Sismicidade do sistema transformante de São Paulo, no Atlântico Equatorial, de janeiro - julho de 2013

Abstract

The Equatorial Atlantic is formed by several slow-slipping ridges and transform faults also associated with inactive fracture zones. Among these, the St. Paul Transform System (SPTS) is a complex multi-fault with four faults and three intra-transform segments with a long-offset of 630 km. In the northern transform, the 200 km long and 30 km wide Atobá Ridge is a major topographic feature that reaches the sea level at the St. Peter and St. Paul Archipelago (SPSPA). We have determined the hypocentral location of 62 minor-moderate earthquakes of SPTS. The earthquakes occurred in 2013 and were recorded by a seismometer installed in SPSPA and three hydrophones deployed during the COLMEIA cruise. A seismogenic zone with a deep brittle-ductile transition was identified in SPTS, with hypocenters reaching 18 km beneath the seafloor. We observed that this lithospheric structure presents relation with the offset age and controls the maximum hypocentral depths of oceanic transform faults. Besides, the earthquakes indicated the existence of a broad serpentinization depth reaching 18 km beneath the Atobá Ridge. We interpreted this as an effect of deep-water percolation into the mantle in the SPTS, which cause a fluid-mantle rocks interaction and allowed the growth of faults into the mantle. Some hypocenters were in the central fracture zone (CFZ) segment of SPTS and their depths reached 8.8 km beneath the seafloor. We interpreted this seismicity as reactivation of a weakness zone existent in CFZ, which occurred due to the transpressive load-induced stress from the Atobá Ridge.