In this paper, a Hilbert-Huang Transform data-processing technique is successfully used to characterize the seismic responses of soil-quay wall systems using measured data in a series of geotechnical centrifuge shaking-table tests. The predominant frequency of a liquefied deposit shifts down to a low frequency level; however, "de-liquefaction" leads to frequent, local higher-frequency spikes in the time histories of predominant instantaneous frequency (PIF). A lower amount of seaward displacement was found if the combined translation and rotation modes resulted in lower excess-pore-water pressure when the wall accelerated seaward. Cyclic changes in the PIF of the wall during shaking are directly related to the stiffness of the soil in which the wall is embedded. Thus, PIF at any given instant provides a superior indicator for characterizing the occurrence of liquefaction and the time-varying soil dynamic property. This advantage assists in evaluating the degradation of soil dynamic properties at any given instant based on the acceleration time histories measured in model tests or even in the field. (C) 2010 Elsevier Ltd. All rights reserved.