About Earthquakes, different types of waves (P,S,L), Magnitude and frequency, effects of earthquakes, distribution and prediction.

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Earthquakes occur almost continuously over the Earth, with an estimated 18 earthquakes of magnitude 7.0 or
larger every year
When there is a build up of pressure within the Earth's crust the ground shakes violenty, an earthquake
The point where the release happens is called the focus, it can be found at a range of depths
-Shallow focus: 0-70km deep
-Intermediate focus: 70-300km deep
-Deep focus: 300-700km deep
At the focus, the seismic shock waves have their highest level of energy, this is because energy decreases as
the waves spread outwards
The place on the Earth's surface immediately above the focus is called the epicentre, it recieves the highest
amount of energy and is potentially the most hazardous location
There are 3 main types of seismic waves:
-P waves (primary): These are the fastest and shake the Earth backwards and forwards. These travel fastest
and move through solids and liquids
-S waves (secondary): These are slower and move with a sideways motion, shaking the Earth at right angles
to the direction of travel. They cannot move through liquids, but do much more damage than P waves
-Surface waves: These travel much closer to the surface and more slowly than both the 2 other waves,
surface waves are more destructive than both. They include L waves (long waves), these cause the ground to
move sideways and Raleigh waves, which make it move up and down
Magnitude and frequency
The magnitude of an earthquake is the amount of energy released by the event and is usually measured on
the Richter scale. It is a logarithmic scale, each unit represents a 10-fold increase in strength and a 30-fold
The frequency of earthquake events varies greatly between seismically active regions and seismic zones
within the shield areas of ancient crust
Seismometers are instruments that measure and record the shock waves created by earthquakes, they also
locate and measure the size of the shockwaves. After this patterns can be seen and worked out, which
therefore helps in earthquake prediction
Aftershocks are earthquakes that follow on from the main event and may last for months afterwards. These
are generated by the Earth settling down after the main earthquake/movement
Effects of Earthquakes
The effects of earthquakes range from the ground shaking to landslides, avalanches and tsunamis. The
severity of the effects depends on a range of factors such as the magnitude of the earthquake
Secondary effects include:
o Tsunamis: These are enormous sea waves.
o Liquefaction: This is where violent disruption of the ground causes it to become liquid-like when
strongly shaken. In some cases it increases pore water pressure which reduces the effective stress
and therefore reduces the shear strength of the soil so it fails more easily. Although the surface
appears dry, cracks would appear and water seep through, bringing liquefied soil with it, creating

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Buildings can `sink' as a result. It is estimated that US$100,000 million worth of
damage was caused by this secondary effect of the 1989 earthquake.
o Landslides and avalanches: Where slope failure occurs as a result of ground shaking
o Human impact: This depends on the population density and distance from the epicentre. Strong
shaking of the ground can cause buildings, roads and bridges to collapse, and disruption to gas,
electricity and water supplies.…read more

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There is currently no reliable way to accurately predict when an earthquake will occur, there are many
methods which include:
-Seismic Records: Studying patterns of earthquakes and using these to predict the next event will occur. The
shockwaves are recorded on a seismometer or seismograph.…read more


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