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On 2012 May 19, an mb = 4 earthquake shook the town of Montes Claros, Brazil in the\udmiddle of the S˜ao Francisco Craton. Because of the scarce seismicity in the area, an event\udlike this could provide valuable information to characterize the governing seismotectonics and\udstress field for the region. Here, we present the results of more than 1 yr of local seismic\udmonitoring after the main shock. We found that the seismicity originated at approximately\ud1-km depth in an NNW-oriented blind reverse fault, dipping to the E. The magnitude of\udthe main shock was 4mb, with aftershocks reaching up to 3.6mb. Focal mechanisms from\udfirst motion polarities and waveform moment tensor inversions indicate a reverse faulting in\udagreement with the orientation of the aftershock locations. In addition, we derived a new 1-D\udlocal velocity model using a simultaneous inversion of hypocentres and velocity layers. The\udresults indicate P-wave velocities of 4.5 km s−1 for the upper layer of carbonate rocks and 5.23\udand 5.69 km s−1 for the lower fractured and compact crystalline basement layers, respectively.\udHigher Vp/Vs ratios were obtained for the upper two layers compared to the lowermost layer,\udpossibly indicating presence of rock fracturing and percolated water. The calculated stress drop\udfor the main event is 0.33 MPa, which is a relatively low value for an intraplate earthquake\udbut still within the observed range. The inversion of the main shock focal mechanism and\udpreviously published focal mechanisms suggests a compressional stress regime in the central\udpart of the S˜ao Francisco Craton, which is different from the strike-slip regime in the southern\udpart, although both have an EW-oriented σ1. On the other hand, focal mechanisms of events\udlocated to the west of the craton indicate an NW–SE oriented σ1 for central Brazil. This\udvariability highlights the importance of local sources of stresses (e.g. flexural stresses) in\udmid-plate South America, unlike other mid-plate areas of the world, such as central and east\udNorth America, where a more uniform stress field is observedPetrobras - BRASIS projec

geophysical journal international

The 2012–2013 Montes Claros earthquake series in the São Francisco Craton, Brazil: new evidence for non-uniform intraplate stresses in mid-plate South America