Superposition of local and regional stresses in northeast Brazil: evidence from focal mechanisms around the Potiguar marginal basin

Abstract

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.

Seismogenic faulting in the Meruoca granite, NE Brazil, consistent with a local weak fracture zone

“…(Ferreira and Assumpção 1983). These events are felt over several hundred kilometers away from the epicenter (Ferreira et al 1998). Active faults in Borborema Province are usually strike-slip and generate events of up to 5.2 m b .…”

“…1), established at the end of the Neoproterozoic Brasiliano orogeny (Brito Neves et al 2000). By contrast, seismicity in the Palhano area cuts across the NE-trending Precambrian fabric (Ferreira et al 1998, Bezerra et al 2011). …”

Estruturas rasas na margem equatorial ao largo do nordeste brasileiro (Estado do Ceará): análise de relevo e anomalias gravimétricas residuais

“…Esta particularidade e a extensão de ambas as anomalias por toda a plataforma relacionam-se com a grande continuidade lateral do Lineamento Transbrasiliano, a qual foi truncada durante a separação entre América do Sul e 74 ESTRUTURAS RASAS NA MARGEM EQUATORIAL BRASILEIRÁ Africa, e que encontra como contraparte africana o Lineamento Kandi-4 • 30' (Trompette, 1994). A coincidência entre a anomalia de relevo positiva e a bacia cretácea foi interpretada por Silva Filho (2004) como uma inversão inicial da Sub-bacia Acaraú, relacionada a uma deformação transpressiva dextral nas falhas de trend NE-SW, resultado da compressão E-W a NW-SE que tem afetado a faixa litorânea dessa porção da margem equatorial (Assumpção, 1998;Ferreira et al, 1998), possivelmente desde o final do Terciário.…”

Ongoing Compression Across Intraplate South America: Observations and Some Implications for Petroleum Exploitation and Exploration

“…This is shown by stress data compilations (Assumpção 1998(Assumpção , 1992Ferreira et al 1998;Lima et al 1997); intraplate stress field numerical models (Coblentz and Richardson 1996;Meijer 1995) and space-based geodetic results (Lima 1999). All but one the available base lines from the French DORIS geodetic space-based system are shortening.…”