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Estimation of non-linear site response in a deep Alpine valley

机译：高山深谷非线性场地响应的估算

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We simulate non-linear behaviour of soils during strong ground motion in the Rhône valley in southern Switzerland. Previous studies of the site response using weak ground motion, ambient noise and linear 3-D FD simulations suggest that the 2-D structure of the basin will lead to amplification factors of up to 12 in the frequency band between 0.5 and 10 Hz. To estimate the importance of non-linear soil behaviour during strong ground motion in the Rhône valley we simulate the response of a superficial soft layer with a fully non-linear 1-D finite difference code. The non-linear wave propagator is based on an effective stress constitutive soil model capable of predicting pore pressure evolution due to shear. We determine the required dilatancy parameters from laboratory analysis of soil samples using cyclic triaxial tests. In order to include the effect of the strong 2-D structure in our non-linear analysis synthetic seismograms are convolved with the transfer function of the basin and then propagated through a 1-D non-linear layer. We find that reduced amplification due to soil non-linearity can be expected at rock accelerations above 0.5 ms−2, and that de-amplification occurs at ground motion levels of approximately 2 ms−2. Nevertheless, the spectral accelerations simulated for the valley centre are still exceeding the design spectra at about 0.5 Hz for magnitudes above 6.0, which reflects the strong amplification of ground motion by the deep 2-D resonance of the basin. For frequencies above 1 Hz the design spectra are generally in agreement with the strongest simulated accelerations. We evaluate the occurrence of soil failure using the 5 per cent strain criterion as a function of hypocentral distance and magnitude. Results confirm observations of liquefaction reported after the 1855 Mw 6.4 earthquake of Visp, and they suggest that soil liquefaction may occur at distances beyond those predicted by empirical relations in the valley. Near the basin edge, however, the simulated liquefaction occurrence agrees with the empirical relations. These results suggest that the response of the whole structure needs to be simulated in order to estimate the non-linear seismic response of complex basins like the Rhône valley
机译：我们模拟了瑞士南部罗纳河谷强烈地面运动期间土壤的非线性行为。以前使用弱地面运动，环境噪声和线性3-D FD模拟对场地响应进行的研究表明，盆地的2-D结构将导致0.5至10 Hz频带内的放大系数高达12。为了估算在罗纳河谷强烈地面运动过程中非线性土壤行为的重要性，我们使用完全非线性的一维有限差分代码模拟了浅层软层的响应。非线性波传播器基于有效的应力本构土模型，该模型能够预测由于剪切作用而产生的孔隙压力。我们使用循环三轴试验从土壤样品的实验室分析中确定所需的剪胀参数。为了在我们的非线性分析中包括强二维结构的影响，将合成地震图与盆地的传递函数进行卷积，然后传播通过一维非线性层。我们发现，在高于0.5 ms-2的岩石加速度下，可以预期由于土壤非线性而导致的放大率降低，并且在地面运动水平约为2 ms-2时发生去放大作用。尽管如此，对于谷中心，模拟的频谱加速度仍超过了设计频谱，在高于0.5赫兹时振幅大于6.0，反映了盆地深层二维共振对地震动的强烈放大。对于高于1 Hz的频率，设计频谱通常与最强的模拟加速度一致。我们使用5％应变标准作为中心距和震级的函数来评估土壤破坏的发生。结果证实了在1855年发生的维斯普6.4级地震后报告的液化现象，并且表明土壤液化可能发生在山谷中的经验关系所预测的距离之外。然而，在盆地边缘附近，模拟的液化现象与经验关系一致。这些结果表明，需要对整个结构的响应进行模拟，以估算像罗纳河谷这样的复杂盆地的非线性地震响应。

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