An earthquake is the shaking and vibration at the surface of the Earth caused by underground movement along a fault plane or by volcanic activity.
The size of earthquakes is determined by measuring the amplitude of the seismic waves recorded on a seismograph. A formula is applied to these which converts them to a magnitude scale, a measure of the energy released by the earthquake. For every unit increase in magnitude, there is roughly a thirty-fold increase in the energy released. For instance, a magnitude 2.0 earthquake releases 30 times more energy than a magnitude 1.0 earthquake, while a magnitude 3.0 earthquake releases 900 times (30x30) more energy than a magnitude 1.0.
A magnitude 8.6 earthquake releases energy equivalent to about 10 000 atomic bombs of the type developed in World War II.
The effects of an earthquake depend on many factors, such as the distance from the epicentre (the point on the Earth's surface directly above where the earthquake originated within the Earth) and the local ground conditions. Generally, for locations near the epicentre, the following effects may be observed:
|Magnitude||Description of effect|
|less than 3.4||Usually felt by only a few people near the epicentre.|
|3.5 - 4.2||Felt by people who are indoors and some outdoors; vibrations similar to a passing truck.|
|4.3 - 4.8||Felt by many people; windows rattle, dishes disturbed, standing cars rock.|
|4.9 - 5.4||Felt by everyone; dishes break and doors swing, unstable objects overturn.|
|5.5 - 6.1||Some damage to buildings; plaster cracks, bricks fall, chimneys damaged.|
|6.2 - 6.9||Much building damage; houses move on their foundations, chimneys fall, furniture moves.|
|7.0 - 7.3||Serious damage to buildings; bridges twist, walls fracture, many masonry buildings collapse.|
|7.4 - 7.9||Causes great damage; most buildings collapse.|
|greater than 8.0||Causes extensive damage; waves seen on the ground surface, objects thrown into the air.|