HR: 0800h
AN: S51A-05    [Abstracts]
TI: The Intraplate Seismogenic Zone of Porto dos Gauchos in the Amazon Craton, Brazil
AU: * Barros, L V
EM: lucas@unb.br
AF: Universidade de Brasilia, Campus Universitario Darcy Ribeiro, predio SG13, Brasilia, DF 70910900, Brazil
AU: Assumpcao, M S
EM: marcelo@iag.usp.br
AF: Universidade de Sao Paulo, IAG/USP, Sao Paulo, SP , Brazil
AU: Quintero, R Q
EM: obsis@unb.br
AF: Universidade de Brasilia, Campus Universitario Darcy Ribeiro, predio SG13, Brasilia, DF 70910900, Brazil
AB: The Porto dos Gauchos Seismogenic Zone (PGSZ), in the center north of Mato Grosso State, in the contact between the southern Amazonian Craton and northern Parecis Basin, represents one of the most important area of seismic activity in Brazil, with the largest magnitude ever observed in the stable continent of the South America plate (6.2 m_b on January 31, 1955). Focal mechanism studies indicated a pure reverse faulting regime with compressional S_Hmax. oriented in SE-NW direction. After the 1955 earthquake, located in Serra do Tombador, a recurrent seismicity has been detected in Porto dos Gauchos, 100 km northeast of Serra do Tombador. No recent events have been detected in the area of the 1955 epicenter, suggesting a long recurrence time or mislocation of Serra do Tombador earthquake. The Porto dos Gauchos recurrent seismicity has been observed since 1959, when a 4.5 macroseismic estimated magnitude was felt by local inhabitants settled in that remote area two years earlier. In subsequent years, with deployment of regional stations in Brazilian Amazon region earthquakes were detected in 1981 (3.8 m_b), 1989 (3.3 m_b), 1993 (3.8 m_b), 1996 (4.4 m_b), 1997 (3.3 m_b), and finally on March 10, 1998 (5.1 m_b). The aftershocks of 1998 main shock were studied with a local network with up to eight 3- component stations. Such network detected more than 2500 events until December of 2002, when the network was deactivated, but only 100 were accurately located. Based on this set of events and a controlled source experiment we determined a 1-D velocity model for the area, a composite focal mechanism with P wave polarities, spectral analysis studies to estimate the source dimension, stress drop and moment magnitudes for the main shock and some others events of the set. On March 23, 2005 another shock occurred in the same seismogenic area of Porto dos Gauchos, with magnitude 4.7 to 5.0. One week later five stations were installed again to monitor the aftershock activity, detecting more than 2000 events in three months. Hypocentral determination, with different hypocentral location programs (Hypocenter, Hypo71, Hypoinverse, Velest and NonLinLoc) and a composite focal mechanism indicated the same source location and similar fault regime (dextral transcurrent fault for both 1998 and 2005 sequences). In this work, besides a brief review of the seismotectonic of the PGSZ we intend to present additional results of better constrained composite focal mechanisms including the P, SH, and SV amplitudes ratios for the two sequences and an attempt to get focal mechanism solution for the main shock of March 1998 based on regional stations.
DE: 7205 Continental crust (1219)
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: 2007 Joint Assembly