We present examples of a controlled numerical experiment that contribute towards understanding of the physical phenomena that lead to the reduction of coherency of strong earthquake ground motion. We show examples for separation distance of 100 m between the two points on the ground surface, which is in the range of engineering interest. Our examples illustrate the consequences of: (a) standing waves that result from interference of the incident and reflected waves from a near vertical contrast in material properties, (b) standing waves within a concave inhomogeneity (a semi-circular valley in our examples), and (c) smaller motions in the diffraction zone, behind the inhomogeneity. We show that it is possible to reduce coherency, to the extent observed for recorded strong earthquake ground motion, even by a single inclusion in a half space, for incident ground motion that is coherent. We also illustrate the combined effects of geometric spreading and finite fault width, superimposed on the otherwise dominating effects caused by interference. Our examples show reduction of coherence for specific angles of incident waves, while, for other angles of incidence, the coherence remains essentially equal to one.

我们提供了一个受控数值实验的示例，这些示例有助于理解导致强烈地震地面运动的相干性降低的物理现象。我们以地面上两点之间100 m的分隔距离为例，这在工程上是有意义的。我们的示例说明了以下结果：（a）入射波和材料性质接近垂直的对比度的反射波对入射波和反射波的干扰导致的驻波；（b）凹面不均匀性内的驻波（在我们的示例中为半圆形凹谷） （c）在不均匀性后面的衍射区中的较小运动。我们表明，即使在半个空间中仅包含一个夹杂物，也有可能降低相干性，达到记录强地震地面运动所观察到的程度，用于相干的入射地面运动。我们还说明了几何扩展和有限断层宽度的综合影响，叠加在其他方面由干扰引起的主要影响上。我们的示例表明，对于特定入射角，相干性降低，而对于其他入射角，相干性基本上等于1。