There are thousands of earthquakes across the Earth each day. Most are too small to be detected without monitoring equipment, but some are powerful enough to destroy a city. Sendai, Los Angles, Kobe, Christchurch and Bam are the names of some of the places that have been affected by powerful earthquakes in recent years.
Earthquakes are most common at plate boundaries. The relative motion of two plates releases energy into the crust and causes faults to form and move. When two plates or a fault lock up, with little or no movement over a long time period, the result is a large earthquake when they eventually unlock and quickly release a large amount of energy.
Learn more with the BBC News animated earthquake guide.
Image: The San Andreas fault, California (Peter Menzel/SPL)
Iain Stewart visits New Zealand to explain how mountains form.
Dr Iain Stewart visits New Zealand to explain how the collision of two plates results in mountain ranges forming over millions of years.
Scientists recreate the San Andreas fault in a laboratory.
Scientists create a scaled-down version of the earthquake causing San Andreas fault to see if they can find ways to predict future quakes.
Why are some parts of the world so earthquake prone?
Scientists explain how the movement of the Earth's plates creates faults in the Earth's crust where the plates meet. Earthquakes are caused by movement along these networks of faults and the movement of the plates themselves.
Earthquakes are common at subduction zones, points where one plate moves below another.
Earthquakes are common at subductions zones, points where the Earth's plates meet and one plate moves below the other. The powerful earthquakes that are caused by this type of plate movement are known as megathrust earthquakes. The 2004 Boxing Day earthquake was a megathrust quake that triggered the subsequent deadly tsunami.
An 8.1 quake kills thousands and causes buildings to sink into the ground.
The 19 September 1985 Mexico City earthquake killed at least 9,000 people and injured 30,000. The greatest destruction occurred in an area where buildings were built on an old lakebed. The soft sediments below the buildings amplified the shockwaves and reduced the structures to rubble. A phenomenon called liquefaction caused seemingly solid ground to take on the properties of a liquid, and buildings sank into the ground.
An earthquake (also known as a quake, tremor or temblor) is the perceptible shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes can be violent enough to toss people around and destroy whole cities. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.
Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter magnitude scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly imperceptible or weak and magnitude 7 and over potentially cause serious damage over larger areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.
In its most general sense, the word earthquake is used to describe any seismic event — whether natural or caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter.