Images of the Earth?s interior are primarily produced by two types of techniques providing snapshots of the Earth at two end-members of resolution. (1) Tomographic imaging provides a whole-mantle view of Earth?s large-scale (> 2000-5000 km) seismic velocity anomalies; (2) Waveform modeling provides a small-scale (~100-500 km) view for specific geographic locations. Because the scale length of features resolved in these imaging techniques varies by an order of magnitude it remains challenging to reconcile these different pictures into one single model of the Earth?s interior. This project aims to reconcile the disparities observed in these images by: (1) Analyzing new seismic data recorded in North America from Earthscope?s Transportable Array (TA) and FlexArray deployments, ANSS Backbone stations, Canadian National Seismic Network Stations, and recent PASSCAL deployments; (2) Analyzing global- and regional-scale models previously introduced to the community by full waveform modeling techniques using the PSVaxi software. The primary goals are to assess the accuracy of the global and regional scale seismic models, and to decipher how complex structures in the deep mantle at all spatial scales affect broadband seismic waveforms.
Seismological modeling of deep mantle structure is crucial in understanding the dynamic processes taking place within the Earth. This project will further develop software that is capable of simulating earthquake motions on the global scale. Ultimately this will aid in elucidating the Earth?s interior structure by comparing the simulated waveforms with the newly recorded data. They will test the validity of previously derived Earth models, assessing how structural features in the deep mantle affect the seismic waveforms. Ultimately, this will provide valuable waveform propagation tools and a rigorous assessment of the accuracy of seismic models and images to the community.
The collaborative research between Michael Thorne (University of Utah) and Jeroen Ritsema (University of Michigan) addressed the seismological of the structure and processes in Earth's deep mantle. Our region of interest has primarily been the core-mantle transition region (at a depth of 3000 km) directly above the boundary between the solid mantle and the liquid outer core. Our research has focused on reconciling seismological images (ie., "cat-scans") and profiles made at different spatial scales. We have shown that sharp boundaries of large-scale "blobs" in the deep mantle can produce rapid wave traveltine variations recorded in regional network. This corroborates previous suggestons that compositional heterogeneity is present in the deep mantle. We have also demonstrates that strong wave speed gradients with depth (on a 100-km scale) affact global wave diffraction, and that the thickness and horizontal extent of thin (< 40 km) layers of, presumably, pertial melts at the base of the mantle can be constrained using ampliutdes of the SKS and SKKS core waves. Related work has let to new global-scale tomographic images of the mantle, new measurements of the splitting (i.e., frequency shifts) of Earth free oscillations, and 3D computer simulations of seismic waves, which illustrate the limitations of imaging techniques to resolve narrow upwellings, such as plumes, in the mantle with seismic data.
