Seismic wavefield calculation for laterally heterogeneous earth models-II. The influence of upper mantle heterogeneity

Abstract

There is increasing evidence that the Earth's mantle is laterally heterogeneous on a broad range of scales, but the character of smaller-scale heterogeneity has to be deduced indirectly. The aim of the present paper is to examine the influence of a variety of stochastic representations of heterogeneity on seismic wave behaviour to help constrain the nature of the variations in seismic properties in the upper mantle. For each of the models, the seismic wavefield is simulated using a pseudospectral method in a 2-D cylindrical coordinate system. The presence of stochastic heterogeneity is particularly important for those parts of the seismic wavefield where a significant portion of the propagation path in the upper mantle is close to horizontal, such as the PP and SS phases, and fundamental-mode and higher-mode surface waves. The effects are noticeable traveltime anomalies and waveform changes for the body waves (particularly associated with phase triplications), and significant phase shifts for Rayleigh waves. A variety of styles of stochastic heterogeneity models are compared for the same source and station configurations using wavefield snapshots and the character of the calculated seismograms. The influence of heterogeneity on body waves and on longer-period Rayleigh waves increases as the scale length increases compared to the wavelength of the seismic waves. The aspect ratio of the heterogeneity has a pronounced effect on the coherence and amplitude of traveltime fluctuations and waveform changes across stations at the surface, which depend on the structures encountered along the propagation paths to the specific receivers. The effect of nearly isotropic heterogeneity is to induce small, short-scale variations in traveltime fluctuations and waveform changes. As the heterogeneity becomes more 'plate-like' the fluctuations are on a broader scale and of larger amplitude because the individual patches of heterogeneity have a stronger influence. The effects of broad-scale and stochastic heterogeneity are compared for a model built from a slice through a tomographic model derived from delay-time inversion for the Himalayan region. As would be expected the influence of the deterministic heterogeneity derived from the tomography study has the result of introducing systematic traveltime variations for body waves and noticeable phase shifts for surface waves when compared with the results for the background reference model. The addition of a moderate level of small-scale stochastic heterogeneity, which could not be resolved in the tomography study, has a limited effect on the seismic wavefield at longer periods but is much more significant for periods of less than 4 s when the heterogeneity scale is of the order of 40 km.