Intraplate earthquake swarm in Belo Jardim, NE Brazil: reactivation of a major Neoproterozoic shear zone (Pernambuco Lineament)

Abstract

S U M M A R YIntraplate earthquakes in stable continental areas have been explained basically by reactivation of pre-existing zones of weakness, stress concentration, or both. Zones of weakness are usually identified as sites of the last major orogeny, provinces of recent alkaline intrusions, or stretched crust in ancient rifts. However, it is difficult to identify specific zones of weakness and intraplate fault zones are not always easily correlated with known geological features. Although Northeastern Brazil is one of the most seismically active areas in the country (magnitudes 5 roughly every 5 yr), with hypocentral depths shallower than ∼10 km and seismic zones as long as 30-40 km, no clear relationship with the known surface geology can be usually established with confidence, and a clear identification of zones of weakness has not yet been possible. Here we present the first clear case of seismic activity occurring as reactivation of an old structure in Brazil: the Pernambuco Lineament, a major Neoproterozoic shear zone. The 2004 earthquake swarm of Belo Jardim (magnitudes up to 3.1) and the recurrent activities in the nearby towns of São Caetano and Caruaru (magnitudes up to 4.0 and 3.8), show that the Pernambuco Lineament is a weak zone. A local seismic network showed that the Belo Jardim swarm of 2004 November occurred by normal faulting on a North dipping, E-W oriented fault plane in close agreement with the E-W trending structures within the Pernambuco Lineament. The Belo Jardim activity was concentrated in a 1.5 km (E-W) by 2 km (downdip) fault area, and average depth of 4.5 km. The nearby Caruaru activity occurs as both strike-slip and normal faulting, also consistent with local structures of the Pernambuco Lineament. The focal mechanisms of Belo Jardim, Caruaru and S. Caetano, indicate E-W compressional and N-S extensional principal stresses. The NS extension of this stress field is larger than that predicted by numerical models such as those of Coblentz & Richardson and we propose that additional factors such as flexural stresses from the nearby Sergipe-Alagoas marginal basin could also affect the current stress field in the Pernambuco Lineament.

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

“…Lopes et al 2010;Bezerra et al 2011). The earthquakes coincide with shear zones containing prominent foliations.…”

“…Ferreira et al (2008 and Lopes et al (2010) documented seismicity with a range of focal mechanism orientations spatially coinciding with the Pernambuco shear zone towards the centre of the Borborema province. They concluded that the earthquake locations and kinematics were controlled by the shear zone mylonitic foliation.…”

Seismogenic faulting in the Meruoca granite, NE Brazil, consistent with a local weak fracture zone

“…Seismically-defined faults either reactivate or cut across preexisting structures (Bezerra et al 2011). For example, Ferreira et al (2008) and Lopes et al (2010) concluded that the seismicity in the Caruaru and Belo Jardim areas reactivated the Pernambuco lineament, a continental-scale ductile shear zone ( Fig. 1), established at the end of the Neoproterozoic Brasiliano orogeny (Brito Neves et al 2000).…”

Analyzing the temporal fluctuations of the reservoir-triggered seismicity observed at Açu (Brazil)

“…In 1930, another event was felt with intensity IV to V MM in Açu but the epicenter is not known. Additionally, swam-like seismic activity has been extensively reported and investigated in NE Brazil using local seismographic networks (Ferreira et al, 1998(Ferreira et al, , 2008Lopes et al, 2010;Bezerra et al, 2007).…”