Earthquakes
An earthquake is a sudden and violent shaking of the earth’s crust.
Earthquakes occur where two plates collide (destructive plate boundary) or slide past
one another (transfrom plate boundary).
Huge amounts of pressure builds up between the two plates until one of the plates
suddenly slips resulting in the pressure being released cauing an earthquake.
This pressure is released as energy in the form of seismic waves, also known as
tremors.
The point in the earth’s crust where the earthquakes occurs is called the focus.
This is the source of the seismic waves and the radiate from the focus through the
crust and toward’s the earth’s surface.
The epicentre is the point on the surface directly above the focus. The tremors are
strongest here as they have the shortest distance to travel to reach the surface. As
you travel further from the epicentre the temors decease in strength.
Smaller tremors, aftershocks, may occur in the hours or days following an
earthquake.
Foreshocks, small tremors, may sometimes occur before a major earthquake and
provide warning, but this is not always the case.
How to measure an earthquake?
Earthquakes are measured using an instrument called a seismometer or seismograph.
The strength of an earthquake is referred to as its magnitude.
The magnitude of an earthquake is shown on the Richter scale. The higher the number
on the scale the more powerful the earthquake. An increase of one unit on the Richter
scale means the earthquake is ten times stronger.
Seimologists, scientists who study earthquakes and seismic activity, can predict where
an earthquake is likely to occur. However they cannot accurately predict when an
earthquake will strike.
Methods used to predict when an earthquake is likely to happen.
Laser beams are used to show/detect plate movements. This is used along the San
Andreas Fault in California.
Seismometer can be used to pick up vibrations in the earth’s crust. Unfortunately
foreshocks do not always occur before a larger magnitude earthquake.
Monitoring levels of radon gas is also used. If the levels of this gas escaping from the
surface increase it could mean an earthquake is likely to occur.
Recording the number of years between similarly large earthquakes occurring in
one location. San Francisco experienced a 6.9 magnitude earthquake in 0ctober 1989.
An earthquake of similar magnitude occurred in 1906. San Fransico expects to
experience a large earthquake approxiametly once every one hundred years.
In 1975, China evacuated an entire city before an earthquake destroyed it.
The effects of earthquakes.
Earthquakes impact socially, economically and environmentally.
Earthquakes can result in loss of life or serious injury to people.
Damage to or the collapse of buildings. Destruction of homes result in displacement
of people and refugee camps need to be established.
Damage to shops/businesses means loss of income and looting may occur.
Rebuilding costs can be very high and an economic strain to these regions.
Water pipes can be damaged often leading to contamination and disease. This
disease is easily spread through refugee camps.
Communication networks, eg roads, can be disrupted making it difficult for aid to
enter to the affected the area and victims, delay may result in higher casualties.
Trade in the region suffers if communication networks are significantly disrupted.
Gas and electricity damage may cause fires that could cause further loss of life or
spread to nearby woodland damaging the environment.
Factors affecting the impact of an earthquake.
The higher the magnitude of the earthquake the more likely it is to cause substantial
damage.
Distance from the epicentre – if a town/city is closer to the epicentre it is more likely
to experience greater levels of destruction.
Level of development – developed countries are more likely to be better prepared for
the impact of an earthquake than developing countries. Developed countries, unlike
developing regions, will have buildings that are designed to withstand high magnitude
earthquakes.
Population density –the greater the amount of people near the epicentre of an
earthquake the more likely the deathtoll will be higher, especially if it is a large
settlement in a developing country.
Communications – high mountainous regions do not allow easy access for aid to victims
of an earthquake.
Aftershocks – the number and magnitude of aftershocks can cause further damage.
Buildings that have been damged during the initial earthquake may collapse as a result
of aftershocks.
Time of day – the time of day can have a significant bearing on the impact. An
earthquake that occurs during a large city’s rush hour can be devastating.