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

Based on the strong motion records from the topography array of 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 wavelength, the strong motion is more likely to be affected by the soil site and the hill topography, therefore the difference 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 surrounding 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.