Geoscience Reference
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Intraplate seismicity in Brazil
MARCELO ASSUMP¸ AO, JOAQUIM FERREIRA, LUCAS BARROS,
HILARIO BEZERRA, GEORGE SAND FRAN¸A, JOSE ROBERTO BARBOSA,
EDUARDO MENEZES, LUIS CARLOS RIBOTTA, MARLON PIRCHINER,
ADERSON DO NASCIMENTO, AND JOAO CARLOS DOURADO
Abstract
We describe the development of the Brazilian earthquake catalogue and the
distribution of seismicity in Brazil and neighbouring areas in mid-plate South
America. This large mid-plate region is one of the least seismically active stable
continental regions (SCR) in the world: the maximum known earthquake had
a magnitude of 6.2 m
b
and events with magnitudes 5 and above occur with
a return period of 4 years. Several seismic zones can be delineated in Brazil,
including some along craton edges and in sedimentary basins. Overall, the
exposed cratonic regions tend to have half as many earthquakes compared to
the average expected rate for all of mid-plate South America. Earthquakes tend
to occur in Neoproterozoic foldbelts especially in areas of thin lithosphere, or
near craton edges around cratonic keels. Areas with positive isostatic gravity
anomalies tend to have more earthquakes, indicating that flexural stresses from
uncompensated lithospheric loads are an important factor in explaining the
intraplate seismicity. We also found that earthquakes are two to three times
more likely to occur within 20 km of mapped neotectonic faults, compared to
events at larger distances. On closer examination, however, we observe that
most of these events occur near but not directly on the major neotectonic faults.
This discrepancy could be explained by the model of stress concentration near
intersecting structures. The Brazilian passive margin is also a region of higher
than average seismicity. Although clear differences are found between different
areas along the passive margin (extended crust in southeast Brazil having
especially high seismicity compared to thin continental shelves elsewhere),
overall the Brazilian passive margin has 70% more earthquakes (magnitudes
above 3.5) than the average stable continental region.
Intraplate Earthquakes,
ed. Pradeep Talwani. Published by Cambridge University Press.
C
Cambridge University Press 2014.
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