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station NAT in northeastern Brazil and general increase of seismological research in Brazil;
magnitudes above 3.5 since 1980 due to the installation of several stations in various parts
of the country by the University of Brasılia (including the Amazon region) and the Uni-
versity of Sao Paulo. The magnitude thresholds defined above are our best estimates of the
magnitude completeness of our catalogue. The geographically “uniform” catalogue (shown
in Figure 3.2b ) is not declustered. Both the raw ( Figure 3.2a ) and the uniform catalogues
( Figure 3.2b ) show several seismically active areas and large regions almost completely
aseismic.
The stress field in Brazil is still poorly known due to the small number of well-determined
focal mechanisms and few in-situ stress measurements. Data from the compilation of
Assump¸ ao ( 1992 ) , other studies (Lima et al ., 1997 ; Ferreira et al ., 1998 ; Barros et al .,
2009 , 2012; Chimpliganond et al ., 2010 ; Lima-Neto et al ., 2013 ) , and recent unpublished
results were combined to provide a preliminary estimate of the stress patterns in Brazil,
shown in Figure 3.2c . Compressional stresses predominate with a trend of roughly east-
west-oriented maximum horizontal stress (S Hmax ) in eastern Brazil, probably changing to
a more NW-SE orientation in the Amazon region. A strong influence of the continental
margin can be seen, making S Hmax roughly parallel to the coast along most of the Atlantic
shore. The effect of the continental margin has been explained as due to lateral density
contrasts between continental and oceanic crust (e.g., Coblentz and Richardson, 1996 ) and
flexural stresses (Assump¸ ao, 1992 , 1998 ) .
Combining the distribution of seismicity ( Figure 3.2a , b) with the estimated stress
patterns ( Figure 3.2c ) , the main active areas could be delineated as follows: (1) southern
part of the Guyana shield and middle Amazon Basin, the largest known event being a 5.5 m b
in 1983; (2) a north-south-trending zone along the eastern border of the Amazon craton;
(3) northern part of the Borborema Province, in northeastern Brazil around the Potiguar
marginal basin, with the largest known event occurring in 1980 with 5.2 m b ; (4) the Porto
dos Gauchos seismic zone with activity known since the largest event of 1955 (Barros
et al ., 2009 ) ; (5) a NE-SW zone in the Tocantins province possibly continuing towards the
Pantanal Basin; (6) southern Minas Gerais zone, in and around the southern tip of the Sao
Francisco craton; and (7) the southeast offshore zone with activity concentrated along the
continental slope from the Pelotas basin in the south to the Campos basin, from 33
°
Sto
20
S (e.g., Assump¸ ao, 1998 ; Assump¸ ao et al ., 2011 ) .
Remarkably, Branner ( 1912 ) , on the basis of historical accounts, had already delineated
the main seismic zones in Brazil (excluding the Amazon region) such as: northeastern
Brazil (zone 3), Central Brazil and Mato Grosso (zone 5), and southeast Brazil (Minas
Gerais and Rio de Janeiro; roughly zone 6). Most of the largest earthquakes (magnitude
°
5) in the second half of the twentieth century occurred roughly in the same areas as
defined by historical events in the late nineteenth and early twentieth centuries.
An interesting aspect of Brazilian seismicity is the different characteristics of the earth-
quake sequences in northeastern Brazil compared with other parts of the country. In north-
eastern Brazil, recurrent swarms and aftershock sequences, some lasting for several years,
are very common (Takeya et al ., 1989 ; Ferreira et al ., 1998 , 2008; Bezerra et al ., 2007 ) ,
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