Faults and Earthquakes - Earthquake Engineering for Structural Design - page 184

Seismic zones corresponding to the subduction are the most dangerous, because it
happens near the surface. These subduction sources produced also the largest
world's tsunamis during the last period, like the 1960 Chile earthquake (M 9.5),
1964 Alaska (M 9.2), 1952 Kamchatka (M 9.0) and 2004 Sumatra (M 9.0) events.
The history mentions that on January 1700 one of the largest earthquakes (known
as the Cascadia earthquake) occurred along the Western Canadian coast of North
America, due to the sliding of the Kula and Juan de Fuca plates under the North
American plate (see Figures 5.14). The same subduction process produced in 1964
the second known mega earthquake in Alaska (M 9.2). The geological evidence
indicates that 13 great earthquakes occurred in this region during the last 6000
years. While large earthquakes have not been observed in the last short period (150
years), there is compelling evidence that they will happen sometime in the future; it
represents a considerable hazard for people living in the very urbanized zones of
this region. For these reasons, these faults are very carefully examined by
seismologists.
Figure 6.4 shows the main world's subduction zones marked by lines with sags.
They are: Aleutines, Cascades, Central America, Andes, South America, Kurile,
Mariana, Lau Basin, New Zealand (at the borders of the Pacific Tectonic Plate
along the Ring of Fire) and North Africa, Sicily, Crete and Indonesia (at the
borders of the Euroasian Tectonic Plate with the African and Indian Tectonic
Plates, respectively, along the Alpide-Himalayan belt). In all these zones crustal
subduction earthquakes are very frequent.
The Western coasts of both the American continents are constantly affected by
earthquakes due to the subduction of different tectonic plates. One of the most
important zones, for exemplification of this earthquake type, is the coast of South
America. Figure 6.5a shows the region where the Cocos Plate in the North, the
Nazca Plate in the Centre (with the majority of important earthquakes) and the
Antarctic Plate in the South subduct the Southern American Plate. The Nazca Plate
subduction forms the very long Peru-Chile Trench and the Andes Mountains (with
its highest elevations over 6000 m representing the second largest orogen after the
Himalaya). The 1960 Chile earthquake (M 9.5), the strongest recorded in the
world, had a rupture length along the fault line of about 1300 km. It is very
important to observe in Figure 6.6b that there are two subductions, the first along
the Peru-Chile Trench and the second inland of the continent, corresponding to the
collision type which will be discussed in the next section. This particular
characteristic of subduction zones produces very large diffuse areas, especially in
the Peru zone and in the Northern areas of Chile.
The second very important subduction activity, due to the subducting of the
Pacific Plate beneath the North American Plate, is along the Aleutian and Alaska
basins (Fig. 6.6), where the second strongest earthquake was recorded: the 1964
Alaska M 9.2. The Aleutian-Alaskan arc is extended for over 4000 km (Black,
2000, Burns 2005)