Seismic Response of Horizontal Soil Layers
In many cases the ground motions developed near the surface of a soil deposit during an earthquake may be attributed primarily to the upward propagation of shear waves from an underlying rock formation. If the ground surface, the rock surface, or the boundaries between different soil layers are inclined, analyses of the response of the soil deposit can be made only by techniques such as the finite-element method. If the ground surface, the rock surface, and the boundaries between the soil layers are essentially horizontal, however, the lateral extent of the deposit has no influence on the response, and the deposit may be considered as a series of semi-infinite layers.
Idriss, I.M. and Seed, H.B., 1968, Seismic response of horizontal soil layers: Journal of the Soil Mechanics and Foundations Division, v. 94, p. 1003-1031.