Earthquake basics

1. Earthquakes.
An earthquake is the shaking and vibration of the
Earth's crust due to movement of the Earth's
plates . Earthquakes are caused by the release of
built-up pressure inside the Earth's crust. An
earthquake's power is measured on the Richter
scale using an instrument called a seismometer.
The point inside the crust where the pressure is
released is called the focus. The point on the
Earth's surface directly above the focus is called
the epicentre.

3. TYPES OF EARTHQUAKE WAVES
• 1. PRIMARY OR P WAVES: these are body waves which propagate within the
body of a rock. As it spreads out, it alternately pushes (compresses) and pulls
(dilates) the rock.It is a very fast wave. These waves are relatively weak and
cause the surface to move in a back and forth motion horizontally.
• 2. SECONDARY OR S-WAVES: This is the slower of the body waves. As an S
wave propagates, it shears the rock sideways at right angles to the direction of
travel. these waves cause the crust to move from side to side at right angles to
the outward motion of the main wave. They are also called TRANSVERSE waves
and are known to cause the most damage.
• 3. SURFACE WAVES: This wave is called a surface wave because its motion is
restricted to near the ground surface. Such waves correspond to ripples of
water that travel across a lake. Surface waves in earthquakes can be divided
into two types. The first is called a Love wave. Its motion is essentially that of S
waves that have no vertical displacement; it moves the ground from side to side
in a horizontal plane but at right angles to the direction of propagation. The
horizontal shaking of Love waves is particuly damaging to the foundations of
structures. The second type of surface wave is known as a Rayleigh wave. Like
rolling ocean waves, Rayleigh waves wave move both vertically and horizontally
in a vertical plane pointed in the direction in which the waves are travelling.
•

4. Measuring Earthquakes
• Earthquake magnitude can be measured using
the Ritcher Scale while the Mercali Scale
measures the intensity of an earthquake.. The
Richter Scale measures the amount of seismic
energy released in an earthquake, While the
Mercali scale measures the intensity of shaking
and the amount of damage done by an
earthquake.

9. Effects of an Earthquake.
• Effects are often classified as primary and secondary impacts. Primary
effects occur as a direct result of the ground shaking. Examples of
primary effects include collapse of buildings, Death, injuries and
disruption of transport and communication routes. Secondary effects
occur as a result of the primary effects, e.g. tsunamis or fires due to
ruptured gas mains and homelessness.
• Earthquake Hazards
• I. Ground Shaking: The first main earthquake hazard is the effect of
ground shaking. Buildings can be damaged by the shaking itself or by
the ground beneath them settling to a different level than it was before
the earthquake (subsidence).
• II. Liquefaction: This is the mixing of sand or soil
and groundwater during the shaking of a moderate or strong
earthquake. When the water and soil are mixed, the ground becomes
very soft and acts similar to quicksand. If liquefaction occurs under a
building, it may start to lean, tip over, or sink several feet. The ground
firms up again after the earthquake has past and the water has settled
back down to its usual place deeper in the ground. Liquefaction is a
hazard in areas that have groundwater near the surface and sandy soil.

10. • III. Tsunamis: A tsunami is formed in a body of
water, such as an ocean or sea, by an event that
causes enough of a disturbance so that a vertical
displacement of the water column occurs. Tsunamis
can be caused by undersea earthquakes, volcanic
eruptions or landslides. An earthquake-induced
tsunami is caused by the shifting of the sea floor. A
tectonic earthquake; that is, one that is caused by
changes in the earth’s crust, but happening under
the sea, causes the water above the area where the
movement occurred to be displaced. As it attempts
to get back to normal, in accordance with
gravitational pull, the sea floor either raises or
lowers, which can cause the tsunami.

13. • IV. Ground displacement(ground movement)
This usually occurs along a fault. If a structure (a
building, road, etc.) is built across a fault, the
ground displacement during an earthquake
could seriously damage or rip apart that
structure

14. Factors affecting the impact of an earthquake.
• Distance from the epicentre – the effects of an earthquake
are more severe at its centre.
• The higher on the Richter scale, the more severe the
earthquake is.
• Level of development (MEDC or LEDC) – MEDCs are more
likely to have the resources and technology for
monitoring, prediction and response.
• Population density (rural or urban area). The more densely
populated an area, the more likely there are to be deaths
and casualties.
• Communication - accessibility for rescue teams.
• Time of day influences whether people are in their
homes, at work or travelling. A severe earthquake at rush
hour in a densely populated urban area could have
devastating effects.

15. Predicting and monitoring earthquakes
• 1. Animal Behaviour: some animals are very
sensitive to sound, temperature, touch, light
intensity and even magnetic fields, so it is perfectly
possible that they can indeed detect the seismic
activity which precedes an earthquake.
• 2. Foreshocks. These are small earthquakes before
a major one that can give warnings.
• 3. Seismic History: Seismologists can study the
seismic history of earthquakes and try and make
predictions of when future earthquakes are likely to
happen.
• 4. Radon: The release of radon has been studied as
a precursor to a major earthquake. However, this is
not exactly conclusive.

16. • LEDC’s often suffer more from the effects of
volcanoes and earthquakes than MEDC’s.This is
because of a number of reasons such as:
• Construction standards tend to be poor in LEDCs.
Homes and other buildings may suffer serious
damage when a disaster occurs.
• Buildings collapsing can cause high death tolls.
• Evacuation and other emergency plans can be
difficult to put into action due to limited funds and
resources.
• Clearing up can be difficult. There may not be
enough money to rebuild homes quickly and safely.
Many people could be forced to live in emergency
housing or refugee camps.