Local shear wave observations in João Câmara, northeast Brazil

Abstract

We present shear wave splitting data from a 10 km aperture, eight‐station digital three‐component seismograph network, which operated from December 1992 to August 1994 near João Câmara (5°33′S, 35°51′W), to record shear waves from continuing microearthquake activity persisting 6 years after a main shock in November 1986. Previous aftershock recording had shown remarkably impulsive simple seismograms and hypocentral errors of typically 200 m in a structure of near‐uniform wave speed, but only one station was three component. The pattern of polarization directions shown is consistent with the trend of Precambrian foliation observed in the field. It is therefore concluded that these shear wave observations are controlled by the seismic anisotropy associated with the Precambrian foliation and hence bear the signature of the “paleo‐stress field,” i.e., the Precambrian deformation regime. This conclusion has important implications for the interpretation of shear wave splitting observations made in crustal crystalline rock, because other authors have asserted that shear wave splitting normally results from the effect of stress‐aligned fluid‐filled cracks (extensive dilatancy anisotropy or EDA). EDA is, by contrast, a signature of the present‐day stress field. In this region, there is firm evidence that the present‐day maximum compressive stress is E‐W. This evidence comes from earthquake focal mechanisms and borehole breakout data and is summarized in the World Stress Map. We therefore conclude that our present data are in maximum conflict with an interpretation in terms of EDA. Our interpretation is supported by the observation that delay times are consistent with those expected from modeling of elastic waves in the crust.