A discussion about the causes of the intraplate seismicity in the Central Brazil from P-wave travel-time tomography results

Abstract:

The study of the intraplate seismicity is very important to help the understanding of the neotectonic processes. We present a discussion of the possible causes of intraplate seismicity in the region of Tocantins Province in Brazil from recent P- and S-wave seismic tomography results. The regional stress field in South America (SA) is dominated by compressions in east-west direction (Zoback, 1992). The origin of this system of tensions is mainly related to the formation of the South American plate from the mid-Atlantic ridge, and resistive forces exerted by the subduction of Caribbean and Nazca plate (Mendiguren & Richter, 1978; Coblentz & Richardson, 1996). Assumpção et al. (2004) that high seismicity regions in SA is due to the weakness of the lithosphere with his thinning. This hypothesis is based on the coincidence between seismic tomographic low-velocity anomalies and events locations, which could be indicate regions of lithospheric thinning. According to this model, stresses would be concentrated uniformly in depth along the lithosphere. In regions with thicker lithosphere, the intensity of the focused stress would be smaller than the thinner regions. Thus, the crust in regions of thinner lithosphere concentrates more intense stresses promoting a higher seismicity. We observed in the Tocantins Province, in Central Brazil, the same pattern observed in the work of Assumpção et al. (2004). The region of low velocity in the Tocantins Province center is usually accompanied by a high concentration of seismic events following the direction of the Transbrasiliano Lineament. Thus, this lineament could be reactivated by stresses accumulated in the crust due to thin lithosphere. Assumpção & Sacek (2013) proposed that flexural stresses from uncompensated lithospheric loads are high enough to explain the seismic zone, which continues for the northern part of the Tocantins Province. The improvement of the tomography results suggest that lithospheric thinning is still important to explain intraplate seismicity in the Central Brazil.