Tectonic Plates and Faults - Earthquake Engineering for Structural Design - page 157

5.3.2 Plate Boundary Zones: Extension of Seismic Zones
The areas affected by the ocean-continent convergence boundaries (see Figure
5.7a) are larger than the zone of plate boundaries, containing very complex
geological formations of the subduction zone (Fig. 5.14). The extension of seismic
zone depends on subduction slope. The reduced slope produced a long extension of
seismic zones. So, the subduction process induces plate boundary zones,
characterized by the presence of multiple faults, distributed on the large area, with
the potential for some crustal earthquakes.
A very illustrative example is the subduction of Kula Plate under the North
American Plate (Fig. 5.14a). After the almost complete subduction, the remainder
plate is the Juan de Fuca Plate (Fig. 5.14b). The spreading of faults in the case of
this subduction refers to the diffuse zone of Cascadia of Western Canada. At some
time in future, this seismic zone will generate a huge subduction earthquake, with
magnitude greater than M 9.0. Geological evidence indicates that one huge
earthquake strikes this Canadian coast every 300-800 years.
Another illustrative example of this diffuse zone is the seismic area of Los
Angeles City, crossed by the San Andreas fault (see Fig. 2.13a), around an
important zone with a network of diffuse faults. It must be noticed that the well-
known 1994 Northridge earthquake is produced by a secondary fault of this
network. A similar example is the 1995 Kobe earthquake, occurred in a fault which
not belonging to the tectonic plate boundaries (see Figure 2.14a). The faults from
these diffuse zones have characteristics very different from the ones corresponding
to the tectonic plate boundaries
The formation of the seismic Californian zone is dominated by the complete
subduction of the ancient Farallon and Pacific plates under the Western coast of the
North American Plate (Fig. 5.16a), creating the San Andreas fault (Wikipedia, nd).
The remains of the Farallon plate are the Juan de Fuca plate, subducting under the
Northern part of North American plate, the Cocos plate subducting under the
Central America, and the Nazca plate, subducting under the South American plate.
It is very important to notice that the very shallow angle of subduction creates a
large extension of seismic zone (Fig. 5.16b), characterized by the presence of many
parallel faults (see Fig. 3.5) and the mountains in the subduction areas (as the
impressive American Rocky Mountains in the Western North America).
In the same time it very interesting to understand the formation of the volcanic
Andes Mountains in South America, as the result of subduction of Nazca plate
under the South American plate, forming the Peru-Chile trench.