Abstract: The reconstruction of Green function by cross-correlating long time ambient noise has been extensively used by seismology community and found its applications in many fields such as structural inversion and stress-related velocity monitoring. Analysis on the ambient noise energy, especially its azimuthal distribution and seasonal variation is now becoming more and more important to obtain reliable and precise information from noise cross-correlation function(NCF). In this paper, more than four years vertical records of 33 broadband stations in Ningxia and its adjacent region are cross correlated and stacked monthly to obtain the distribution and variation of noise energy for both 5～10s and 10～20s periods range using normalized background energy flux method. Seasonal variations of strength for both ranges are observed and agree well with the ocean wave activity, which are strong in winter in the northern hemisphere and relatively weak in summer for same hemisphere. But the azimuthal variation are different. For 5～10s noise, the energy mainly comes from the costal line of southeast China. For 10～20s noise the azimuth of the dominant energy has strong seasonal variation. Back projections of the corresponding dominant noise energy azimuth range indicate that the noise field in Ningxia is controlled by several oceans simultaneously but certain ocean may take the main control on the overall noise energy distribution. Due to the none-uniform and none-random properties of noise filed there, we suggest that evaluation of noise field characters should be made before further studies are conducted, especially for time lapse based investigation.

Key words: ambient noise, seasonal variation, normalized energy flux

摘要： 由背景噪声互相关重建格林函数的技术已在地震学中得到了广泛应用，对背景噪声能量的空间分布、强度的季节变化的研究有利于认识噪声互相关函数对格林函数的收敛性。文中利用宁夏及其周边地区33个宽频带台站2008年1月至2012年11月垂直分量的连续记录，计算了两两台站间的互相关函数，并对该地区5～10s及10～20s的背景噪声能量的优势方位及强度随季节的变化进行了分析。结果表明，5～10s的能量主要来自于中国东部的海岸线，其强度具有明显的季节性变化；10～20s的噪声能量不仅强度具有季节性的变化，其优势方向也随季节具有明显变化，受到多个大洋的交替影响。研究表明，宁夏及其邻区的背景噪声场具有较为复杂的特征，在进行后续研究时要予以充分考虑。

关键词: 背景噪声, 季节性变化, 优势能量