An earthquake time series consists of the (possibly weighted) number of events of a given class of earthquakes per unit time as a function of time. The spectrum of a single earthquake time series and the cross spectrum between different series determine the usefulness of past events in predicting future ones. The spectra of earthquakes might also shed light on the possible triggering of earthquakes by such things as tidal potential changes and other earthquakes.
Much of the past work on earthquake time series has dealt with seeking significant periodicities in the data. In most cases [Davidson, 1938] significance tests have failed to take into account the correlation between events that result from the aftershock phenomena. In the spectral domain, nonrandom bunching of events due to aftershocks results in a nonwhite spectrum.