NEOTECTONIC MOVEMENTS IN NORTHEASTERN BRAZIL: IMPLICATIONS FOR A PRELIMINARY SEISMIC-HAZARD ASSESSMENT

Abstract

Northeastern Brazil is located on the Brazilian passive margin within the intraplate part of South America. The region comprises a Precambrian crystalline basement deformed mainly by shear zones, overlain by Cretaceous sedimentary basins formed during the South America-Africa breakup and by Cenozoic units. There are two main sets of faults in the region, trending NE and NW. The maximum horizontal compression deduced from focal mechanisms is oblique to the NE-and the NW-trending faults, in a compressive direction which favors right-lateral and left-lateral strike-slip movement respectively. Widespread faulting from the beginning of the deposition of Barreiras Formation to the Quaternary indicates that the neotectonic period in northeastern Brazil started in the Miocene. Judging from the historical and instrumental seismicity, surface rupture and liquefaction is unlikely anywhere in the region. But the neotectonic record shows widespread evidence of liquefaction, surface faulting and coseismic uplift. Liquefaction in Quaternary alluvial sediments, for example, points to strong paleoearthquakes. The recognition of strong and pervasive seismic faulting from the Miocene to the Holocene indicates that the seismic hazard in northeastern Brazil has been underestimated.