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

Abstract

an m b = 4 earthquake shook the town of Montes Claros, Brazil in the middle of the São 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 4m b , with aftershocks reaching up to 3.6m b . 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 results indicate P-wave velocities of 4.5 km s −1 for the upper layer of carbonate rocks and 5.23 and 5.69 km s −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 São Francisco Craton, which is different from the strike-slip regime in the southern part, although both have an EW-oriented σ 1. On the other hand, focal mechanisms of events located to the west of the craton indicate an NW-SE oriented σ 1 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.

Causes of intraplate seismicity in central Brazil from travel time seismic tomography

“…4 shows the effective elastic thickness (Te), available from Bizzi et al (2003) and the strain/ stress regime in the study area, available from Assumpção et al (1985Assumpção et al ( , 1997, Lima et al (1997), Assumpção (1998), Chimpliganond et al (2010), Marotta et al (2013), andAgurto-Detzel et al (2015). Fig.…”

“…b) Strain rate estimated by geodetic observations (Marotta et al, 2013). c) Stress estimated by focal mechanism (FM), breakout (BO), in situ measurements (IS) and geological fault analysis (GF), showed by Assumpção et al (1985Assumpção et al ( , 1997, Lima et al (1997), Assumpção (1998), Chimpliganond et al (2010), Marotta et al (2013), andAgurto-Detzel et al (2015). d) Predominant directions of strain rate and stress for the profile (vertical cross-section Z-Z') region shown in Figs.…”

Earthquake source properties of a shallow induced seismic sequence in SE Brazil

“…The main event in the March 2005 sequence was a m R = 2.9 event [ Assumpção et al ., ]. For SE Brazil, it has been found the relation M w = m R − 0.4 ± 0.1 [ Drouet and Assumpção , ; Agurto‐Detzel et al ., ]. Hence, if we consider the main event to have M w = 2.5, and extrapolating the relation found for corner frequency versus seismic moment (Figure a), we obtain a fc = 9.4 Hz for this event.…”

Lithospheric Features of the São Francisco Craton

“…The southern part of the SFC is characterized by E-W compressional and N-S tensional stresses (Assumpção, 1998;Assumpção et al 2014). Stresses change in the central part of the craton towards E-W compression (Agurto-Detzel et al 2015b). In the Tocantins fold belt province, stresses are also compressional but the orientation of the major compressional axis rotates towards SE-NW, as seen from inversion of focal mechanisms (Carvalho et al 2016) as well as in situ measurements of hydraulic fracturing (Caproni and Armelin 1990).…”