Seismogram recording of an earthquake
Credit: U.S. Geological Survey.
Epicenter and hypocenter, Credit: U.S. Geological Survey.
An earthquake is a sudden slip on a fault, followed by ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the Earth. The immediate area where the earthquake takes place is the focus or hypocenter, the point immediately above it on the Earth's surface is the epicenter, and the shock waves emanating from the earthquake are called seismic waves. Depending on the character of the focus, the epicenter may be a small circle or a line.
Earthquakes occur to relieve a stress that has built up within the crust or mantle of the Earth; fracture results when the stress exceeds the strength of the rock. The reasons for the stress build-up are to be found in the theory of plate tectonics. If a map is drawn of the world's earthquake activity, it can be immediately seen that earthquakes are confined to discrete belts. These belts signify the borders of contiguous plates; shallow earthquakes being generally associated with mid-ocean ridges where creation of new material occurs, deep ones with regions where one plate is being forced under another.
Seismic waves are of two main types. Body waves travel from the hypocenter, and again are of two types: P (compressional waves), where the motion of particles of the Earth in the direction of propagation of the wave; and S (shear) waves, where the particle motion is at right angles to this direction. Surface waves travel from the epicenter, and are largely confined to the Earth's surface; Love waves are at right angles to the direction or propagation; Rayleigh waves have a more complicated, backward elliptical movement in the direction of propagation.
The experienced intensity of an earthquake depends mainly on the distance from the source. Local intensities are gauged in terms of the Mercalli Intensity Scale, which runs from I (detectable only by seismograph) through to XII ("Catastrophic"). Comparison of intensities in different areas enables the source of an earthquake to be located. The actual magnitude of the event is gauged according to the Richter scale.
Earthquake hazard is anything associated with an earthquake that may affect the normal activities of people. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunamis, and seiches.
Earthquake risk is the probable building damage, and number of people that are expected to be hurt or killed if a likely earthquake on a particular fault occurs. Earthquake risk and earthquake hazard are occasionally incorrectly used interchangeably.
· Chile, 27 Feb 2010: 8.8 magnitude quake
· Haiti, 12 Jan 2010: About 230,000 people die after shallow 7.0 magnitude quake
· Sumatra, Indonesia, 26 Dec 2004: 9.2 magnitude. Triggers Asian tsunami that kills nearly 250,000 people
· Alaska, US, 28 March 1964: 9.2 magnitude; 128 people killed. Anchorage badly damaged
· Chile, south of Concepcion, 22 May 1960: 9.5 magnitude. About 1,655 deaths. Tsunami hits Hawaii and Japan
· Kamchatka, NE Russia, 4 Nov 1952: 9.0 magnitude