Intraplate seismicity in Brazil - Intraplate Earthquakes - page 51

3.1 Introduction
Explaining the genesis of intraplate seismicity is a great challenge. Several models have
been proposed to explain the cause of earthquakes far away from the active plate bound-
aries. These can be broadly divided into two kinds of models: those involving weakness
zones and those involving stress concentrations. Crustal weak zones are usually due to
the last major orogenic process and involve ancient rifts or failed rifts (e.g., Sykes, 1978 ;
Johnston and Kanter, 1990 ; Assump¸ ao, 1998 ; Schulte and Mooney, 2005 ) , or suture zones
(i.e., around craton edges as shown by Mooney et al . [2012]). Stress concentrations can
arise from lateral density variations (e.g., Stein et al ., 1989 ; Assump¸ ao and Araujo, 1993 ;
Zoback and Richardson, 1996 ; Assump¸ ao and Sacek, 2013 ) , contrasts of elastic properties
(e.g., Campbell, 1978 ; Stevenson et al ., 2006 ) , or fault intersections (Talwani, 1999 ; Gan-
gopadhyay and Talwani, 2003 , 2007 ) . Quite often, weak zones and stress concentrations are
linked in the same process. For example, lithospheric thin spots between ancient cratonic
areas can be regarded as a weak zone but the seismicity is due to stress concentration
in the elastic upper crust (e.g., Assump¸ ao et al ., 2004 ) . Evidence for higher seismicity
along Phanerozoic suture zones in North America can be interpreted as concentration of
stresses and deformation (“crumpling” zones) near craton edges (Lenardic et al ., 2000 ) ,
which are ultimately caused by lateral variations of lithospheric thickness (Mooney et al .,
2012 ) .
If lateral density variations are not isostatically compensated, flexural stresses can arise
in the upper crust and reach large magnitudes to significantly contribute to intraplate
seismicity (e.g., Cloetingh et al ., 1984 ; Stein et al ., 1989 ; Zoback and Richardson, 1996 ;
Assump¸ ao et al ., 2011 ; Assump¸ ao and Sacek, 2013 ) . Flexural stresses can be caused
by ice-sheet retreat (e.g., Mazzotti et al ., 2005 ) , sediment load in the continental margin
(Stein et al ., 1989 ; Assump¸ ao 1998 ; Assump¸ ao et al ., 2011 ) , or intracrustal loads from
past geological processes (e.g., Zoback and Richardson, 1996 ; Assump¸ ao and Sacek,
2013 ) .
As reviewed by Mazzotti ( 2007 ) , it is usually agreed that several different factors can
contribute to produce an intraplate seismic zone. However, one of the major difficulties with
most models for intraplate seismicity is the fact that the same geological/structural features
are also present in areas with no current seismic activity. For example, not all continental
shelves are equally active, despite having similar geological structures and potentially the
same sources of stress. This has contributed to the debate of whether long-termmigration of
intraplate seismic zones occurs, with significant implications for seismic hazard assessment
(Stein et al ., 2009 ; Li et al ., 2009 ) and highlighted the importance of further studies of
intraplate mechanisms and a more detailed comparison of seismicity patterns between
different regions and continents.
Here we present the spatial distribution of seismicity in Brazil and discuss possible
correlations with geological and geophysical features, trying to assess some of the proposed
models mentioned above. Assessing the applicability of general models for intraplate
seismogenesis is important to better delineate seismic zones in mid-plate South America.