One of the most important seismic areas in Brazil is the northeastern region with earthquake activity occurring mainly in the exposed Precambrian basement around the Potiguar Mesozoic marginal basin. Since 1986, temporary seismographic networks have been deployed at many sites near the border of the Potiguar basin and also further inland. This instrumental effort has allowed a better understanding of the seismicity patterns and crustal stresses in the region.  The seismicity in NE Brazil occurs mainly in swarms (lasting from months to many years) with shallow earthquakes (depths <12 km). These features and the large number of granitic/gneissic outcrops of the Precambrian basement allowed us to obtain good quality seismographic records, with clear P and S arrivals. Even with few stations and analogue recordings, reliable hypocentres and composite focal mechanisms have been determined with simple velocity models. The seismicity shows poor correlation with mapped faults. Several composite and single focal‐mechanism solutions were determined: strike‐slip faulting predominates in the area. Most nodal‐plane solutions are in good agreement with the fault planes determined from the distribution of hypocentres.  The principal stress directions, obtained with inversion of focal mechanisms, show a strike‐slip stress field in the upper crust with maximum stress orientation ranging from SE–NW to E–W, roughly parallel to the northern coastline. Well‐bore breakouts in the Potiguar basin also show that the maximum horizontal compression is roughly parallel to the northern coast line. It is suggested that this pattern is the superposition of E–W compressional regional stresses, generated mainly by ridge push and collisional boundary forces on the South American plate, with local extensional stresses perpendicular to the coast, generated both by the continent—ocean structural transition and by flexural forces from sediment loading at the continental shelf.

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[1] We analyzed borehole breakout data and drilling-induced tensile fractures derived from resistivity image logs run at 10 oil wells to derive the orientation of the maximum horizontal stress S Hmax from the Potiguar Basin in the continental margin of Brazil. Stress magnitudes are derived from density logs for the vertical stress, mini-frac tests for the minimum horizontal stress S hmin , and rock strength laboratory analysis to estimate the S Hmax magnitudes. We compared these results with the stress regime and S Hmax orientation derived from nine earthquake series located in the crystalline basement, where seismicity is concentrated, and previous breakout data from the basin. In the basin, the S Hmax gradient is 20.0 MPa/km, and the S Hmax /S hmin ratio is 1.154, indicating a normal tectonic stress regime from 0.5 to 2.0 km, whereas the S Hmax gradient of 24.5 MPa/km and S Hmax /S hmin ratio of 1.396 indicate a transition from a normal to strike-slip stress regime at 2.5 to 4.0 km. The deeper stress regime in the basin is similar to that in the basement at 1-12 km depth. This transition of the tectonic stress regime is consistent with an incipient tectonic inversion in the basin. We note that the S Hmax orientation rotates from NW-SE in the western part of the Potiguar Basin to E-W in its central and eastern parts, roughly following the shoreline geometry, indicating that local features such as flexural stresses influence the local (scale < 100 km) stress pattern. We also conclude that the basement is critically stressed, but not the basin.

Abstract-Ambient noise correlation analyses are largely used in seismology to map heterogeneities and to monitor the temporal evolution of seismic velocity changes associated mostly with stress field variations and/or fluid movements. Here we analyse a small earthquake swarm related to a main m R 3.7 intraplate earthquake in North-East of Brazil to study the corresponding post-seismic effects on the medium. So far, post-seismic effects have been observed mainly for large magnitude events. In our study, we show that we were able to detect localized structural changes even for a small earthquake swarm in an intraplate setting. Different correlation strategies are presented and their performances are also shown. We compare the classical auto-correlation with and without pre-processing, including 1-bit normalization and spectral whitening, and the phase auto-correlation. The worst results were obtained for the pre-processed data due to the loss of waveform details. The best results were achieved with the phase cross-correlation which is amplitude unbiased and sensitive to small amplitude changes as long as there exist waveform coherence superior to other unrelated signals and noise. The analysis of 6 months of data using phase auto-correlation and cross-correlation resulted in the observation of a progressive medium change after the major recorded event. The progressive medium change is likely related to the swarm activity through opening new path ways for pore fluid diffusion. We further observed for the auto-correlations a lag time frequency-dependent change which likely indicates that the medium change is localized in depth. As expected, the main change is observed along the fault.