The Earth's outer layer or crust is broken into pieces called tectonic plates which are constantly moving towards, away from or past each other. Because continents are part of these plates, they also move. An earthquake occurs when the rocks break and move as a result of stresses caused by plate movements.
Most earthquakes occur on the edge of plates, especially where one plate is forced under another such as happens off Sumatra or past another as occurs in California. Some regions have more earthquakes than others with 80% of all recorded earthquakes taking place around the edge of the Pacific Plate, in New Zealand, Papua New Guinea, Japan, Canada, USA and South America.
Earthquake vibrations travel very fast, up to 14km/s. The fastest seismic waves take less than 20 minutes to reach the other side of the earth, a distance of almost 13 000km!
The epicentre is the point on the Earth's surface directly above the source of the earthquake. The source, also known as the focus, can be as deep as 700km. Smaller earthquakes occur much more frequently than large ones and most cause little or no damage. A very large earthquake can be followed by a series of smaller events called aftershocks during a period of adjustment which may last for several months.
Earthquakes also can cause a tsunami, or a series of waves which can cross an ocean and cause extensive damage to coastal regions. In areas where there are steep slopes, vibrations resulting from earthquakes may cause landslides.
Where do earthquakes occur?
No part of the Earth's surface is free from earthquakes, but some regions experience them more frequently. They are most common at tectonic plate boundaries where different plates meet. The largest events usually happen where two plates are colliding, particularly around the edge of the Pacific Plate in New Zealand, Papua New Guinea, Japan and the Americas.
Intraplate earthquakes occur in the relatively stable interior of continents away from plate boundaries. They are less common and do not follow easily recognisable patterns. This type of earthquake generally originates at shallow depths.
Although Australia is not on the edge of a plate, the continent experiences earthquakes because the Indo-Australian plate is being pushed north and is colliding with the Eurasian, Philippine and Pacific plates. This causes the build up of stress in the interior of the Indo-Australian plate which is released during earthquakes.