局部山体地形对强地面运动的影响研究

Effect of local mountain topography on strong ground motion

基于曲线网格有限差分方法研究了地震波在不同坡度的山体地形及水平地表模型中的传播,得到了各模型速度波形及地表峰值速度特征,从地形自身特征及震源特征两方面出发讨论了地形效应:一是相同的震源模型下地形坡度、形状对地震动的影响;二是同一山体模型下地震动对不同震源机制的点源以及相对复杂的有限断层的响应.主要结论如下:(1)一般情况下,地形放大效应在坡度较大的地方比较明显,并随着坡度的增加而增大,但在某些特定情况下,放大效应与坡度并不满足正相关,且这种情况的发生与震源性质无关,可能仅受地形形态自身的影响;(2)对于不同的震源机制,地面运动各分量受地形影响程度不同,总体上水平分量受地形影响程度更大;(3)震源机制和震源激发的波的频率会影响放大效应最大值出现的位置,放大效应最大值不一定出现在山顶处,有可能会出现在起伏地形的震源对侧,出现位置可能与波的相互作用有关;(4)有限断层模型下,地面运动特征相对更为复杂,地形效应不仅受断层模型几何特征的影响,同时断层破裂过程对其也有着重要的影响.

In this study, we used the curved-grid-based finite-difference method to investigate the effect of local mountain topography on strong ground motion by earthquakes. The ground motions were simulated with different topographic irregularities in a three-dimensional homogeneous half-space. The topographic effect was analyzed in two aspects：one is the topographic slope and shape with the same source model; the other is the response of ground motion to the point source model with different focal mechanisms and finite fault model with the same topography. Our results show that （1） Generally, topographic amplification is obvious in the areas with large slopes, which increases with the slope. However, in some special cases, the amplification effect is not related to the focal mechanism, but probably only affected by topography itself. （2） Topography has different influences on three components of ground motions for different focal mechanisms. Overall, the horizontal component are more affected by topography than the vertical component. （3） The position of maximum PGV varies with focal mechanism and frequency. It may appear on the side of the ridge opposite of the source rather than the top of the ridge, which is probably associated with the interaction of incident waves and diffracted or scattered waves. （4） The ground motion characteristics are relatively more complex for a finite fault model. Besides the local topography, not only the fault geometry, but also the fault rupture process has a strong influence on the features of ground motion.