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Bulletin of the Seismological Society of America; February 1998; v. 88; no. 1; p. 160-169
© 1998 Seismological Society of America
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Seismicity and stresses in the Brazilian passive margin

Marcelo Assumpção

Institute of Astronomy and Geophysics University of São Paulo, P.O. Box 3386, São Paulo, SP, 01065, Brazil

Abstract

Seismicity and stresses along the Brazilian continental margin show different patterns between the northeastern and southeastern regions. In the northeastern margin, earthquakes tend to occur onshore under a strike-slip regime with horizontal compression parallel to the northern coast line. In the southeastern margin, higher seismicity is observed offshore, in areas where the continental crust was highly extended during the South Atlantic rifting in the Mesozoic. Three new events in the southeast continental shelf are shown to have reverse-faulting mechanisms, from waveform modeling of short-period teleseismic P waves. Multiple depth phases (reflections from the top and bottom of the water layer and double reflections in the water layer) could be identified and better constrained the hypocenters to middle and upper crustal depths. In South America, models of intraplate stresses caused by plate boundary forces and spreading effects due to the continental/oceanic crustal transition indicate higher compressional stresses in the SE offshore area, as compared with the continental area, in agreement with the observed higher seismicity and reverse-faulting mechanisms. The combination of regional stresses, local flexural effects from thick sedimentary loads, and a presumably weaker crust from Mesozoic thinning explains the main patterns of seismicity in the northeastern and southeastern Brazilian margins.




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