Running head: EARTH SCIENCE: EARTHQUAKES
Earth Science: Earthquakes
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EARTH SCIENCE: EARTHQUAKES
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Definition
Earthquakes refer to the way that the Earths releases stress. It is sudden and quick rolling
or rolling of the earth’s surface. This occurs whereby the plates of the Earth move against each
other leading to stress being put on the lithosphere part of the earth. This causes the lithosphere
to either break or shifts when the stress gets more enough. Earthquakes occur when the rocks
lapse or break along the fault lines found in the Earth’s crust, leading to the release of energy
which causes movement of the ground. The movement is in form of waves known as seismic
waves which is felt and thus referred to as an earthquake. The region where the earthquake
waves originate from is called the focus and the point on the Earth surface that is directly above
the focus is called the epicenter.
Figure 1. Shows how seismic waves that cause earthquakes are generated
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Formation of Earthquakes
The breaking of rocks is referred to as faulting and it leads to energy release as a result of
conversion of stored stress into movement in a sudden manner. This results to the production of
vibrations referred to as seismic waves which move outwards in many directions in a speed of 14
kilometers per second. The fastest waves take an approximate of twenty minutes to cover about
13000 kilometers and thus reach the other side of the Earth and this is by passing straight
through the Earth’s center. The effect of these waves is that they distort every rock which they
pass through and the rock returns to its original shape afterwards.
Earthquakes don’t occur very deep and the reason is that rocks are always not rigid at
very high pressures and also high temperatures. Thus in in deeper places the rocks move
plastically and thus they cannot store stress. Most earthquakes that occur cause less damage or no
damage at all. However very large earthquakes are followed by aftershocks and also minor
faulting might happen during the adjustment period and can last for some months.
Places Where Earthquakes Occur
Earthquakes occur frequently in some areas as compared to others. They mostly
occur at plate boundaries. This is a place where different tectonic plates meet. Large Earthquakes
occur where two plates are colliding as at such a place, there is a very high buildup of stress and
it occurs very rapidly.
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Earthquakes also occur very frequently at the circum-Pacific belt. The intaplate
earthquakes happen less frequently. They occur in relatively stable interior of the continents and
this is away from plate boundaries and such earthquakes originate from shallow levels.
Types of Earthquakes
Earthquakes are of three types which include tectonic, explosion and volcanic.
This types of earthquakes are due to the regions in which they occur and the regions’ geological
make-up. Mostly the earthquakes that occur are tectonic earthquakes. Tectonic earthquakes
happen due to breaking of the rocks of the Earth’s crust which is caused by geological forces
which are caused when the tectonic plates move. The volcanic earthquakes happen during the
volcanic activity. Another type of earthquakes is the collapse earthquakes is the collapse
earthquakes and the explosion earthquakes. The collapse earthquakes are small and they occur in
underground caverns and mines while the explosion earthquakes happen as a result explosion of
nuclear and chemical mediums.
Figure 2. Shows how faulting tectonic forces causes earthquakes
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Figure 3. Diagrams a and b both shows how earthquakes are caused by volcanoes. In
situations as shown in the diagrams, earthquakes are formed in the zones of subduction
where the overriding lithosphere layer sticks onto the descending lithosphere layer which
slips abruptly afterwards.
Types of Earthquake Waves
Earthquake waves are a total of three which are the P-wave, S-wave and the
surface waves. These waves are caused a result of released stress which is in the form of energy
from earthquakes. The P-wave is also referred to as the primary wave and it is considered the
fastest wave when detected by seismographs. These type of earthquake waves move both in
liquid and solid rocks. They are also compressional as they compress and expand the rocks while
they move in through them.
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Figure 4. Shows P-wave motion
On the other hand, the S-waves also referred to as the secondary waves have the
characteristics of shearing the rocks which they pass through them. However they don’t travel
through liquid as liquid cannot shear and it can only compressed. These waves are taken to be so
dangerous as compared to the P-waves and this is because they are so large and they also cause
vertical and horizontal motion in the ground to be produced.
Figure 5. shows S-wave motion
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The P and S waves are taken to be body waves and they both occur within the Earth’s
interior. The speed of these waves is determined by the density and the elastic nature of the rocks
which they pass through. However these waves are amplified as they near the surface.
Another type of earthquake waves is the surface waves. These are the type of waves is
considered the slowest of all waves. They occur close to the Earth’s surface or to the outside of
the ground. The surface waves consist of the love waves and the Rayleigh waves. The Love
waves are waves that move like the S-waves but they move only horizontally while the Rayleigh
waves move in both horizontal and vertical manner.
Figure . Shows Love wave motion
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Figure 6 shows Rayleigh wave motion.
Determination of the Depth of an Earthquake.
Earthquake occurrence range is about 700 kilometers below the Earth’s surface.
This depth range is divided into the shallow, intermediate and deep zones. The shallow zone
earthquakes are estimated to occur between 0 to 70 kilometers deep. For the intermediate zone
earthquakes they occur between 70 to 300 kilometers and the deep zone earthquakes occur
between 300 to 700 kilometers deep. The seismogram usually indicates that large earthquakes
have deep focus as a result of the small amplitude of the surface waves which have been
recorded and also as a result of the behavior of the P and S waves.
Determining Severity of Earthquakes
This is achieved by measuring the highest heights of the seismic waves and this is
done at a distance of 100 kilometers from the Earth’s epicenter. The possible heights achieved
are applied to the Richter scale. This scale is used to categorize earthquakes into divisions known
as magnitudes. The earthquake’s magnitude is the estimation of the amount of energy which is
produced by the earthquake.
One scale used is the Richter scale. The Richter scale is an instrument used in measuring
the amount of energy produced by any earthquake. This scale cannot be altered by the observer’s
size of distance from the earthquake. However there are some things that can alter the scale such
as population density and underlying rocks. The reading of the Richter scale is interpreted as, the
larger the reading, the larger the closeness of the damage of the earthquake to the epicenter.
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The other scale which is used is the Mercalli Scale. This instrument is used to measure
the intensity of an earthquake in a given place. It measures how strongly an earthquake is felt at a
given region where it occurs.
Determination the Size of Earthquakes.
Measurement of the size of earthquakes is done by recording the magnitude of the
earthquakes on the seismographs. The effects of earthquakes in populated areas are as a result of
factors such as how far the observer is from the epicenter of the earth. The magnitudes of the
earthquakes that occur each year on the earth are magnitude 8 which is for one earthquake,
magnitude 7 for ten earthquakes that occur and magnitude 6 for one hundred earthquakes that
occur on Earth.
Impacts of Earthquakes
Earthquakes cause different effects to the environment. The damage caused by an
earthquake depends on different factors such as the type of construction, the distance, the type of
underlying rock and the strength of the earthquake.
Landslides
This is an effect caused by earthquakes in that there is movement of the ground leading to
a change in the whole landscape. This is caused when there is damage of buildings when the
ground surface paves way underneath the buildings. This paving of way is always in the form of
a landslide down a hill or it can also be liquefaction of the soils.
Collapsing of Buildings
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This is an effect of earthquakes which leads to the worst casualties. It occurs whereby
people may be hit by buildings when an earthquake occurs or when people get trapped in
buildings when an earthquake occurs leading to the collapse of the building. At times when
earthquakes occur, people can get hit by falling glasses or debris and this is very damaging.
Earthquakes with very large magnitudes get to an extent of knocking buildings off their
foundations.
Start of Fires
There have been fire outbreaks due to earthquakes. At times earthquakes may also trigger
fires to get out of control. This leads to lots of property to be damage and even death of people
involved. The emergency response to fires caused as a result of earthquakes are so demanding
and this leads to slow response to such fire thus increasing the damages caused.
Occurrence of Tsunamis
These are caused as a result of earthquake, volcano, or landslide occurrence underwater.
Tsunami is a wave and it travels so fast across the ocean covering a very long distance. When it
starts to approach the land, it builds to a height of 10 to 20 meters as the water is shallower. It
then floods the areas around the coast. The tsunamis cause a lot of effects as they carry a lot of
energy which causes massive erosion and breaks down buildings along the coast. These waves
last for some hours.
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References
Engdahl, E. R & A. Villasenor, (2002) Global Seismicity. In International Handbook of
Earthquake and Engeneering Seismology, New York :Academic Press
Hyndman, D. & Hyndman, D. (2009), Earthquakes and their causes: Natural Hazards and
Disasters. Brooks/Cole. ISBN 0-495-31667-9