Earthquakes

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Earthquakes
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How earthquakes occur
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Almost all earthquakes are the result of plate
movement. Rock on either side of a fault or a
crack that mark plate boundaries becomes
gradually distorted or bent because of the forces
that drive plate tectonics. Elastic strain
energy builds up in this deformed rock until
finally it reaches the breaking point. The rock
then snaps back (elastic rebound) into its
unstrained position by slipping along the
fault. The strain of this energy is released and
the energy moves outward as seismic shock waves,
producing an earthquake.
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If the strain energy is released regularly, it
results in what is called tectonic creep. This
usually results in a large number of minor
earthquakes or tremors, that cause little or no
damage. If movement along a fault does not occur
regularly, the fault is said to be locked. The
famous San Andreas Fault is said to be in this
state. In these situations strain energy builds
up for long periods, creating a potential for a
strong earthquake. Not all earthquakes occur
along plate boundaries. Volcanoes and landslides
can cause earthquakes. There are also faults
found within a single plate, that is, far from a
plate boundary.
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The actual location in the earth where an
earthquake occurs is called its focus. These
points range in depth from a shallow-focus, which
are just below the earths surface to deep-focus
quakes which have been found to be as far down as
700km (in subduction zones). The point on the
surface directly above the earthquake's focus is
called the epicentre. Most research is done on
shallow-focus earthquakes, they are the most
common and cause the most damage.
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Types of seismic waves
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The seismic waves produced by an earthquake
radiate out from its focus. These waves cause the
earth to vibrate in response to the pushing and
pulling forces that are applied to them. Two
major types of waves are emitted 1. Body
waves 2. Surface waves
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• Body waves arrive first since they are the
  fastest. Body waves are broken down into two
  types
• Primary (P) waves, which cause the rock particles
  through which they pass to shake back and forth
  (compression effect) and can result in a noise
  similar to a sonic boom.
• Secondary (S) waves make the particles vibrate
  both vertically and horizontally (shearing
  effect). The S waves can lead to extensive damage
  if buildings are not designed to withstand the
  shearing effects.

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Surface waves arrive last and as their name
implies they travel along the surface of the
earth. It is these waves that cause damage.
Again, there are two types. Love (L) waves move
the ground from side to side in a shearing
effect. Rayleigh (R) waves - similar to ocean
waves in that they cause surface materials to
move in a circular motion just as a floating
object would move as a sea wave passes under it -
responsible for most of the damage to buildings
and structures.
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The energy released by an earthquake is measured
by a seismograph.
http//www.911supplies.com/images/earthquakes.jpg
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The violence of an earthquake can be measured
using two different scales The Richter scale -
devised in 1932 by Charles Richter The Mercalli
scale - devised in 1905 by Mercalli. The Richter
scale actually uses a numerical value. It is
important to note that this scale is logrithmetic
in nature. In other words a magnitude four
earthquake releases ten times the energy that a
magnitude three earthquake does and 100 times the
energy of a magnitude two, and so on. The
Mercalli scale is a visual scale based on the
destruction that occurs. It is subjective,
determined by observers. The next two slides
illustrate the two scales.
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The next few slides illustrate the destructive
power of South Asia earthquake of October 2005.
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Effects of Earthquakes
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• Damage to structures buildings, towers,
  bridges, etc. if they are not built to resist the
  shaking. Many earthquake prone areas are in
  developing countries and as a result the loss of
  life and the extent of property damage is
  especially great.
• The change in land height can lead to flooding of
  coastal areas, changes in the water table. In the
  Alaskan earthquake of 1964 more than 250 000 km2
  was vertically deformed and in some areas the
  shore line rose by more than 10 metres.

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• Fires often break out as electrical and gas
  services are disrupted. Often difficult to fight
  because water lines are also damaged. Tokyo
  earthquake in 1926 lead to huge fires, and 40 of
  the 143 000 people that died perished in the
  firestorm that followed.
• Landslides or mudflows can be triggered, damaging
  communities and blocking rivers, leading to
  flooding.
• Can set in motion tsumanis.

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When earthquakes occur under an ocean, they can
create waves called tsunamis. Tsunami waves can
have tremendous destructive powers. The picture
below is a famous Japanese painting depicting a
tsunami.
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A tsunami (pronounced soo-nahm'ee) is a series
of waves generated by an undersea disturbance
such as an earthquake or landslide. From the
area of the disturbance, the waves will travel
outward in all directions, much like the ripples
caused by throwing a rock into a pond. The time
between wave crests may be from 5 to 90 minutes,
and the wave speed in the open ocean will average
more than 600 kilometres per hour. Tsunamis
reaching heights of more than 35 metres have been
recorded. As the waves approach the shallow
coastal waters, they appear normal and the speed
decreases. Then as the tsunami nears the
coastline, it may grow to great height and smash
into the shore, causing much destruction.
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Tsunamis can originate hundreds or even thousands
of miles away from coastal areas. Local geography
may intensify the effect of a tsunami. Areas at
greatest risk are less than 15m above sea level
and within one kilometre of the shoreline.
Especially at risk are coastal areas where the
continental shelf is very narrow and quite
shallow. Hawaii is a classic example. People who
are near the seashore during a strong earthquake
should listen to a radio for a tsunami warning
and be ready to evacuate at once to higher
ground.
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Rapid changes in the water level are an
indication of an approaching tsunami. Although
tsunamis are rare along the Atlantic coastline, a
severe earthquake on November 18, 1929 in the
Grand Banks of Newfoundland generated a tsunami
that caused considerable damage and loss of life
at Placentia Bay, Newfoundland. See the slide
show on the South Asian tsunami of 2004