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The activity near the SW-NE-trending Jaguaribe Lineament (JL in
Figure 3.5
)
actually
occurs on an E-W-trending strike-slip seismogenic fracture (“Palhano” earthquakes in
Ferreira
et al
. [1998]). The activity near the Senador Pompeu Lineament (SPL in
Figure
trending Sobral-Pedro-II shear zone actually occur on a small E-W-trending fracture, as
shown in
Figure 3.6b
.
In the Amazon basin, a similar inconsistency can be found with the
“Rio Negro” fault (BR-10 of Saadi
et al
.,
2002
;
labeled “RN” in
Figure 3.5
)
: the event less
than 10 km from the fault is the 5.1 m
b
Manaus earthquake of 1963, 45 km deep, which had
a well-determined reverse faulting mechanism with the two nodal planes oriented SW-NE
Negro fault.
Our statistical tests showing higher seismicity up to about 20 km from faults interpreted
as being neotectonic may, in fact, indicate a zone more prone to seismic activity, but not
necessarily a direct relationship. The presence of a neotectonic feature might increase the
chances of stress concentration in intersection zones involving other smaller fractures, such
the major lineament may not be favourably oriented with respect to the present stresses
while other smaller faults may be optimally oriented, such as shown in
Figure 3.6b
.
3.4.5 Flexural stresses
Flexural stresses are known to contribute significantly to earthquakes in the continental
Flexural stresses can reach high magnitudes, and even apparently small loads can con-
effect of Late-Pleistocene erosion in the New Madrid seismicity. In Brazil, the SW-NE-
trending seismic zone in the Tocantins province (
Figure 3.2
)
coincides with high gravity
anomalies (
Figure 3.7
)
. Assump¸ ao and Sacek (
2013
)
showed that the uncompensated
lithospheric load, which causes the high gravity anomalies, produces large compressional
stresses in the upper crust (due to flexural bending of the lithosphere) and explained the
seismicity distribution in Central Brazil. Here we test whether the correlation between
gravity anomalies and seismicity can be extrapolated to all of the mid-plate area in South
America.
Assump¸ ao and Sacek
(
2013
)
.
Figure 3.7
shows the epicentres of the “whole” catalogue
and the gravity anomalies. It can be seen that most epicentres tend to be located in areas
of positive gravity anomalies, and large areas without seismicity (such as in the middle
of the Parana and Parnaiba basins, as well as in the Guapore shield) tend to have nega-
tive gravity anomalies. The average isostatic anomaly in the continental area shown in
Figure 3.7a
is
−
13 mGal (
Figure 3.7b
)
. The distribution of gravity anomalies at the