Coherence of dispersed synthetic strong earthquake ground motion at small separation distances

•Coherency of strong earthquake ground motion for short separation distances.•Role of dispersion on coherency of earthquake wave motions.•Displacements of soil beneath large foundations.•Coherency of synthetic body and surface waves in parallel layers.

It is shown that the observed loss of coherency of synthetic strong motion on ground surface for separation distances less than about 100 m can be described in terms of the dispersion of strong motion waves. Additional contributions to the loss of coherency from variations of material properties in the soil and from geometrical departures from perfectly flat ground surface and irregular layer geometries are not considered in this paper. It is also shown that the synthetic surface displacements over a large rectangular area (100×100 m) on ground surface can be described well by a flat surface undergoing translations and rotations only, and with only minor departures from the plane flat surface.