Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998geoji.134..341f&link_type=abstract
Geophysical Journal International, Volume 134, Issue 2, pp. 341-355.
Astronomy and Astrophysics
Astronomy
12
Fault-Plane Solutions, Intraplate Earthquakes, Ne Brazil, Stress Distribution
Scientific paper
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.
Assumpção Marcelo
Ferreira Maria Joao
Oliveira T.
Takeya Mario Koechi
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