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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SUMMARY We analyse a catalogue of earthquakes recorded near João Cǎmara, Brazil, an intraplate area with a simple velocity structure and low attenuation. The evolution of seismicity is characterized by the seismic b value and the fractal dimension of earthquake epicentre locations measured from sliding 100 event windows. The events are divided into a number of spatial clusters using a nearest centroid sorting technique, and two of these clusters are examined in detail. One cluster, characterized by intense activity, shows a clear negative correlation between b value and fractal dimension of epicentre locations, as has been predicted from simple mechanical models. A second cluster shows a weak positive correlation, and it is suggested that this might be explained in terms of fluid flow in the fault zone.

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Shallow seismicity along a well‐defined section of reactivated Precambrian basement near João Câmara, Brazil (5.5°S, 33.7°W) has continued since August 1986. A magnitude 4.3 event on 21st August with a decaying aftershock sequence and another main shock‐aftershock sequence which started in September were followed by a magnitude 5.1 event on 30th November and 11 events of magnitude 4 or above up to February 1987. A network of up to four vertical component short period seismometers with smoked paper recorders has been deployed since late August 1986. Several thousand events have been recorded above magnitude 0, and activity was still being recorded (about 10 events per day) in May 1987. Simple velocity structure, good transmission and impulsive S arrivals on almost all seismograms combine to facilitate reliable hypocentral locations even for this sparse network, and epicenters define a 25 km linear pattern of seismicity with northeasterly strike, approximately coincident with a Precambrian structural trend. A composite fault plane solution shows dextral shear along this strike with a small normal component. Fault plane solutions and epicenter distribution in Northeastern Brazil suggest an overall picture of E‐W regional compression.