三角形)分布图
Figure 1. Location of portable array(triangles)in the southeastern Gansu region
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图 2 经过12个月叠加的噪声互相关函数波形图
实线与虚线之间为信号窗口；时间轴上正负分支分别表示同一路径上沿相反方向传播的面波延时
Figure 2. The 12-month stacked cross-correlation function waveforms in the periods of 5-10 s and 10-20 s
Signal window is marked by the solid line and the dashed line; the positive and negative lags present surface wave lapse time along the same path in the opposite direction，respectively
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图 3 5—10 s(a)和10—20 s(b)周期内台站对信噪比方位的冬季(左)和夏季(右)分布图
Figure 3. Azimuth distribution of SNRs for the station-pairs in the winter(left)and in the summer(right)in the periods of 5—10 s(a)and 10—20 s(b)
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