TIME-DOMAIN APPROACH FOR THE TRANSIENT RESPONSES IN STRATIFIED VISCOELASTIC EARTH MODELS

We have developed the numerical algorithm for the computation of transient viscoelastic responses in the time domain for a radially stratified Earth model. Stratifications in both the elastic parameters and the viscosity profile have been considered. The particular viscosity profile employed has a viscosity maximum with a contrast of O(10(2)) in the mid lower mantle. The distribution of relaxation times reveals the presence of a continuous spectrum situated between O(10(2)) and O(10(2)) years. The principal mode is embedded within this continuous spectrum. From this initial-value approach we have found that for the low degree harmonics the non-modal contributions are comparable to the modal contributions. For this viscosity model the differences between the time-domain-and normal-mode results are found to decrease strongly with increasing angular order. These calculations also show that a time-dependent effective relaxation time can be defined, which can be bounded by the relaxation tomes of the principal modes.