Faults and Earthquakes - Earthquake Engineering for Structural Design - page 198

But the Eastern part configuration is more complex, being composed by two
blocks, Sino-Korean and Yangtze blocks. Events during the pre-Pangaea cycle
(150 Ma) in this region resulted in the collisions and sutures between these blocks.
The great earthquakes (i.e, the 1976 Tangshan and 2008 Sichuan earthquakes
with magnitudes exceeding M 7.0) which shook these regions show that intraplate
earthquakes occurring in collision zones and along the ancient faults can produce
very damaging effects.
6.3.4 Earthquakes in Weak Crustal Zones
Concerning the seismic hazard, the major faults and sources of these earthquakes
are not well known and the hazard assessments are more difficult than for the
interplate earthquakes. Fortunately, low to moderate earthquakes occur in general
in these intraplate faults. Only in some special cases, such as the soil liquefaction
in densely built zones, can these earthquakes be very devastating. There are some
important and specific seismic areas producing important intraplate earthquakes.
Eastern North America is the example of the earthquake produced in a rift.
Although this zone of North America is far from the plate boundary, earthquakes
occur frequently (Fig. 6.17). Moreover, the 1811 and 1812 New Madrid
earthquakes (M 7.8 and 8.1) were two of the strongest earthquakes produced in
USA. This is due to the fact that when the super-continent Pangaea was formed, a
collision was produced between the North American and the African plates, giving
rise to the Appalachian Mountains in the Eastern North American continent. The
New Madrid earthquakes produced, due to the liquefaction phenomena, landslides,
settlements, ejection of sand and water. Another seismic area is the Charleston
region, where in 1886 a great earthquake (M= 7.6) shocked the Eastern US
seaboard.
Another very important zone of intraplate earthquakes is the European area ,
which is divided in three distinct zones: Western, Central, and Northern areas.
The Western area is composed by the territories of France, the UK and
Belgium.
France is a country of low-to-moderate seismic hazard but which, in the past,
suffered strong earthquakes producing damage and casualties (Pequegnat et al,
2008). The earthquakes are located in four main very different tectonic regions
(Fig. 6.18). The first two are the zones of the Pyrenees in the Southwest, due to the
collision with the Iberian Plate and of the Alps in the Southeast, due to collision
with Italian Plate. The earthquakes occurring in these zones are the result of the
effect of these collisions. In the central and Western regions these seismic events
have the characteristics of crust fractures.
The UK has a moderate rate of seismicity (earthquakes of magnitude 5 are rare),
but sufficiently high to pose a potential hazard for some old buildings with a poor
structure. The characteristics of these earthquakes correspond very well to the crust
fracture ones, being produced by the influence of the Mid-Atlantic Ridge. The
distribution of earthquakes on the territory of the UK is presented in Figure 6.19.