A palestra será apresentada pelo doutorando Lúcio Quadros de Souza.
Resumo:
The development of new automated techniques to measure earthquake-based interstation phase velocities allows for more detailed imaging of the South American Lithosphere.
We calculated Rayleigh-wave phase velocities using earthquake records from 1.022 broad-band seismic stations operated between 1990 and 2020 in South America, Antarctica, and the Caribbean. The earthquakes were selected with the following criteria:
1. Events aligned within 10° of the great circle path between two stations;
2. A linearly increasing minimum magnitude between 4 and 6 Mw as a function of the epicentral distance;
3. Epicentral distances between 2.5° and 30°.
We automatically extracted the Rayleigh-wave fundamental mode from dispersion curves using a new implementation of the 2-station cross-correlation method and three strict quality criteria:
1. Phase velocity curves are selected based on proximity to a 3D background model
2. Curve smoothness
3. Width of the considered frequency range.
We obtained 46.763 broad-band dispersion measurements between 4 and 315 s following this process. Finally, the single-event dispersion curves were averaged for each interstation pair with associated error estimates. Further quality control was applied to evaluate the average dispersion curves' smoothness and standard deviation.
The dispersion curves were simultaneously inverted for isotropic and anisotropic (2ψ and 4ψ) phase-velocity maps parameterized on a triangular grid with a knot spacing of 30 km.
The isotropic phase velocity maps at periods of 15 and 30 s indicate around 8% high-velocity perturbations in the regions of: (1) cratonic blocks of the South American platform (Brazilian Shield, São Francisco and Rio Apá cratons); and (2) under the Pantanal basin, possibly related to a high-velocity lower crust. We also observed between
-8 to -4% low-velocity perturbations associated with the Andean Mountain range root below the Altiplano Boliviano region (central Andes). The Paraná, Chaco and Parecis intracratonic basins also have lower velocities in relation to the neighboring cratonic areas.
At periods of 60 and 100 s, we observed around 4% high-velocity perturbation associated with the deep roots of the oldest region of the Amazonian craton (eastern area of the Brazilian Shield) and the São Francisco craton.
Azimuthal anisotropy is laterally and vertically variable within the South American lithosphere. At longer periods, fast directions point to asthenospheric flow guided by LAB topography, for example, below the Pantanal basin (central-west Brazil).
We also stochastically inverted the isotropic component for a 3D shear wave velocity model based on a particle swarm optimization technique. The results at 100 km show a -4% Vsv anomaly below and to the southwest of the Pantanal basin, possibly related to a LAB thinning, corroborating with previous tomographies for South America. We also observe crustal and lithospheric thinning near the Transbrasiliano Lineament in the Tocantins Province. The thin crust was also observed from seismic refraction profiles and
receiver functions. It explains the higher seismicity in the area due to increasing flexural stresses in the upper crust. The lithospheric thinning also agrees with previous low-velocity anomalies in the upper mantle from a P-wave tomography in Central Brazil. The lithospheric thinning was suggested to contribute to accumulating stresses in the upper
crust.
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