S U M M A RYA method for simulating seismic wave propagation in a laterally heterogeneous whole earth model is presented by solving the elastodynamic equations in 2-D cylindrical coordinates using the pseudospectral method (PSM). The PSM is an attractive timedomain technique that uses the fast Fourier transform for an accurate di¡erentiation of ¢eld variables in the equations. Since no dispersion error arises in Fourier di¡er-entiation, even when using a large grid spacing, computer memory and time are reduced by several orders of magnitude compared to traditional ¢nite-di¡erence methods. In order to examine body-wave phases with current computing resources, a slice through the sphere is approximated with a 2-D cylindrical model. An irregular grid spacing is used in the vertical coordinate to improve the treatment of the various structural boundaries appearing in the earth model by matching the heterogeneity in the model. Synthetic seismograms obtained by the PSM calculation are compared with those calculated from an exact simulation method for a spherically homogeneous (1-D) earth model and achieve good agreement.The PSM method is illustrated by constructing the seismic P^SV wave¢eld for strongly heterogeneous earth models including a shield structure near the free surface and velocity perturbations just above the core^mantle boundary. The visualization of the evolution of seismic P and S waves in time and space is tracked using a sequence of snapshots and synthetic seismograms. These displays allow direct insight into the nature of the complex seismic wave behaviour in the Earth's interior.
Summary 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.
Summary Broad‐band seismograms observed at regional distances from deep earthquakes display large‐amplitude Rayleigh and Love waves with a predominant period of about 20 s, although deep earthquakes in laterally homogeneous structures do not generate such surface waves. Since the propagation direction of the surface waves inferred from particle motion is approximately radial and the group velocity of the surface waves observed at each station is similar, it can be assumed that the surface waves have travelled close to the great‐circle paths. Back projection allows us to determine the region in which these surface waves are generated. Many of the zones from which the surface waves appear to originate are located close to the Norfolk Ridge, where crustal structure has strong lateral heterogeneity. This suggests that the surface waves are generated by the interaction of body waves from the deep earthquakes with this strongly heterogeneous crustal structure. In order to investigate the effect of such heterogeneity in crustal structure on seismic wavefields, we have modelled the P –SV wavefield using a pseudospectral method. The results show that surface waves such as those observed can be generated by ridge structures and indicate that sedimentary layer thickness plays a very important role. We suggest that such unexpected surface waves from deep earthquakes may prove useful for delineating heterogeneity in shallow structure.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.