Intraplate seismicity in Brazil - Intraplate Earthquakes - page 60

For simplicity, we did not include the onshore marginal basins, as different criteria could
be used to define the onshore limit of the extended crust. We traded a more detailed and
complex analysis for a simpler, more objective statistical test.
The continental region ( Figure 3.2b ) and the passive margin ( Figure 3.4 ) have a total of
274 events with magnitudes 3.5 or greater in the uniform catalogue (238 in the continent
and 36 in the passive margin). Taking into account the different sizes of both areas, we
would expect only 21 events (7.8%) in the passive margin if the seismicity were uniformly
distributed. The larger number of observed events (36) indicates that the Brazilian passive
margin has a seismicity rate 70% higher than the average SCR seismicity, with a confidence
limit better than 98%. A test using m b

4.5 also shows a larger number of events in the
passive margin (six observed events compared to an expected number between two and
three), but the small sample prevents a reliable measure of the statistical significance.
Although the Brazilian passive margin has a higher seismicity rate, on average, Figure
3.4 shows a high concentration in two areas: one in southeastern Brazil where the continental
crust was subjected to an extreme degree of extension and thinning, and the other just
north of the Amazon fan, both characterized by thick sedimentary sequences. Areas of
short continental shelf (most of the eastern and northeastern margin), with generally thin
sedimentary layers, have almost no earthquakes. This indicates that crustal weaknesses
caused by high levels of extension, as well as flexural stresses, are important factors in
defining seismicity rates in passive margins (e.g., Assump¸ ao, 1998 ; Assump¸ ao et al .,
2011 ) . Concentration of seismicity in the continental margin is a common pattern of
global seismicity, as shown by Schulte and Mooney ( 2005 ) , but local conditions can cause
significant variations along the margin, as shown by Sandiford and Egholm ( 2008 ) for
Australia.

3.4.4 Influence of neotectonic faults

Neotectonic has been described as the period when young tectonic events have occurred and
are still occurring in a given region after its last orogeny or after its last significant tectonic or
stress field set up (e.g., Pavlides, 1989 ) . The neotectonic period varies in different regions.
There is no consensus on the onset of the neotectonic period in some passive margins,
such as the Brazilian margin, where neotectonic studies are patchy. Therefore, we choose
to analyze a preliminary neotectonic map of Brazil and show two examples of pre-existing
fabric such as ductile shear zones and their relationship with the present-day seismicity.
Neotectonic activity in Brazil has been clearly observed in several areas (e.g., Riccomini
and Assump¸ ao, 1999 ; Bezerra and Vita-Finzi, 2000 ; Bezerra et al ., 2006 , 2011), with
observed geological faulting dated as recent as Holocene (e.g., Riccomini and Assump¸ ao,
1999 ) . A preliminary map of Neotectonic faults in Brazil was presented by Saadi et al .
( 2002 ) , with most of the mapped faults and lineaments likely to be active in the neo-
tectonic period based on geomorphological criteria. While this compilation is still very
preliminary and very few faults have geochronological dates, it is the only compilation
spanning the whole country, and for this reason may be useful for some initial tests. For