The 2012–2013 Montes Claros Earthquake Series in the S˜ao Francisco Craton, Brazil: New Evidence for Non-uniform Intraplate Stresses in Mid-Plate South America

levy, weitz;

On 2012 May 19, an mb = 4 earthquake shook the town of Montes Claros, Brazil in the middle of the S˜ao Francisco Craton. Because of the scarce seismicity in the area, an event like this could provide valuable information to characterize the governing seismotectonics and stress field for the region. Here, we present the results of more than 1 yr of local seismic monitoring after the main shock. We found that the seismicity originated at approximately 1-km depth in an NNW-oriented blind reverse fault, dipping to the E. The magnitude of the main shock was 4mb, with aftershocks reaching up to 3.6mb. Focal mechanisms from first motion polarities and waveform moment tensor inversions indicate a reverse faulting in agreement with the orientation of the aftershock locations. In addition, we derived a new 1-D local velocity model using a simultaneous inversion of hypocentres and velocity layers. The resultsindicateP-wavevelocitiesof4.5kms−1 fortheupperlayerofcarbonaterocksand5.23 and 5.69kms−1 for the lower fractured and compact crystalline basement layers, respectively. Higher Vp/Vs ratios were obtained for the upper two layers compared to the lowermost layer, possibly indicating presence of rock fracturing and percolated water. The calculated stress drop for the main event is 0.33 MPa, which is a relatively low value for an intraplate earthquake but still within the observed range. The inversion of the main shock focal mechanism and previously published focal mechanisms suggests a compressional stress regime in the central part of the S˜ao Francisco Craton, which is different from the strike-slip regime in the southern part, although both have an EW-oriented σ1. On the other hand, focal mechanisms of events located to the west of the craton indicate an NW–SE oriented σ1 for central Brazil. This variability highlights the importance of local sources of stresses (e.g. flexural stresses) in mid-plate South America, unlike other mid-plate areas of the world, such as central and east North America, where a more uniform stress field is observed.