Focal mechanisms of small earthquakes in the southeastern Brazilian shield: a test of stress models of the South American plate

Abstract

Focal mechanisms of small earthquakes with magnitudes of about 3 in the SE Brazilian shield are calculated using S/P amplitude ratios. Low attenuation (Qp from 400 to 800) in the shield upper-crustal layers allowed sharp S arrivals to be recorded up to distances of 100km. Besides P-wave polarities, SH-wave first motions were also used to constrain the nodal-plane orientations. Normal and reverse faulting mechanisms with strike-slip components were found. The inversion of four mechanisms to estimate the stress tensor indicated a strike-slip stress regime with roughly E–W-orientated σ1 and N–S σ3. Both the orientations and the shape factor (φ=0.7) of the inverted stress are in excellent agreement with theoretical predictions for that part of Brazil from the driving-force model of Coblentz & Richardson (1996). Good agreement with the nature of the stress, as well as its orientation, was also found for the model of Meijer (1995). Both of these theoretical models include spreading stresses along the continent/ocean lithospheric transition. Because the earthquakes are more than 300km from the continental shelf they should not be affected by the local flexural forces caused by sediment load in the marginal basins. The agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses.