Ambient noise and earthquake surface wave phase velocity tomography of the South American lithosphere

• Ano de defesa: 2024
• Autor(a) principal: Souza, Lúcio Quadros de
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Anisotropia sísmica; Crosta; Crustal imaging; Moho; Moho depth; Seismic anisotropy; Seismic tomography; Tomografia sísmica
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/14/14132/tde-16122024-115104/

Rayleigh-wave phase velocities are automatically determined using earthquake records of 1,022 stations throughout South America, Antarctica and the Caribbean between 1990 and 2020 for 10,799 earthquakes resulting in 19,522 interstation measurements. Isotropic and anisotropic phase-velocity maps are presented for periods between 5 and 200 s. For depths between 0 and 300 km, the isotropic components were used to calculate a 3-D shear-wave velocity model for the continent based on a stochastic particle-swarm-optimization inversion technique. We also obtain a Moho map for South America that shows good agreement with the most recent crustal thickness map. Azimuthal anisotropy is observed in areas of previously poor coverage by SKS studies within the South American Platform, including the Amazonian Basin, Amazonian Craton, and Pantanal Basin. For periods above 60 s, we observed a NE-SW oriented fast direction of azimuthal anisotropy in the regions of the Pantanal and Chaco-Paraná sedimentary basins. This trend coincides with a low-velocity zone (-4% V_ at 100 km) observed in this and other studies interpreted as thinned lithosphere. This result suggests that mantle flow is channeled by the lithospheric topography in this area. At crustal depths, beneath the Andes, azimuthal anisotropy is oriented parallel to the strike of the orogeny, which is consistent with the observed compression of the South American Plate from the subduction of the Nazca Slab. We also observe a systematic difference between the Guyana and Brazilian Shields at lithospheric depths. Our model shows that, on average, shear-wave velocities are approximately 3% lower in the Guyana Shield than in the Brazilian Shield which may result from a lithospheric reworking in the Central Atlantic Magmatic Province. Finally, thin crust and lithosphere is observed in the Tocantins Province in Brazil in accordance with previous seismic refraction and receiver function studies that might explain the high seismicity observed in this area. Ambient noise dispersion curves were calculated similarly to the earthquake methodology. We used 138 seismic stations from 1998 to 2022 from the Brazilian Seismographic Network and additional temporary deployments to compute 1,477 ambient noise phase-velocity dispersion curves. Rayleigh-wave isotropic and anisotropic maps, between periods of 2 and 200 s, were calculated by combining dispersion curves from the earthquake dataset with ambient noise. For the isotropic phase velocities, the results show good agreement with previous tomographies in the crust. At 2 s, higher phase velocities are observed to the west of the Pantanal Basin relative to the east. This result agrees with a joint inversion of Receiver Function, surface waves and H/V data and indicates that the basin\'s basement is shallower in the west. For the azimuthal anisotropies and crustal depths (5 to 20 s), we observed a NE-SW fast axis trend to the north of the Pantanal Basin and NW-SE to the south of it, well correlated with the Paraguay fold belt strike under the basin. At the same depths, N-S fast axis anisotropies were observed mainly inside the Paraná Basin and those could be associated with the collision of the Paranapanema, Rio Apa and Amazonian Cratons during the assemblage of west Gondwana during the Neoproterozoic as mentioned by a previous study. Fast axis anisotropies parallel to the passive margin in Mantiqueira Province were observed and correlated well with a Pms splitting study in this area. This result helps confirm the interpretation that crustal and lithospheric anisotropy in the Ribeira belt is due mainly to shear deformation during the Brasiliano orogeny.