The Rio Grande do Sul Offshore Earthquake, February 12th 1990

The occurremce of earthquakes offshore the Rio Grande ela SuI state, in Brazil, are uncommon. This last one, from February 12, 1991, reached magnitude 5.2 mb and was recorded by several stations world-wide. A focal depth of 28 km was determined using pP and sP phases from digitized seismograms from Europ,Africa and North America. A focal mechanism solution, using P-wave first motion and the negative polarity of pP phases to determine the focal planes, showed a 32° reverse faulting, 40° dip, and P axis oriented WNW-ESE, normal to coastal alignement. The depth and this compression direction suggest a legional spreading stress due to the different densities of continental and oceanic lithosphere that can cause reverse faults resulting from the combination of integrated plate driving forces, "ridge-push" and basal drag.