Shear wave velocities in the upper crust of the Quadrilátero Ferrífero, Minas Gerais: Rayleigh-wave tomography.

We applied a combination of ambient seismic noise and classical earthquake-receiver techniques to characterize the shallow crustal shear-wave velocities in the Quadrilátero Ferrífero (QF), Minas Gerais state, SE Brazil, to a depth of about 4 km. Ambient seismic noise was recorded by up to 26 stations. To improve the signal of the extracted empirical Green's function (EGF), we correlated short time windows of 10 min with 70% overlapping before stacking. To test the accuracy of the retrieved EGF signals, we compared the results obtained from ambient seismic noise correlation with results from an earthquake occurred near FABR station. After measuring dispersion using frequency-time analysis (FTAN), we applied strict quality criteria (e.g., eliminating paths with residuals larger than two standard deviations, or lengths smaller than 3 wavelengths). The Fast Marching Surface wave Tomography (FMST) method was used to obtain group velocity maps. Then, the local dispersion curves were inverted to obtain a 3D Vs model. The resulting 3D model shows low velocity anomalies in the middle of the QF, compared with high velocities in the Archean part of the São Francisco Craton to the west. The low velocity metasedimentary layer in the QF is about 1.5 km thick.