Towards High-Resolution Waveform Tomography of South America

The South American continent is composed by diverse geological units, from its ancient Precambrian stable platform to the now active Andean orogenic belt. However, the structure of its lithosphere and the dynamics of its interaction with the underlying mantle are still unclear and debated. Waveform tomography can resolve 3D distribution of shear-wave velocity, sensitive to temperature and composition in the crust, upper mantle and transition zone, but, until recently, seismic data sampling was scarce and uneven.

The unprecedented recent growth in the coverage with broadband seismic stations all across the continent now enables us to resolve regional-scale structure such as the subducting slab and the boundaries between tightly spaced cratons. We assemble all available data across the continent, including the stations of the Brazilian Seismic Network, sampling the Amazon area and the northeastern Brazil, and the stations recently deployed in the Pantanal and Paraná-Chaco basins as part of the FAPESP "3-Basins Thematic Project." We then complement them with our very large global waveform dataset, which maximizes the data sampling in a very broad frequency range and yields detailed structural information from the crust down to the mantle transition zone.

Our S-velocity tomographic model is constrained by vertical-component waveform fits, computed using the Automated Multimode Inversion of surface, S- and multiple S-waves. Each seismogram fit provides a set of linear equations describing 1D average velocity perturbations with respect to a 3D reference velocity model within the source-receiver sensitivity volume. All the equations are then combined into a large linear system and inverted jointly for a model of isotropic S- and P-wave velocities and azimuthal anisotropy. The isotropic shear speeds offer important new information on the structure and evolution of the continent's lithospheric blocks, including cratons, intra-cratonic basins and the subduction zone region. Azimuthal anisotropy provides evidence on the past and present deformation, which we interpreted together with other evidence, including geological structure and shear wave splitting.