Earthquakes, like most volcanoes, take place along tectonic plate boundaries.

When two plates move past each other, the jagged parts of the plate boundaries get stuck while the rest of the plates keep moving. Eventually, when the plates have moved far enough, the edges suddenly become unstuck, causing an earthquake.

An estimated 1.4 million earthquakes occur every year, according to the U.S. Geological Survey (USGS). The vast majority are very slight and have no impact on the surface, or they occur in sparsely populated areas of the world.

But an estimated 100 a year cause some degree of damage.

Scientists cannot predict exactly when they will occur, but they have been able to map where they are most likely to strike.

Interestingly, anecdotal evidence suggests that animals show unusual behaviour before an earthquake. There have been reports of creatures leaving their homes ahead of an earthquake, but more research needs to be done in this field.

Earthquakes usually have the greatest impact in poorer countries. The main reason has to do with building quality and regulations - now buildings can be designed to withstand significant levels of shaking. Poorer people's livelihoods are also likely to be more vulnerable.

In December 2003, an earthquake in the southern Iranian city of Bam took just 12 seconds to annihilate a city of 120,000 people. More than a quarter of the population died, and nearly all survivors were left homeless.

The quake measured just magnitude 6.5. But its devastating effect was due to three factors:

The earthquake occurred just below the city at a depth of 10 km (6 miles)

Almost everyone was asleep in bed with the earthquake struck

The city's traditional architecture meant walls and roofs crumbled as they collapsed, leaving no air pockets and suffocating those inside.

Time was of the essence in saving lives. Local rescue teams deployed within minutes, despite losing four crew members and their headquarters in the disaster. Working with 10 sniffer dogs, they saved 157 lives. Residents saved hundreds more using their bare hands.

In contrast, a major international effort involving 34 teams from 27 countries found just 22 people alive.

Terrain is an important factor in the impact of earthquakes.

The India/Pakistan earthquake of 2005 was magnitude 7.6. It killed nearly 75,000 people - including 16,000 children who were crushed when their classrooms collapsed on top of them. Aftershocks in the mountainous region caused countless landslides, blocking roads and hampering relief efforts.

Earthquake size is measured by magnitude, usually based on the scale worked out in 1935 by Charles Richter. But the "Richter scale" is unreliable for measuring larger earthquakes and has been heavily modified. The USGS favours describing quakes merely by "magnitude" and gives readings consistent with the Richter scale.

Magnitude is the same no matter where you are, or how strong or weak the shaking is on the surface. Every increase of one whole number of magnitude represents a 10-fold increase in intensity.

Below is a very rough guide to how magnitudes relate to the amount of shaking on the surface. However, if an earthquake of high magnitude occurs deep below the earth's surface the amount of shaking will be less than if it occurs nearer the surface.

Magnitude 2.5 or less - the earthquake is usually not felt

Magnitude 7.0-7.9 - major earthquake, serious damage

Magnitude 8.0 or greater - can totally destroy communities near the epicentre.

Source: UPSeis

WHAT TO DO DURING AN EARTHQAUKE

Turkish rescue workers search rubble with sniffer dogs. REUTERS/Fatih Saribas

The American Red Cross recommends the following:

Find a safe place under a sturdy table or desk or against an inside wall where nothing can fall on you.

Drop, cover and hold on. Protect your eyes by pressing your face against your arm.

Stay indoors until the shaking stops and you're sure it's safe to exit. Stay away from windows.

If you are in bed, hold on and stay there, protecting your head with a pillow.

If you are outdoors, find a clear spot away from buildings, trees and power lines. Drop to the ground.

If you are in a car, slow down and drive to a clear place. Stay in the car until the shaking stops.

After the shaking stops, check for serious injuries. Use the telephone only to report life-threatening emergencies.

Expect aftershocks.

JARGON BUSTING

Fault, fault plane - This is the surface where two tectonic plates move past each other.

Hypocentre - This is where the earthquake starts below the earth's surface.

Epicentre - This is the location directly above the hypocentre.

Foreshock - Some large earthquakes are preceded by a series of smaller earthquakes in the same place.

Aftershock - These are smaller earthquakes that happen after the main quake. If the mainshock (see below) is large, aftershocks can continue for weeks, months or even years.

Mainshock - This is the main earthquake.

Earthquake magnitude - This is the measured value of the earthquake size. It is a measurement of the size of the largest seismic wave recorded during a quake. The magnitude is the same no matter where you are, or how strong or weak the shaking was in various locations. Magnitudes are based on the Richter scale.

Earthquake intensity - This is a measure of the shaking created by the earthquake and varies with location.

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