基于台阵记录的局部场地条件地震动效应分析

Analyses on the effect of the local site conditions on the strong motion based on the array records

基于自贡西山公园山体地形影响台阵和唐山响堂土层场地台阵的地震动记录,采用互相关分析、相干系数分析、傅里叶谱比法和考虑相干系数的傅里叶谱比法,对比研究了山体地形和土层场地两种局部场地条件对地震动的影响.结果表明,局部场地条件下的地震动效应与地震波的频率成分相关.低频地震波由于其波长较长,易于穿过土层场地和山体地形发生衍射,从而导致二者地震动显著相关,但二者在该频段的地震动场地效应差异不大;较高频率的地震波由于其波长较短,更容易受土层场地条件和山体地形条件的影响,造成二者地震动场地效应差异显著.此外,由于山体顶部的尺寸变小,低频长波长地震波容易在该位置发生衍射,导致山顶周边测点的地震动相关性增加,而较高频率的地震波则容易引起山顶局部场地发生共振效应.山体地形上相邻位置的地震动差异一般较大,相关性较低,这与山体地形不同位置的几何形状对较高频率的地震波影响有关;山底基岩的地震动受山体地形的影响较小,该位置的地震动效应机制尚需作进一步研究.

Based on the strong motion records from the topography array ot Xishan Park in Zigong and the soil site array of Xiangtang in Tangshan, this paper analyzes the site effects of soil site and hill topography on the ground motion by using the cross-correlation function, coherence function, the Fourier spectral ratio methods with and without consideration of coherence function.The results show that the strong motion effect of the local site conditions is relevant to the frequency content of the input seismic wave. For the low frequency component, its wavelength is so long that it is easy to spread across the soil site and the hill topography, and the diffraction makes the strong motion of the two different local site conditions more related; however, the difference of the site effects in the low frequency band for the two local site conditions is weak. For the seismic wave with high frequency and short wave- length, the strong motion is more likely hill topography, therefore the difference to be affected by the soil site and the of the site effects on the strong motion with high frequency is obvious. In addition, with the decrease of the mountaintop dimension, the diffraction is easy to happen when the seismic wave with low frequency and long wavelength spreads to the mountaintop, leading to increase of the cross-correlation of the ground motions at the observing points surround- ing the mountaintop. The resonance effect likely happens when the seismic waves with higher frequency spread to the mountaintop. The strong motion of the adjacent points on the hill topography changes greatly, and their correlation is low, which is caused by the effect of the geometrical shape on the strong motion with higher frequency on the different locations of the hill topography. The strong motion of the bedrock at the bottom of hill is weakly affected by the existence of the hill topography, and its mechanism of the strong motion needs to be further studied.