Coherence of SH-waves near a semi-circular inclusion——the role of interference and standing waves

Coherence of SH-waves near a semi-circular inclusion - the role of interference and standing waves

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.