Taking a multidisciplinary approach, this book provides coverage from seismological and engineering, deterministic and stochastic, as well as physical and mathematical points of view. The author describes the estimation of spatial variability from recorded data, its physical interpretation, and the development of spatial variability/coherency models. She uses random variation analysis to illustrate the effect of differential ground motions on the quasi-static and dynamic response of extended structures. Worked-out applications demonstrate the significance of seismic ground strains and differential motions on the seismic response of foundations, bridges, and dams.