Fault plane solutions of intraplate earthquakes in brazil: Some constraints on the regional stress field

Abstract

Determination of intraplate stress fields is fundamental for a better understanding of the forces driving the plates. In the South American plate, few earthquakes away from the plate boundaries have been recorded by enough stations to have the nodal planes well determined. The fault plane solutions of four recent events in central and northern Brazil were determined. These solutions were compared with previously determined solutions of other events in central and southeast Brazil. A statistically reliable estimation of the stress field for all of Brazil is not yet possible on the basis of these data. Near the Atlantic coast, however, the data are compatible with a regional stress field with the maximum compressive stress oriented parallel to the expected direction of ridge-push or of South America's absolute plate motion (Mendiguren and Richter, 1978). Farther to the west, the stress field changes; the direction of the maximum compressive stress seems to be aligned N-S rather than E-W. Almost all earthquakes analysed show reverse or strike-slip focal mechanisms; one event in central Brazil, however, shows normal faulting. Thus, the stress field in Brazil is certainly not uniform and, in some areas, local sources of stress may predominate over regional fields.