Study On Site Effects Of Earthquake Ground Motions Of The Capital Region, China

Site effect means the amplification of earthquake ground motions by earth's surface medium; it is an important physical index for depicting the quality of under-ground superficial medium around station. Different sites have significantly different response to disaster caused by earthquake. Furthermore, the nature of site directly affects the distribution of the disaster mentioned above. Site effect is an important reference for forecasting disaster caused by earthquake. It can also be used to measure the source parameters and earthquake magnitude precisely. Moreover, it is a momentous factor for the research of seismic wave attenuation. The capital region is one of the most remarkable seismic active area in China mainland. In order to provide the necessary foundation for the research of earthquake damage forecasting and earthquake prediction, we calculated site effect of the stations in the capital region.In this paper, it is expatiated that the principles of studying site effect by using the data of earthquake ground motions and how to distill site effect from seismic observation spectrums. It is prerequisite to eliminate spreading path effect and free surface effect to separate site effect from seismic observation spectrums. We introduce the method of eliminating the spreading path effect with trilinear geometrical spreading model and Atkinson's method, and introduce the method of revising free surface effect with SH-wave. The principle of studying site effect by generalized inverse matrix inversion method is expatiated as an emphasis in this paper, along with weighting factor combined with signal-to-noise; mathematic principles of generalized inverse matrix and the law of least squares are introduced independently; moreover, the error analysis approach is indicated.In this study, we utilize the small and medium-scaled earthquake data (SH_wave) recorded by stations in the capital region from 2001 to 2006. The target zone is divided into mountainous zone and plain zone. We compute by generalized inverse matrix inversion method and obtain site effect of 68 stations ranging from 1.0 Hz to 20 Hz, and source spectra for 83 earthquakes. As a result, the relationship of source spectra and the corner frequencies indicated that it exists and they are in inverse ratio; but the seismic moment and local magnitude M Lremains a correlation, and the seismic moment increases with the raise of ocal magnitude M L. About site of stations, we found that site effect of rock is stable, and about 1.0 in the 1.0~10.0 Hz range; site effect of deposit is about 2~7 in low frequency, and site effect reduces with the increase of frequency when the frequency goes beyond 10 Hz. Site effect of deposit is remarkably higher than site effect of rock, and it increases with the raise of deposit depth. Site effect of the station is in inverse ratio with the density of the pedestal medium. On the other side, the stations with the same lithology of basement have significant consistency in site effect, and site effect of deposit has an obvious raise in the frequency of 4.0~5.0 Hz. The result in all showed that the lithology of station floor is the leading factor, and influenced by geological conformation.