Intraplate Stress Field in South America from Earthquake Focal Mechanisms: New data from waveform inversion of small regional events.

We updated the previous compilation of earthquake focal mechanisms in Brazil and in the subAndean region. For Brazil, about ten new focal mechanisms were determined by waveform inversion using path-specific velocity models derived from group velocity inversion of Rayleigh and Love waves, checked against P-wave polarity data. The new dataset is used to derive the stress tensor orientation in mid-plate South America. Most events in the sub-Andes show reverse or strike-slip mechanisms. Focal mechanisms in Brazil are also mainly reverse and strike-slip faulting. In the sub-Andes, stresses are compressional with the principal major compression (S1) E-W, on average. A slight rotation of S1 is observed, controlled by the orientation of the Andean plateau. In the sub-Andes the intermediate principal stress (S2) is also compressional. In mid-plate South America, stresses vary in nature and orientation. In SE Brazil and near the Chaco-Pantanal basins, S1 is oriented roughly E-W with S2 approximately equal to S3. This stress pattern changes to purely compressional (compressional SHmax and Shmin) in the north of the São Francisco craton. A rotation of SHmax from E-W to SE-NW is suggested towards the Amazon region. Along the Atlantic margin, the regional stresses are affected by coastal effects, which tend to make SHmax parallel to the coastline and Shmin (usually S3, extensional) perpendicular to the coastline. The few breakout data and in-situ measurements in Brazil are generally consistent with the pattern derived from the focal mechanisms. Although ~E-W trending SHmax is usually well explained by global and regional numerical models of plate stresses, the SE-NW orientation across the Amazon craton has not been explained by any current numerical model of intraplate stresses.