HR: 1400h
AN: S33A-09    [Abstracts]
TI: Spatial distribution of hypocentres and crustal model in Joao Camara, Northeastern Brazil
AU: * Fernandes, C
EM: celia@iag.usp.br
AF: University of Sao Paulo, Rua do Matao, 1226 Geophysics Departament, Sao Paulo, SP 05508-090, Brazil
AU: Berrocal, J
EM: berrocal@iag.usp.br
AF: University of Sao Paulo, Rua do Matao, 1226 Geophysics Departament, Sao Paulo, SP 05508-090, Brazil
AU: Takeya, M
AF: University of Rio Grande do Norte, Campus Universitario Lagoa Nova, Natal, RN 59072- 970, Brazil
AU: Bezerra, F H
EM: bezerrafh@geologia.ufrn.br
AF: University of Rio Grande do Norte, Campus Universitario Lagoa Nova, Natal, RN 59072- 970, Brazil
AU: Sislva, J
EM: josimar_usp@yahoo.com.br
AF: University of Sao Paulo, Rua do Matao, 1226 Geophysics Departament, Sao Paulo, SP 05508-090, Brazil
AU: Barros, P
EM: pedro@iag.usp.br
AF: University of Sao Paulo, Rua do Matao, 1226 Geophysics Departament, Sao Paulo, SP 05508-090, Brazil
AB: An important seismic cycle that begun in August 1986 occurred in João Câmara, Northeastern region of Brazil, which lasted for several years producing more than 60.000 small to medium magnitude earthquakes. The more important event of this cycle (m_b 5.1 and intensity VII MM) occurred on November 30, 1986. The epicentral area is on the border of the Late Cretaceous sedimentary Potiguar basin that overlays the Precambrian shield. Large Late Precambrian SW-NE transcurrent ductile shear zones divide the shield in this region. These shear zone have been reactivated in the Cretaceous and again in the Cenozoic as brittle structures. Data for this study were collected from a local nine-station network installed by Cardiff and UFRN universities, obtained during June to September 1987. Using a simultaneous inversion method, hypocentres of some earthquakes of that sequence and a local structural model, were obtained with the VELEST program. Those hypocenters were also relocated with the HYPODD program for comparing the results obtained with both methods. An elongated SW-NE feature around represents the seismogenic source 40 km long. Stable solutions for the simultaneous inversion were obtained after about 400 iterations for the northern portion and 53 for the southern portion. The mean structural model beneath the seismographic network show tree layers with increasing VP from around 5.8 km/s to around 6.2 km/s, at the bottom of the third layer in 6.5 km of depth, showing also a probable low velocity layer between 3 and 5.5 km of depth. The hypocentres present depths between 0.9 and 9.0 km. The spatial distribution of those hypocentres and the composite focal mechanism solutions suggest a correlation with a dextral transcurrent fault, passing close to the east of Joao Camara, with a small normal component segmented in at least three no- coplanar portions with a dominant strike of around 41°. The dip angle, oriented to the NW, changes from 76° in the northern portion to 66° in the southern one; whereas the slip angle increases from 10° to 35° from northern to southern portions. The spatial distribution of those events seems to correlate well with the surface tectonic features described above, and with the regional stress pattern. Focal mechanism and breakout data indicate that the region is under a roughly E-W-trending SH max related to a strike-slip faulting regime.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: 2007 Joint Assembly