Every day an earthquake happens somewhere in the world.
Many are so light that they cannot be detected. Only a small proportion of the more than a million quakes that occur every year actually cause damage. The vast majority are very small and have no impact on the suface, or they occur in sparsely populated areas of the world.
Scientists cannot predict when an earthquake will strike, but they have been able to map where earthquakes are most likely to happen.
Most of the largest earthquakes occur within the Pacific "Ring of Fire", a horseshoe-shaped band of volcanoes and fault lines circling the edges of the Pacific Ocean.
The Earth’s crust is divided into tectonic plates which are constantly moving. Earthquakes, like volcanoes, take place along the 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.
Underwater earthquakes, or landslides caused by an earthquake, can trigger tsunamis – large water waves that can cause widespread damage when they hit land.
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 is building quality and regulations - buildings can now be designed to withstand significant levels of shaking, at slightly higher cost. Poorer people's livelihoods are also likely to be more vulnerable.
If people aren’t trained in what to do in an earthquake, they are more likely to be killed or injured.
In January 2010, Haiti experienced the biggest urban disaster in modern history, when a 7.0 magnitude quake killed more than 200,000 people and left 1.5 million homeless. The main quake was followed by several strong aftershocks.
The tremors occurred near the earth’s surface, near a crowded capital city with poorly constructed buildings, and among people who had had little or no training in what to do in an earthquake.
In December 2003, an earthquake in the southern Iranian city of Bam wiped it out in just 12 seconds. More than a quarter of the 120,000 population died, and nearly all the survivors were left homeless. The quake measured magnitude 6.5, but it was very shallow, and the city’s traditional architecture meant walls and roofs crumbled as they collapsed, leaving no air pockets and suffocating people inside.
Although no structure is 100 percent quake-proof, buildings can be made much safer relatively cheaply, adding less than 10 percent on average to building costs. The cost can be as little as 3 to 4 percent higher when building a safe school, and a 5 to 10 percent increase when building a hospital, the U.N. Secretary-General’s special representative for disaster risk reduction, Margareta Wahlstrom, said soon after the Haiti earthquake.
Time is of the essence in saving lives. Usually locals digging with their bare hands save more lives than well-equipped international rescue teams who arrive days after the quake.
Terrain is an important factor in the impact of earthquakes. Building on steep slopes and on soft soil foundations increases the chance of buildings sinking or tipping over during an earthquake.
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.
Measuring a tremor
Earthquake size is measured by magnitude, a measure of the amount of energy a tremor releases. The magnitude is 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. 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.
- The largest recorded earthquake in the world was magnitude 9.5 in Chile, May 22, 1960.
- Most earthquakes occur at depths of less than 80 km (50 miles) from the Earth's surface.
- The world's deadliest recorded earthquake occurred in 1556 in central China, where most people lived in caves carved from soft rock. An estimated 830,000 people died.
- The earliest recorded evidence of an earthquake dates back to 1831 BC in China's Shandong province.
Source: The U.S. Geological Survey's Earthquake Facts page.
What to do during an earthquake
Here's what the American Red Cross says people should do during an earthquake:
If you are inside when the shaking starts:
- Drop, cover and hold on. Move as little as possible.
- If you are in bed, stay there, curl up and hold on. Protect your head with a pillow.
- Stay away from windows to avoid being injured by shattered glass.
- Stay indoors until the shaking stops and you are sure it is safe to exit. When it is, use stairs rather than the elevator in case there are aftershocks, power outages or other damage.
- Be aware that fire alarms and sprinkler systems frequently go off in buildings during an earthquake, even if there is no fire.
If you are outside when the shaking starts:
- Find a clear spot (away from buildings, power lines, trees, streetlights) and drop to the ground. Stay there until the shaking stops.
- If you are in a vehicle, pull over to a clear location and stop. Avoid bridges, overpasses and power lines if possible. Stay inside with your seatbelt fastened until the shaking stops. Then, drive carefully, avoiding bridges and ramps that may have been damaged.
- If a power line falls on your vehicle, do not get out. Wait for assistance.
- If you are in a mountainous area or near unstable slopes or cliffs, be alert for falling rocks and other debris. Landslides are often triggered by earthquakes.
Fault, fault plane – The edges where two tectonic plates move past each other.
Hypocentre – The point at which the earthquake starts below the earth's surface.
Epicentre - The point on the earth’s surface directly above the hypocentre.
Foreshock – One of a series of smaller earthquakes that sometimes precede a big earthquake in the same place.
Aftershock – One of the 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 - The main earthquake.
Earthquake magnitude - 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 is in various locations.
Earthquake intensity - A measure of the shaking on the earth’s surface created by the earthquake.
For resources, including photos and topographical maps, see the U.S. Geological Survey's Learning Links.
The USGS has also produced High Quality Earthquake Animations.
For a full set of links to information about earthquakes, see the USGS's Earthquake Topics.
The USGS also has information on animals and earthquake prediction.
For what to do in an earthquake, see the American Red Cross's Earthquake Safety Checklist
To see the latest earthquake alerts, visit: