利用背景噪声估计华北地区场地放大效应

用噪声互相关函数聚束方法得到中国华北地区噪声源的极值方位和慢度,然后将噪声傅里叶谱在该方位和慢度点进行聚束,并计算得到噪声的频率波数功率谱。将该功率谱作为输入,单台站噪声的自功率谱作为响应,二者的比值可用于计算华北地区相对场地放大因子。计算结果显示:平原地区放大效应明显大于山地地区,而平原地区的场地放大效应与渤海湾盆地沉积层厚度有直接关联。本研究可为计算场地放大效应提供一种可行的途径。

Estimation of Site Amplification Effect in North Chinafrom Ambient Seismic Noise

Site amplification of the strong motion of earthquakes is a subject of considerable interest. However, other data are required to produce site amplification factors when available strong motion data are lacking, and this application has been proven to be reliable and practicable. Seismic array observations now cover almost the entire Chinese continent and supply abundant data. Our method is applied in the North China array. Assuming that the seismic stations in a network or array are driven by a forcing function derived from the ambient noise field, each site will respond differently depending upon the elastic and anelastic properties of the underlying medium. With the Steven-Taylor method, the F-K spectrum of ambient noise is regarded as the input and the auto-power spectrum is the response; the ratio between the response and input can thus represent site amplification. In this study, we use the F-K spectrum of the cross-correlation function of ambient noise to deprive the velocity and azimuth of energy extremum, which is different to that in the Steven-Taylor method. We then correct the Fourier spectrum for all stations according to the velocity and azimuth, and the input power spectrum is computed from the stack and average of all the Fourier spectrum. The ratio between the power spectrum of each station and the maximum of frequency-wave number spectrum of ambient noise can then be adopted to estimate site amplification. This method is used to estimate site amplification of the North China array in a period from 10 to 20 s, and the ratio between maximum and minimum amplification is about 7. The sediment thickness of the plain region is basically consistent with that of the Bohai Bay Basin, and site amplification factors of the plain after standardization are found to be larger than those in the mountain region. This study therefore provides an available method to estimate site amplification.