Civil Engineering Reference
In-Depth Information
E
ARTH
D
ISTURBANCES
C
ONTINUOUS
D
ISTURBANCES
S
INGLE
D
ISTURBANCES
A
RTIFICIAL
N
ATURAL
A
RTIFICIAL
N
ATURAL
Traffic
Minor causes
Machinery
Meteorological
Collapse of caves
Blasting
Storms
Large slides and slumps
Volcanic shocks
Explosive tests
Wind
R
ock burst in mines
Superficial
Frost
Demolitions
eteorites
M
(explosive)
Water in motion
Tectonic shocks
Magmatic
(eruptive)
Bombing and bomb tests
Surf
Shallow
Depth less than 60 km
Gunfire
Streams
Intermediate
Depth between 60 and 300 km
Waterfalls
Accidental large
detonations
Deep
Depth between 300 and 700 km
Volcanic tremor
Figure 1.1
Earth disturbances recorded by seismographs
According to the theory of continental drift, the lithosphere is divided into 15 rigid plates, including
continental and oceanic crusts. The plate boundaries, where earthquakes frequently occur, are also called
' seismic belts ' (Kanai, 1983 ). The Circum - Pacifi c and Eurasian (or Alpine) belts are the most seismi-
cally active. The former connects New Zealand, New Guinea, the Philippines, Japan, the Aleutians, the
west coast of North America and the west coast of South America. The 1994 Northridge (California)
and the 1995 Kobe (Japan) earthquakes occurred along the Circum- Pacifi c belt. The Eurasian belt links
the northern part of the Mediterranean Sea, Central Asia, the southern part of the Himalayas and Indo-
nesia. The Indian Ocean earthquake of 26 December 2004 and the Kashmir earthquake of 8 October
2005 were generated by the active Eurasian belt.
The principal types of plate boundaries can be grouped as follows (Figure 1.3):
(i)
Divergent or rift zones
: Plates separate themselves from one another and either an effusion of
magma occurs or the lithosphere diverges from the interior of the Earth. Rifts are distinct from
mid-ocean ridges, where new oceanic crust and lithosphere is created by sea- fl oor spreading.
Conversely, in rifts no crust or lithosphere is produced. If rifting continues, eventually a mid-
ocean ridge may form, marking a divergent boundary between two tectonic plates. The Mid-
Atlantic ridge is an example of a divergent plate boundary. An example of rift can be found in
the middle of the Gulf of Corinth, in Greece. However, the Earth's surface area does not change
with time and hence the creation of new lithosphere is balanced by the destruction at another
location of an equivalent amount of rock crust, as described below.
(ii)
Convergent
or
subduction zones
: Adjacent plates converge and collide. A subduction process
carries the slab-like plate, known as the ' under - thrusting plate ' , into a dipping zone, also referred
to as the ' Wadati - Benioff zone ' , as far downward as 650 - 700 km into the Earth 's interior. Two
types of convergent zones exist: oceanic and continental lithosphere convergent boundaries. The
fi rst type occurs when two plates consisting of oceanic lithosphere collide. Oceanic rock is mafi c,
and heavy compared to continental rock; therefore, it sinks easily and is destroyed in a subduc-
