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Seismometer component orientation is one of the most critical parameters for seismology, especially for methods that take wave field polarization effects into account, such as receiver functions, seismic anisotropy, surface wave dispersion and seismic tomography. Using P-wave particle motion combined with directional statistics, we estimated the orientation for 90 stations of the Brazilian Seismographic Network (RSBR), corresponding to 105 seismometers. The difference between the number of stations (90) and seismometers (105) is due to instrument replacement during station operation. We observed that 81 seismometers have an orientation of ±5° and 19 seismometers greater than ±5°. Only one seismometer has an orientation greater than 90° (BOAV station). ON and BR are the subnetworks of RSBR with the largest number of seismometers with orientation errors bigger than ±10° (6 for each one) and NB is the only subnetwork with no misoriented station. All the estimated orientations were close to the measured in the field, which is evidence that our approach is more accurate to detect misoriented stations than previous studies that analyzed the orientation of RSBR stations. The Watson's U2 statistical test indicated that the back azimuth deviations for Precambrian Basement and Phanerozoic Cover were significantly different, suggesting that the regional geology could affect back azimuth estimates. • P-wave particle motion was effective in detecting seismometer misorientation. • Directional statistics proved to be a suitable approach in back azimuth estimation. • Brazilian Seismographic Network has 14 stations with misoriented seismometers. • Stations deployed on Precambrian terrains tend to produce more accurate back azimuth estimates. [ABSTRACT FROM AUTHOR]

The study of the crust using receiver functions can provide valuable geological information, such as average crustal composition, its formation dynamics and the tectonic evolution of a region, as well as serve as an initial reference for the generation of seismic wave velocity models to improve earthquake location. To fill in gaps in information on the crust of the Amazonian Craton and adjacent provinces in Brazil, we used receiver functions and H-k stacking to estimate crustal thicknesses and the V P /V S ratios. The results indicate that the crust of the study region is predominantly felsic, with an average V P /V S around 1.73 and an average thickness of 38.2 km, with a range of 27.4–48.6 km. Minimum curvature interpolation of the crustal thickness values has made it possible to delimitate of the Amazonian Craton, which corresponds to the area with an average thickness equal to or greater than 39 km. In addition, it was possible to identify its potential cratonic blocks, as well as the Paranapanema Block of Paraná Basin. The geometry of the craton, defined by its crustal thickness, is corroborated by the distribution of natural seismicity that accompanies its edges. These are related to suture zones between the Amazonian, São Francisco/Congo and Paranapanema paleocontinents. The sedimentary basins that have undergone rifting processes have a thinner crust, usually less than 37 km thick. Due to the great variability of the results, it was not possible to determine a characteristic value of crustal thickness or V P /V S ratio for each structural province located in the study region. [ABSTRACT FROM AUTHOR]

The Borborema Province is characterized by a succession of tectonic events, including compressive periods associated with amalgamation of lithospheric blocks during the Brasiliano orogeny in the Neoproterozoic, superimposed by extension events related to the breaking of the continents and the South Atlantic formation in the Cretaceous. Ten seismographic broadband stations belonging to the Geoterm Network were installed in the Transversal Zone and South Domains to collect the data used in this work in the southern Borborema Province, northeastern Brazil. Thus, this study aims to determine the thickness of the Moho and Mid-Crustal ("Conrad") discontinuities and estimate the Vp/Vs ratios to define rock composition using the receiver function over a time window, using the HK-stacking method. The station in the Alto Moxotó Terrain determined a crustal thickness of 35.4 km. The two stations in the Pernambuco-Alagoas Terrain estimated thickness of approximately 37 km and no thinning trend in the geotectonic context. The four stations in the Sergipano Folding Belt indicated thickness ranging between 34.6 and 37.2 km, and a South-North crustal thickening. Mid-crustal discontinuity depth estimates ranged between 10 and 19 km, indicating a WNW-ESE thinning of the upper crust in the middle of the area. The P and S wave velocity ratios appeared to have no geological or tectonic correlation and ranged between 1.68 and 1.77, indicating that the crustal composition is predominantly felsic. [ABSTRACT FROM AUTHOR]

On 2012 May 19, an mb = 4 earthquake shook the town of Montes Claros, Brazil in the middle of the Sãao Francisco Craton. Because of the scarce seismicity in the area, an event like this could provide valuable information to characterize the governing seismotectonics and stress field for the region. Here, we present the results of more than 1 yr of local seismic monitoring after the main shock. We found that the seismicity originated at approximately 1-km depth in an NNW-oriented blind reverse fault, dipping to the E. The magnitude of the main shock was 4mb, with aftershocks reaching up to 3.6mb. Focal mechanisms from first motion polarities and waveform moment tensor inversions indicate a reverse faulting in agreement with the orientation of the aftershock locations. In addition, we derived a new 1-D local velocity model using a simultaneous inversion of hypocentres and velocity layers. The results indicate P-wave velocities of 4.5 km s-1 for the upper layer of carbonate rocks and 5.23 and 5.69 km s-1 for the lower fractured and compact crystalline basement layers, respectively. Higher Vp/Vs ratios were obtained for the upper two layers compared to the lowermost layer, possibly indicating presence of rock fracturing and percolated water. The calculated stress drop for the main event is 0.33 MPa, which is a relatively low value for an intraplate earthquake but still within the observed range. The inversion of the main shock focal mechanism and previously published focal mechanisms suggests a compressional stress regime in the central part of the Sãao Francisco Craton, which is different from the strike-slip regime in the southern part, although both have an EW-oriented σ1. On the other hand, focal mechanisms of events located to the west of the craton indicate an NW-SE oriented σ1 for central Brazil. This variability highlights the importance of local sources of stresses (e.g. fkexural stresses) in mid-plate South America, unlike other mid-plate areas of the world, such as central and east North America, where a more uniform stress field is observed. [ABSTRACT FROM AUTHOR]

Abstract: We present a crustal thickness map of Brazil and adjacent areas based on a compilation of data published in the literature as well as new measurements. We used crustal thicknesses mainly derived from seismic datasets such as deep seismic refraction experiments, receiver function analyses, and surface-wave dispersion velocities. Crustal thicknesses derived from modelling gravity anomalies commonly depend on assumptions, such as constant density contrast across the Moho interface, which are not always easily verifiable and were considered only along the continental shelf to fill large gaps in the seismic data. Our compilation shows that the crust in the stable continental area onshore has an average thickness of 39 ± 5 km (1-σ deviation) and that no clear difference can be observed between low altitude, intracratonic sedimentary basins, NeoProterozoic foldbelts (except for the Borborema Province), and cratonic areas. The thinnest crust is found in the Borborema Province of NE Brazil (30–35 km) and along a narrow belt within Tocantins Province (∼35 km), roughly parallel to the Eastern border of the Amazon craton, while the thickest crust is found in the Amazon and São Francisco cratons (41 ± 4 km), and the Paraná Basin (42 ± 4 km). Both the Ponta Grossa and the Rio Grande Arches are areas of thinned crust, and the western border of the Brazilian platform, near the sub-Andean region, seems to be characterized by a crustal thickness of less than 40 km. Although sparse in data coverage, we expect the resulting crustal thickness map to be useful for future studies of isostasy, dynamic topography, and crustal evolution of the country. [Copyright &y& Elsevier]