earthquake

A man photographing the ruins of a building against a backdrop of devastation after the great San Francisco earthquake of 1906.

The propagation of seismic waves through the Earth. Waves vibrate outwards from the focus of the earthquake, deep within the Earth. As well as the movement of waves from the epicentre the wave frequency contracts and dilates.

Abrupt motion of the Earth's surface. Earthquakes are caused by the sudden release in rocks of strain accumulated over time as a result of plate tectonics. The study of earthquakes is called seismology. Most earthquakes occur along faults (fractures or breaks) and Benioff zones. As two plates move past each other they can become jammed. When sufficient strain has accumulated, the rock breaks, releasing a series of elastic waves (seismic waves) as the plates spring free. The force of earthquakes (magnitude) is measured on the Richter scale, and their effect (intensity) on the Mercalli scale. The point at which an earthquake originates is the focus or hypocentre; the point on the Earth's surface directly above this is the epicentre.

The Alaskan (USA) earthquake of 27 March 1964 ranks as one of the greatest ever recorded, measuring 8.3–8.8 on the Richter scale. The 1906 San Francisco earthquake is among the most famous in history. Its magnitude was 8.3 on the Richter scale. The deadliest, most destructive earthquake in historical times is thought to have been in China in 1556. In 1987 a Californian earthquake was successfully predicted by measurement of underground pressure waves; prediction attempts have also involved the study of such phenomena as the change in gases issuing from the crust, the level of water in wells, slight deformation of the rock surface, a sequence of minor tremors, and the behaviour of animals. The possibility of earthquake prevention is remote. However, rock slippage might be slowed at movement points, or promoted at stoppage points, by the extraction or injection of large quantities of water underground, since water serves as a lubricant. This would ease overall pressure. Human activity can create earthquakes. Mining, water extraction, and oil extraction can cause subsidence that helps generate earthquakes, while the building of large dams and underground nuclear testing have also been linked with earthquakes.

Most earthquakes happen at sea and cause little damage. However, when severe earthquakes occur in highly populated areas they can cause great destruction and loss of life. A reliable form of earthquake prediction has yet to be developed, although the seismic gap theory has had some success in identifying likely locations.

The San Andreas Fault in California, where the North American and Pacific plates move past each other, is a notorious site of many large earthquakes.

Earthquakes have been responsible for moving the North Pole towards Japan at a rate of about 6 cm/2 in every 100 years. This is because most major earthquakes occur along the Pacific Rim, and tend to tilt the pole towards the earthquake epicentres.