Structural collapse on soft ground is one of several emergent effects in the nonlinear complex soil-building system. The gravilastic stress-strain equation is derived from first principles, and we show that it may lead to a phase transition from Rayleigh to gravity waves at large cyclic strains. Instability of high-rise buildings may be caused by a combination of prograde particle motion and short wavelengths. The effect of large amplifications on soft ground may be less critical in terms of damage than is the emergence in specific geological environments of prograde surface waves.

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