Earthquake relocation, focal mechanism and stress field determination in central Brazil

Abstract

In this work, we perform a seismological investigation in the central Brazil region for focal mechanisms and stress. This large area (910.000 km2) is surrounded by several geological provinces: Eastern Amazon craton; southwestern Parnaíba basin; western São Francisco craton; northern Paraná basin and almost the whole Tocantins province. The area is crossed, in an SW-NE direction, by the continental-scale discontinuity - Transbrasiliano lineament (LTB). In the Brazilian seismic catalog (1720–2019), it is possible to distinguish a seismic belt following the LTB lineament and some dispersed events in the northern, western and eastern areas. The study area is characterized mainly by low magnitude events (M < 4); only 11 events reported in the catalog with M > 4 occurred, and among them just one with magnitude 5. Based on quality criteria a set of 118 events from 338 present in the catalog were selected for a thorough analysis. This work was divided in three parts: (i) Events relocation with the iLoc code and Regional Seismic Travel Time (RSTT) velocity model; (ii) Focal mechanisms determination of 10 events using waveform envelopes inversion and polarities, and; (iii) Inversion of focal mechanisms for stress field. The study shows that the seismicity of the area is mainly concentrated in two relatively narrow belts and that the major compressional stress axis of the whole zone is well resolved with azimuth about 133° which is compatible with previous studies of stress in central Brazil.