深厚松软场地卓越周期与地面峰值加速度的空间变异特征:以苏州为例

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

基于区域地震构造与活动性特征,苏州存在遭遇近场中强地震、中远场强地震和远场大地震影响的风险。根据250 m深钻孔信息与111个不同深度原状土样的动三轴试验结果,采用修正Matasovic本构模型描述土的动剪切模量折减和阻尼比增长特性是适宜的。基于393个钻孔的剪切波速资料,以剪切波速不小于700 m/s的岩土层为地震基岩,采用集中质量模型非线性分析法进行钻孔土柱地震反应分析,提出了场地卓越周期的弱震预测法,给出了场地卓越周期和地面峰值加速度(PGA)的空间变化特征:(1)在近场中强地震、中远场强地震和远场大地震的基岩弱震动作用下,场地卓越周期等值线图的空间变异性相似,但PGA放大系数等值线图的空间变异性与基岩地震动特性密切相关。(2)基岩峰值加速度小于0.1g时,PGA的大小主要取决于基岩峰值加速度;反之,基岩地震动频谱特性的影响不可忽略。

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 1 D 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 0.10 g, otherwise, the effects of frequency characteristics of bedrock motions on the PGA amplifications are significant.