Spatial variation of predominant periods of site and amplifications of peak ground accelerations for deep sediment layers: case study of Suzhou City

Based on the regional tectonic environment, the urban region of Suzhou City may be subjected to the effects of near-field mid-strong earthquakes, middle-field strong earthquakes and far-field large earthquakes. According to the cyclic triaxial test results of 111 undisturbed soil samples from a 250 m-deep borehole, the modified Matasovic model can be used to well characterize the tendency of reduction of shear modulus and increase of damping ratio. Based on the 393 sets of borehole shear wave velocity data, and by identifying the weathered rock layer with shear-wave velocity larger than 700 m/s as the seismic bedrock, the nonlinear 1D site response analysis is performed using a discretized lumped mass model. A method for evaluating the site predominant period using the weak motion records is proposed. The spatial variation of the predominant periods and the amplifications of peak ground accelerations (PGAs) in the urban region of Suzhou City are given: (1) Using the seismograms during the near-field mid-strong earthquakes, the middle-field strong earthquakes and the far-field large earthquakes as the bedrock motions, the contour maps of the predominant periods of site are quite similar, but the spatial variation of the amplification of PGAs is closely related to the characteristics of bedrock motions. (2) The PGAs are mainly related to bedrock motion intensities less than 0.10g, otherwise, the effects of frequency characteristics of bedrock motions on the PGA amplifications are significant.