Faults and Earthquakes - Earthquake Engineering for Structural Design - page 191

well established, due to the permanent seismic activity of these regions. No
comparable simple description exists yet for quantifying active intraplate
earthquakes in diffuse zones, known as Stable continental interiors . In contrast
with the interplate boundaries, the interior of the plate appears to tectonically
behave as a rigid block, but with some faults in uncertain positions. Due to their
low activity, the geological and geophysical indicators and the seismic data are
limited for this type of earthquake. Broad mountain belts and rifts with diffuse
seismicity and strong variations in the crust structure show that the studies of
intraplate earthquakes are more complex than the interplate ones.
The difficulties come from the fact that the well-known Tectonic Plate Theory
starts from the division of the Pangaea super-continent into the actual continents,
ignoring the fact that, before this, some cycles of gathering and spreading of
continents occurred (see Wilson's Cycle Theory, Section 4.1.5), with different
configurations which are known today. Therefore, the mountain belts and the
diffuse seismic zones in the inland of actual continents are the result of this pre-
Pangaea history. In addition, the tectonic plates are not perfectly rigid and some
inland faults exist in the weakest zones of the crust. The best model for the diffuse
zones must consider that they are composed by some rigid micro-plates, as a result
of (see Section 5.3.1):
(i) Extension of seismic zones under subducted plates;
(ii) Collision along two continental plates or of some ancient tectonic activity;
(iii) Faults due to some weaknesses of the crust.
In fact, the tectonic plates look like a broken plate. Therefore, the so-called
Stable Continental Interiors do not exist in the reality. So, an earthquake can occur
in almost any world zone.
6.3.2 Earthquakes along the Extension of Subduction Plates
Most of the world's great earthquakes occur in the ocean-continent convergence
boundaries, in the extension of subduction zones (see Figs. 5.7a and 5.13). A
subduction zone is a large convergent boundary where two tectonic plates collide.
They are constantly shifting and moving, so when they subduct, one pushes
beneath the other. Subduction zones create geologic formations such as ocean
trenches and mountain ranges, as well as phenomena like earthquakes and
volcanoes. Because subduction zones are gently inclined at shallow depths, they
have the largest seismogenic area, exceeding the plate boundary zones.
While scientists have a general understanding of plate boundaries, many
problems of seismic zones remain obscures, mainly regarding the dimensions of
the subduction areas.
The Figure 6.11 shows the two large subduction areas of South American plate,
and the earthquakes produced in these areas. One can see that the seismogenic
zones are large extended in the continental territory.