Bezerra, Francisco Hilario Rego; (1998) Neotectonics in northeastern Brazil. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The thesis describes neotectonic deformation in the continental intraplate region of northeastern Brazil and explores its links with modern seismicity. The region, which is under E-W-oriented compression and N-S-oriented extension, shows shallow earthquake swarms which last for several years and include 5.0-5.2 mb events. Remote sensing, borehole and geophysical data, in conjunction with field structural information, indicate a continuous faulting process since the Miocene which has reactivated Cretaceous faults and Precambrian shear zones or in places generated new faults which cut across existing structures. Three main sets of faults are recognised across the area: a NE-striking set, a NW-striking set and a N-striking set. The first and the second sets are pervasive and their cross-cutting relationships show that they locally form a conjugate set and display both a strike-slip and a dip-slip component of movement. They have generated troughs filled by as much as 260 m of Cainozoic sediments. Radiocarbon dating shows that some of the faults slipped as recently as 4,041-3,689 cal. yr BP. Although the elevation of coastal deposits is consistent with the predictions of glacioisostatic models for the area, tectonic influence can be detected notably near the Carnaubais fault, where rapid emergence by at least 5 m to the east of São Bento occurred 4,080-2,780 cal. yr BP. Secondary ground failure, which includes hydroplastic deformation, liquefaction and landslides, can be seen in Quaternary alluvial sediments and is reported in the historical record. The present data show that the potential for large earthquakes in northeastern Brazil has been underestimated. Empirical relationships using liquefaction and surface rupture point to events of at least Ms=6.8 compared to a maximum mb = 5.2 recorded instrumentally. The finding that NE- and NW-trending faults are favourably orientated for reactivation in relation to the current stress field is of potential value for seismic hazard assessment.

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