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70°W
65°W
60°W
55°W
50°W
45°W
40°W
35°W
5°N
0°
5°S
PL
10°S
15°S
20°S
> 20km
< 20km
< 10km
25°S
30°S
1000 km
70°W
65°W
60°W
55°W
50°W
45°W
40°W
35°W
Figure 3.5 Correlation between neotectonic faults and epicentres of the “uniform” catalogue. Thick
grey lines are the proposed neotectonic faults compiled by Saadi
et al
.
(
2002
)
. Open circles are
epicentres more than 20 km away from any fault segment; grey and solid circles denote epicentres
in the range 20-10 km from any fault, and closer than 10 km, respectively. Thin solid lines are the
limits of the major geological provinces as in
Figure 3.2
.
Faults discussed in the text: RN, Rio Negro;
PL, Pernambuco Lineament; SP-II, Sobral Pedro-II Lineament; SPL, Senador Pompeu Lineament;
JL, Jaguaribe Lineament.
example, it is natural to wonder if the present seismicity tends to be close to the mapped
neotectonic structures.
Figure 3.5
shows a map of the Neotectonic faults and lineaments as
(
Figure 3.2
)
tend to lie close to the proposed neotectonic features. For these tests, we
excluded the Samambaia fault (fault BR-38 in Saadi's compilation) because it had been
defined only on the basis of the earthquake activity near Joao Camara, northeast Brazil
(5.5
Win
Figure 3.2
)
; the corresponding earthquakes were also removed from the
database. In addition, we corrected the position of the fault BR-15 (Sobral-Pedro II in north-
eastern Brazil, labeled “SP-II” in
Figure 3.5
)
because it was misplaced in Saadi's compila-
tion.
Using the “uniform” catalogue (magnitudes above 3.5 m
b
), we compared the number of
observed epicentres closer than 20 km (or 10 km) to any fault segment, with the expected
number assuming a random distribution of events in the continental area of Brazil. We
used the criterion of 10 and 20 km distance because this is roughly the estimated epicentral
accuracy of most events in the catalogue.
Figure 3.5
shows the events closer than 20 km
(and 10 km) from any fault segment. The chance of a random event being closer than 20 km
°
S 35.5
°
