Events Relocation and Source Parameters of Central Brazil Earthquakes

In this paper, we performed the relocation and moment-tensor determination of events located in Central Brazil within the Goiás-Tocantins Seismic Range (GTSR). The GTSR is a part of the Tocantins Geological Province which includes, among others, the large geological structure (Lineament Transbrasiliano-LTB). The region is characterized, with few exceptions, by low magnitude seismicity (M < 3.5) distributed along a belt parallel to the LTB but not coincident. The events were recorded only by a few stations from the Brazilian Seismograph Network (RSBR) deployed in 2014.

Most of the events are too weak to have their Focal Mechanisms (FM) obtained by the traditional method of P-phase polarities. To overcome this limitation, for some events, we used ISOLA for the standard waveform inversion, the CSPS (Cyclic Scanning of the Polarity Solutions), which makes use of the full waveform inversion to constrain the non-unique focal mechanism solutions obtained by the FOCMEC program from a set of a few available polarities and the recently developed technique of inversion of waveform Envelopes (ENV).

Firstly the work was focused on improving the hypocentral location of the Brazilian seismic catalog using a nonlinear method, to better define the location uncertainty, as well as to relocate the events using the RSTT201404um (Regional Seismic Travel Time) model. This model includes Pg and Lg uncertainty values together with an integration of the crust and Moho depths for South America. This enables the relocation of the horizontal and vertical errors to about 10 km, thus making it possible to correlate the GTSR seismicity with the knowing geological structures including the LTB large scale regional structure.

Secondly, with the help of the codes ISOLA, the CSPS and ENV, we determined the focal mechanisms of some weak events with the aim of obtaining the regional stress field which is important in order to understand the tectonics and intraplate seismicity, particularly at the GTSR.