The Paradigm of Stable Intraplate Regions and Neotectonics in Northeastern Brazil

Abstract

Active faults in stable continental regions are less frequent than those in plate boundaries but cannot be ignored. These faults can generate earthquakes in intraplate settings that cause widespread damage in regions where the population is ill-prepared to cope with them. Intraplate earthquakes are due to many causes such as reactivation of ancient rifted crust, major terrane boundaries, density and rigidity contrast, heat flow, and fault intersection. The present-day stress field in the South American plate roughly follows the trajectories of the absolute plate, i.e., is mainly E-W-oriented. In Northeastern Brazil, the stress regime is mainly strike-slip and the maximum horizontal stresses follow the equatorial coastline. Northeastern Brazil is one of the most seismically active parts of the South American stable continental region and presents earthquakes up to magnitude Mb = 5.2. Seismicity is concentrated in the upper crust down to a depth of 12 km and illuminates active faults up to 40 km long. Active faults either reactivate shear zones or regional foliation and quartz veins or cut across the preexisting tectonic fabric.