Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 140

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Earthquakes

Earthquakes and plate tectonics

The Earth's lithosphere or crust is broken into rigid plates that move
away from, past and into each other (Fig. 6.1). This movement is not usually
continuous but occurs in pulses. During such phases, the energy released is
expressed as an earthquake. The majority of earthquakes worldwide occur at
the boundaries between tectonic plates. As Abbott (1999)notes,there are three
separate processes related to plate tectonic movements that can produce earth-
quakes. These are as follows.

(1) The pull-apart motion at spreading centres causing rocks to fail in ten-

sion. Rocks rupture sooner when subjected to tension. This process yields
mainly smaller earthquakes that do not usually pose an especially great
threat to humans.

(2) The slide-past motion that occurs as rigid plates wrap around the curved

Earth. The plates slide past each other via dominantly horizontal move-
ments of transform faults (transform faults are the fractures generated
by rigid plates moving over Earth's curved surface). These movements
can create large earthquakes because the irregular plate boundary
retards slip along irregularly shaped faults. Considerable stored energy
must be expended to overcome the rough surfaces of rocks and the
bends in the faults. A large amount of seismic energy is released in
overcoming these impediments.

(3) The collisional motions that occur at subduction zones and in conti-

nent--continent collisions. These motions release immense amounts of
energy resulting in Earth's largest tectonic earthquakes. An incredible

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