Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 142

continental plate). Examples of continental plate collision include the Arabian
plate pushing into Eurasia, and India colliding with Asia to create the Himalayas
and the Tibetan plateau.
Three-quarters of the earthquake energy released on Earth occurs along these
convergent plate margins. The majority of these earthquakes are shallow focus
events that occur in the upper 60 km of the crust. Some earthquakes occur up
to 700 km under the Earth's surface. Below this, little or no seismic activity is
found.
Earthquakes also occur away from plate boundaries as crustal stresses are
released due to continental uplift, crustal readjustment resulting from glacial
deloading (isostasy), and strain release of energy by fracturing where the source
of stress is likely to be thermal imbalances in the upper mantle.
Earthquakes and faulting can also occur due to dilatancy in crustal rocks. The
pressure of overlying rocks equals the strength of the unfractured rock at depths
greater than 5 km. Here, sudden brittle failure and frictional slip do not occur
as the necessary shearing forces cannot be obtained. This is the case where rocks
deform plastically. However, sudden rupture can occur if the effective friction
along any crack boundary is reduced as a result of the presence of water. If
rocks in the upper crust strain without undergoing plastic deformation, they
can crack locally and expand in volume. This process is called dilation. Although
dilation can occur too quickly for immediate groundwater penetration, water
will eventually penetrate the cracks and provide lubrication for the remaining
stress to be released.
Four different wave types are transmitted during an earthquake: P-waves,
S-waves, L-waves (I) (Love wave) and L-waves (II) (Rayleigh wave). The P-wave is
the primary wave and can be described as a compressional wave that spreads
out from the epicentre. It consists of alternating phases of compression and
dilation and can travel through gases, liquids and solids, as well as refracting at
fluid--solid boundaries. The velocity of a P-wave is dependent upon rock density
and compressibility. Although these waves have the potential to travel through
the core of the Earth, they are refracted at the core--mantle boundary into two
shadow zones 3000 kilometres wide. P-waves are not detectable on opposite sides
of the Earth due to this refraction.
The S-wave is a shear wave and travels 0.6 times slower than P-waves. The
velocity of S-waves depends upon rock density and rigidity. Although S-waves
can travel through the mantle they cannot travel through the rigid core of the
Earth. The shadow zone this creates on the opposite side of the core overlaps the
shadow zones created by the P-waves. Because of this, the spatial distribution
and time separation between the arrival of P- and S-waves at three separate