An earthquake is a vibration in the rocks of the Earth’s crust caused by a sudden dislocation of the rocks along a fault. Energy spreads away from the focus, carried by seismic waves.

Seismic waves may be classified as P, S and L waves according to the way in which the rock particles oscillate as they transmit the energy.

P and S waves are important because they travel through the body of the Earth (which is why they are called Body Waves) and can be used to "X-ray" the interior of the Earth to reveal its structure.

L waves are confined to the surface of the crust and therefore are responsible for the damage caused to buildings. In strong earthquakes, witnesses describe the ground as being distorted "like a wave".

As usual, there are a number of observations which raise their own questions.

• Earthquakes occur only in brittle rocks which can break rather than bend.
• The maximum depth for an earthquake focus is 720 km.
• Earthquakes occur (only) in certain zones around the Earth. These zones often coincide with areas of active volcanoes. This doesn't mean that earthquakes cause volcanoes, nor that volcanoes cause earthquakes. But what does it mean?

Earthquakes can be highly destructive to the "built environment". Collapsing buildings are the cause of most fatalities and injuries; others are caused by landslides etc. The earthquake itself is not a killer. If we learn how to build earthquake-proof buildings and if we learn not to build houses in areas liable to landslides and liquefaction, then the number of deaths will be reduced. Unfortunately, it is often too expensive or there is not enough alternative building land for this to happen.

The Olympic stadium, Beijing, designed to withstand earthquakes

An alternative is to predict when earthquakes will happen. While we know the areas which suffer frequently from earth movements, and can forecast that an event is likely, we cannot yet predict the exact date when it will happen. Nature sometimes can - if animals start behaving oddly in an earthquake area, be warned!

The media usually describe earthquakes in terms of the Richter scale, although seismologists use a more accurate, more up-to-date scale. Charles Richter devised his scale in 1935 to measure the frequent tremors and earthquakes in California (home of the infamous San Andreas fault). It can be applied easily to the smallest tremor and the strongest earthquake. People would expect to feel a tremor of magnitude 3 or more. There is no limit to the scale, but it is highly unlikely that any earthquake would exceed 9 on the scale.
Note that

• a magnitude 4 earthquake is about 10 times stronger than magnitude 3
• a magnitude 5 earthquake is about 100 times stronger than magnitude 3
• a magnitude 6 earthquake is about 1000 times stronger than magnitude 3
• a magnitude 7 earthquake is about 10000 times stronger than magnitude 3

and so on. (This is because the Richter scale is a logarithmic scale). A magnitude 9 earthquake is really very strong indeed!
In terms of the amount of energy released, each step is 30 times stronger than the last.

Tsunamis are often associated with earthquakes. They result from the same movements within the rocks of the sea floor, but tsunamis are not caused by earthquakes, despite what is said in the media.

Tsunamis can travel right across an ocean at high speeds. If the 'target' area is far enough away from the epicentre of the associated earthquake, there is enough time for a warning to be given - if there is a warning system in place (another expense). If you are sunbathing on the beach and you notice the that sea suddenly recedes, it is time to make your rapid escape to high land, because the water will very soon rush back in a wave 10 metres or so high. And stay at a safe height - until the fourth and fifth wave (at least) have arrived.

Kodiak was inundated by a tsunami from the 1964 Alaska earthquake. Enough warning was given, and the population was able to escape to higher land.

Geopix Study Topic "Earthquakes"

• describes the causes and distribution of earthquakes;
• describes the different seismic waves and their velocities;
• outlines how seismographs and seismogram are used to record earthquakes and to locate the epicentre;
• explains the scales used to measure their strength;
• suggests ways of predicting earthquakes;
• outlines the methods of reducing the effects on buildings and people;
• describes the origins and behaviour of tsunamis;
• uses the Alaska 1964 earthquake and tsunami as a case study.

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The "Earthquakes" Study Topic includes the following pages:

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