Occurrences of vertical seismic ground accelerations greater than gravity have been inferred over the years, based mainly on reports of objects separating vertically from each other during earthquakes. Only one instrumentally recorded such acceleration has been measured. A survey is made of these reports and the observations are divided into three dynamical categories: rigid body rocking and internal accelerations of systems of particles with and without elasticity. Mathematical solutions obtained for two dynamical systems resting on the ground demonstrate that upthrow does not necessarily entail vertical ground acceleration greater than gravity. The results are in agreement with an argument of N. M. Newmark but extend it to systems of purely rigid bodies. The implications in assessing intensity are discussed.

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