原位剪切波速各向异性的评估——以美国克罗基特岩土台阵为例

In-situ shear wave velocity anisotropy assessment: A case study of Crockett geotechnical array, USA

剪切波速是反映土体动力学特性的重要参数之一。受地质地貌和沉积环境的影响,场地的近地表剪切波速可能存在明显的各向异性。本研究利用克罗基特岩土台阵2个子台阵记录的4次浅源、地方震的弱震动数据合成360个方向的加速度分量,使用解卷积的地震干涉测量法评估台阵原位剪切波速的各向异性。结果表明:(1)两个子台阵的上下部分均表现出一定程度的各向异性,剪切波速在水平方向上的差异多在5%到10%之间;(2)两个子台阵上半部分的剪切波速各向异性主要受到河流搬运作用的影响,表现为沿河流入湾方向剪切波速较小,垂直于河流入湾方向剪切波速较大;下半部分的剪切波速各向异性主要受河流对海岸的侵蚀影响,表现为沿台阵近处山体走向的剪切波速较大,垂直于山体走向的剪切波速较小。上述结果与场地的地质地貌和沉积环境的特征相符,且与河流流域磁化率各向异性特征研究结果相一致(即河流走向近似为磁化率最大方向)。一方面验证了本研究所使用的原位剪切波速估计方法的准确性,另一方面也表明位于滨江沿河地区的工程场地的近地表剪切波速结构可能存在较明显的各向异性。该方法和结论具有一定的普适性。场地的原位剪切波速各向异性评估可使场地地震反应分析更符合实际,为工程抗震设防和地震危险性评估提供参考。

Shear wave velocity is one of the important parameters reflecting the dynamic characteristics of soil. Affected by geology,landform and sedimentary environment,the near-surface shear wave velocity of the site may have obvious anisotropy. In this study, the acceleration components in 360 directions were synthesized using the weak motion data of four shallow-focus and local earthquakes recorded by the two sub-arrays of the Crockett geotechnical array,and the anisotropy of in-situ shear wave velocity of the array is evaluated by the seismic interferometry by deconvolution. The results are as follows.(1)The upper and lower parts of the two sub-arrays show a certain degree of anisotropy,and the difference of shear wave velocity of the horizontal direction is mostly between 5% and 10%. (2)The shear wave velocity anisotropy in the upper half of the two sub-arrays is mainly affected by the transport effect of the river,and the shear wave velocity along the river is smaller than that perpendicular to the direction of the river entering the bay. The shear wave velocity anisotropy in the lower part is mainly affected by the erosion of the river on the coast,and the shear wave velocity along the mountain near the array is larger than that perpendicular to the direction of the mountain. The above results are consistent with the characteristics of the geology,landform and sedimentary environment of the site,and are in accord with the results of the research on magnetic susceptibility anisotropy in the river basin,that is,the river trend is approximately the direction of the maximum magnetic susceptibility. It verifies the accuracy of the in-situ shear wave velocity estimated by the method used in this study. It also shows that the shear wave velocity structure in the near-surface of the engineering site located in the riverside area may have obvious anisotropy. The method and conclusion have certain universality. The in-situ shear wave velocity anisotropy assessment of the site can make the seismic response analysis of the site more realistic,and provide a reference for engineering seismic fortification and seismic risk assessment.