Geoscience Reference
In-Depth Information
5°N
anomaly distribution
S−wave 100km
0°
5°S
−7 −6 −5 −4 −3 −2 −1
0
1
2
3
4
5
6
7
S−wave anomaly(%)
10°S
earthqs.
(whole)
15°S
20°S
−7 −6 −5 −4 −3 −2 −1
0
1
2
3
4
5
6
7
25°S
earthqs.
(unif)
% 4.49km/s
30°S
−7.0 −3.5
0.0
3.5
7.0
75°W
70°W
65°W
60°W
55°W
50°W
45°W
40°W
35°W
−7 −6 −5 −4 −3 −2 −1
0
1
2
3
4
5
6
7
Figure 3.3 Comparison of S-wave anomalies with epicentral distribution. (a) S-wave anomalies at
100 km depth (Assump¸ ao
et al
.,
2013
)
from joint inversion of surface waves and receiver function
point constraints. White circles are epicentres from the “uniform” catalogue. (b) Histogram of the
S-wave anomalies within the continent and east of the catalogue limit, corresponding to the grey-
shaded area in (a). (c) Histogram of the S-wave anomalies beneath all continental epicentres of the
“whole” catalogue. (d) Similarly for the “uniform” catalogue. The solid black lines in (c) and (d)
indicate the distribution expected from (b) if the earthquakes had no correlation with lithospheric
structure. For colour version, see Plates section.
Areas with “normal” lithospheric depths (anomalies between
−
1 and
+
3%) seem to be the
least seismic.
3.4.3 Passive margin seismicity
preferentially in rifted crust, that is, in interior continental rifts (31%) or rifted continental
margins (28%), compared with continental non-extended crust (41%). Considering that the
area of non-extended crust in SCRs is several times that of rifted margins, it is clear
that passive margins have a much higher seismicity rate. For magnitudes greater than 6,
the preference for rifted crust is even stronger. Here we test if the extended crust along the
Brazilian passive margin is significantly higher than the average continental area. The
whole Brazilian margin was subjected to rifting during the Pangea breakup in the Jurassic-
