The São Vicente Earthquake Of April 2008 In The Se Continental Shelf, Brazil

The continental margin and shelf of most stable intraplate<br>regions tends to be relatively more seismic than the<br>continental interior. In the SE continental margin of Brazil<br>a seismic zone can be defined from Rio Grande do Sul to<br>Espírito Santo, with activity mainly along the continental<br>slope suggesting a close relationship with flexural<br>stresses caused by the weight of the sediments.<br>Earthquakes with magnitudes larger than about 5 mb<br>occur every 15 to 20 years, on average. Focal<br>mechanism solutions of previous earthquake in this zone<br>showed reverse faulting on planes dipping roughly 45o<br>with horizontal P axes. The recent 5.2 mb earthquake of<br>April 23rd, 2008, occurred 125 km south of São Vicente<br>and was well recorded by many stations in SE Brazil as<br>well as at teleseismic distances in North America and<br>Africa. Its focal depth is 17 km, placing the hypocenter in<br>the lower crust. A well determined focal mechanism<br>solution shows one vertical nodal plane and one subhorizontal<br>nodal plane. The P and T axes have large<br>plunges. This uncommon orientation of the fault<br>mechanism can be attributed to a rotation of the principal<br>stress directions in the lower crust caused by flexural<br>effects due to sediment load.