Plate tectonics (Copyright Geoscience Australia) - More>>
What causes earthquakes?
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.
Interesting Fact
Earthquake vibrations travel very fast, up to 14 kilometres per second. The fastest seismic waves take less than 20 minutes to reach the other side of the earth, a distance of almost 13,000 kilometres!
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 700 kilometres. 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.