HR: 0800h
AN: S51A-07    [Abstracts]
TI: Intraplate Earthquake Swarm in Belo Jardim, NE Brazil: Reactivation of a Major Late Proterozoic Shear Zone (Pernambuco Lineament)
AU: Lopes, A E
EM: afonso@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: * Assumpcao, M
EM: marcelo@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: Nascimento, A
EM: aderson@dfte.ufrn.br
AF: Univ. Fed. Rio Grande do Norte, UFRN, Natal, RN , Brazil
AU: Menezes, E
EM: eduardo@dfte.ufrn.br
AF: Univ. Fed. Rio Grande do Norte, UFRN, Natal, RN , Brazil
AU: Barbosa, J R
EM: jroberto@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: Ferreira, J
EM: joaquim@dfte.ufrn.br
AF: Univ. Fed. Rio Grande do Norte, UFRN, Natal, RN , Brazil
AB: Intraplate 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. Stress concentrations can occur due to basement heterogeneities (including lithospheric thinning), thermal expansion/contraction, and isostatic/flexural deformation. Stresses from continental/oceanic crustal transition and flexure from sediment load in the continental margin have been shown to be a major factor to explain the observed seismicity along the continental margin in NE Brazil. Although Northeastern Brazil is one of the most seismically active areas in the country (magnitudes 5 roughly every 5 years), with hypocentral depths shallower than ~10 km and seismic zones as long as 30-40km, no clear relationship with the known surface geology can be established with confidence, and a clear identification of zones of weakness has not been possible. The 2004 earthquake swarm of Belo Jardim (magnitudes up to 3.0) and the recurrent activity in the nearby town of Caruaru (magnitudes up to 3.8), both located along the major Late Proterozoic shear zone (known as Pernambuco Lineament), are the first clear indication of seismic activity occurring as reactivation of an old structure. A seismic network of 12 stations, operated in Belo Jardim during 20 days in November 2004, showed that the activity occurred by normal faulting on a North dipping, E-W oriented fault plane. This fault plane is in close agreement with the E-W trending structures within the Pernambuco Shear zone. The Belo Jardim activity was concentrated in a 2 km (E-W) by 2 km (downdip) fault area, with an average depth of 5 km. The nearby Caruaru activity occurs as both strike-slip and normal faulting, also consistent with the Pernambuco Lineament. The focal mechanisms of Belo Jardim and Caruaru indicate E-W compressional and N-S extensional stresses. The orientation of this local stress regime cannot be explained only by forces originated near the continental margin, implying that some additional factors are necessary to understand the causes of the seismic reactivation of the old Pernambuco Lineament
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8107 Continental neotectonics (8002)
DE: 8164 Stresses: crust and lithosphere
SC: Seismology [S]
MN: 2007 Joint Assembly