Faults and Earthquakes - Earthquake Engineering for Structural Design - page 209

occur with a magnitude exceeding M 7.0 (Ismail-Zadeh et al, 2005, Cloetingh et al,
2004). In the interval of the last 600 years, 3 to 5 earthquakes per century occurred
with magnitudes larger than M 7.2 (Ciucu and Fulga, 2008). The seismicity of the
Vrancea zone is divided into a crustal domain (depth under 40 km) with lower
magnitudes and an intermediate depth domain larger than 70 km, where all strong
earthquakes are located. These two domains are separated by an almost non
seismic zone ranging from 40 to 70 km in depth, showing that the earthquakes
occurred in these domains belong to very different source types. It is very
interesting to notice that the earthquake magnitudes increase with the depth of
sources.
6.5 WORLD'S SEISMIC ZONES
Generally, the maps showing the world's epicenters (Figure 2.2) do not a make
distinction between the earthquake types, showing only the epicenter locations.
The advances in knowing the nature of faults allows us to establish the expected
earthquake type in each seismic areas of the world and to choose, consequently, the
most adequate design methodology, specific for the respective type of earthquake.
In the following, these seismic areas will be described (Figs. 6.34 and 6.35) by
underlining their main characteristics. The corresponding zones are numbered in
function of the geographical position and the earthquake type. For North and South
American continents, the seismic zones are presented in Figure 6.30.
North American seismic zones (NA). The Western coast is the most seismically
active area, being influenced by the Circum-Pacific Ring. Crustal interplate
subduction earthquakes, resulting from the subduction of the ancient Kula Plate
under the North American Plate, characterize the Alaska zone (NA-a). In this
seismic zone, the Anchorage second larger ever-recorded earthquake occurred in
1964, producing very important avalanches, landslides and tsunami. The character
of this diffuse seismic zone is marked by the existence, in the continental part, of
some intraplate earthquakes, dominated by the Dinali strike-slip fault (like the San
Andreas Fault, but situated in a scarcely populated region). The same interplate and
intraplate earthquake types characterize the Western Canadian coast, being
produced by the subduction of the Kula and Juan de Fuca plates beneath the North
American plate (NA-b). But the most known zone of North America is the
Western coast of the USA (NA-c), dominated by the San Andreas Fault and the
corresponding diffuse zone, where interplate crustal strike-slip earthquakes are the
main ground motion type, due to the oblique movements between the Pacific and
the North American plate. The Eastern part of North America (NA-d) is dominated
by diffuse intraplate earthquakes, resulting from the continental movements during
a previous cycle of the Pangaea formation. Especially Missouri, the Eastern coast
of the USA and Canada are subjected to normal, reverse and thrust earthquake
types.
Central American seismic zones (CA) . The territory of Mexico is dominated by
crustal subduction interplate and intraslab earthquakes, produced by the subduction