Source process of a large deep-focus earthquake and its tectonic implications — the Western Brazil earthquake of 1963

Abstract

A large deep-focus earthquake ( h = 577 km) in the western Brazil region that occurred on November 9, 1963, was followed by a relatively large aftershock. The long-period P wave records of the WWSSN for the main shock reveal three successive P phases. Regarding this earthquake as a triplet, the temporal and spatial relations among the three successive events are examined on the basis of the azimuthal distribution of the difference between the arrival times of the first and the second P phases and that of the first and the third P phases. The separation between the first shock and the aftershock in space and time is determined from the time differences of the first P arrivals which are read on the short-period seismograms of the WWSSN. The second and the third foci and the focus of the aftershock are found to be all situated on one of the P nodal planes of the first shock. This result almost definitely suggests that the earthquake occurred in the form of shear faulting. The primary P wave forms are obtained from the long-period P wave signals after eliminating the influence of the seismograph, crust and mantle. The seismic moment is accurately determined as 2.2 × 10 27 dyne · cm from their time integrations. The fault process is inferred from the spatial and temporal relations among the four shocks and from the primary P wave forms: the rupture which initiated at the first focus spread out with a uniform velocity and covered a more or less fan-shaped surface. The velocity of rupture propagation, faulted area, displacement discontinuity and the stress drop are estimated to be 2.2–2.5 km/s, 510–680 km 2, 265–355 cm and 300–460 b. Some implications are discussed in the light of the plate tectonics. The predominant gap in seismic activity beneath northern Peru, western Brazil has been interpreted in terms of the detachment of the descending lithosphere. The presence of a large-scale normal faulting which extends over the entire thickness of the detached slab of lithosphere is suggested from the present results.