# Study on the Seismic Effect of the Pebble Soil Site in the Zhongwei Basin
## 4 Results
### The Result of Variation in the Silty Clay Thickness  
The above research results show that the overlying silty clay in the Zhongwei Basin has a direct impact on the site PGA amplification ratio. To study the influence of the silty clay layer's thickness on the ground motion parameters, this paper set the thickness of the pebble layer to 30 m and established six one-dimensional calculation models by changing the thickness of the overlying silty clay layer from 0 to 5 m, as shown in Table 3.  
As shown in Table 4, the peak acceleration of the surface response spectrum increases with the increase in the upper silty clay layer thickness. As shown in Figure 7, the maximum shear strain under different strength inputs is less than 3 \(\times\) 10\({}^{-5}\), and the site nonlinearity is small. Therefore, the equivalent linearization method is suitable for calculation and analysis. As shown in Figure 8, with the increase in the silty clay layer thickness, the PGA amplification ratio of the site increases. The amplification ratio of PGA decreases with the increase in the input ground motion. When the overlying soil layer is greater than 3 m, the amplification coefficient increases significantly under different inputs, and the smaller the input, the larger the amplitude. Under the extremely rare earthquake, the PGA amplification ratio of the site is significantly lower than that of the other seismic effects, which accurately reflects the influence characteristics of the site nonlinearity on the ground motion.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**PGA of Site Surface (GaI)**} \\ \cline{2-5}  & **50 y—63\%** & **50 y—10\%** & **50 y—2\%** & **100 y—1\%** \\ \hline Model 2–1 & 73.2 & 258.0 & 534.4 & 693.0 \\ Model 2–2 & 74.8 & 263.6 & 551.0 & 694.6 \\ Model 2–3 & 77.0 & 270.8 & 568.2 & 705.2 \\ Model 2–4 & 80.0 & 277.6 & 571.0 & 721.8 \\ Model 2–5 & 83.0 & 295.8 & 582.4 & 751.6 \\ Model 2–6 & 87.6 & 306.0 & 603.2 & 769.6 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The PGA of site surface under different silty clay layer thicknesses.  
Figure 7: The shear strain under four seismic inputs.  
As shown in Figure 9, the pebble soil site in the Zhongwei Basin has an obvious amplification effect on the short-period seismic waves, and the periodic peak points of the response spectrum are all within 0.1-0.2 s. Under the action of frequent earthquakes, basic earthquakes, and rare earthquakes, the site significantly amplifies the short-period component of the earthquake but has little effect on the long-period component. Under the action of the extremely rare earthquake, the response spectrum curve shows an obvious \"double peak phenomenon\", and the second peak point appears in the period of 0.5-0.7 s. The amplitude of the response spectrum increases with the thickness of the overlying silty clay layer under frequent earthquakes, basic earthquakes, and rare earthquakes. Under the extremely rare earthquake, the response spectrum curve shows strong dispersion before 0.3 s. This result is surprisingly similar to the findings of another recent study on the effects of deep soil sites atop deep geological sediments on surface ground motion, in which a spectral peak at 0.5 s was also found [24].  
Figure 8: Amplification ratios of ground motion for the different thicknesses of silty clay layer under four seismic inputs.  
Figure 9: Spectral accelerations for the different thicknesses of silty clay layer under four seismic inputs.