背景噪声频散曲线测定及其在华北地区的应用

Measurement of Rayleigh wave dispersion from ambient seismic noise and its application in North China

利用华北地震台阵垂直分量的观测资料,采用滑动绝对平均方法对资料进行预处理,通过互相关方法从背景噪声中提取瑞利面波的格林函数,开发了群速度频散曲线的自动提取工具,测量了位于华北盆地、燕山隆起和太行山隆起的3条频散曲线,反演得到了3个区域的S波速度结构.研究分析表明,滑动绝对平均方法可以有效降低地震和台站附近干扰源的影响.为了得到可靠的层析成像结果,应计算格林函数的信噪比,选择高信噪比的格林函数测量其频散曲线,进行层析成像反演.当信噪比大于7时,一般都能得到稳定可靠的频散曲线.群速度频散曲线的最大可信周期(Tmax)与台站间距有关,华北地区最大可信周期以不超过台站间距的1/12为宜,周期大于Tmax时不同月份测得的频散曲线变化较大.

Vertical component ambient seismic noise recorded by North-China Seismic Array was pre-processed with ＂running-absolute-mean＂ normalization method, and Rayleigh wave Green＇s Function was extracted from ambient seis- mic noise records with cross-correlation method. We have developed an auto- matic procedure to measure Rayleigh wave group velocity. Three typical disper- sion curves obtained in the region of North-China basin, Yanshan uplift and Taihangshan uplift, respectively, were inverted for the shear wave velocity structures. Our results indicate that the disturbances coming from earthquakes and the sources near stations can be effectively removed by using the ＂running- absolute-mean＂ normalization approach. In order to obtain reliable tomographic result, we should calculate the SNR of Green＇s Function, and measure group velocity dispersion with high SNR Green＇s Functions. The dispersion curves are stable and reliable when the SNR is greater than 7. The maximum reliable period T has a bearing on the inter-station distance. Tmax should be less than 1/12 of the inter-station distance in North-China. The dispersion curves are significantly variable at periods beyond Tmax This variation should be taken into account in doing surface wave tomography.