Source mechanisms of moderate-size earthquakes and stress orientation in mid-plate South America

Fault plane solutions of six mid-plate earthquakes with magnitudes ranging from 4.8 to 5.5 mb were determined using P-wave first motion data and the relative amplitudes of short-period teleseismic depth phases (pP and sP). These results, together with previous studies, show a predominance of reverse and strike-slip faulting indicating horizontal compressional stresses in mid-plate South America. The orientation of the P-axes, however, is not uniform. In western and southwestern Brazil the P-axes are oriented approximately E-W (probably due to the Nazca-South America plate convergence). In eastern Brazil the direction of the P-axes show more scatter, but an average E-W direction is consistent with the available data. On the contrary, in northern Brazil, the P-axes of two events in the Amazon basin trend approximately N-S, a direction perpendicular to the direction of absolute plate motion as well as to the relative plate motions Nazca-South America and South America-Africa.