Ground motion analysis in seasonally frozen regions

In seasonally frozen regions, the seasonal appearance of the frozen layer may have certain influence on the ground motion features of the area. In this paper, a site in a seasonally frozen region in northwestern China is taken to investigate the ground motion features in different seasons. As an easy method to obtain the linear features of the ground, micro-tremors are measured twice, once without the frozen layer and the other during the maximum frozen depth in the year. It is shown from the micro-tremor analyses that the shape of frequency spectrum changes and the predominant period of the ground is enhanced when the frozen layer is present. To examine the nonlinear properties of soils under strong ground motions, triaxial dynamic tests were performed on undisturbed samples taken from the working site to obtain the soil's dynamic parameters. These parameters, together with the results from seismic risk analysis, are input into a one-dimensional shear beam model and strong ground motion parameters are calculated for the site in seasons with and without frozen layer. This calculation shows that the seasonal frozen layer can reduce the strong motion to some extent, and change the predominant period slightly.