Earthquake

An earthquake is a shaking of the ground, normally caused by movements on a geologic fault. Earthquakes range in size from those barely perceptible by sensitive instruments to "great earthquakes" such as the one which struck Indonesia in 2004.

According to the theory of plate tectonics, pieces of the earth's crust, called plates, are constantly moving relative to each other. Since the crust is brittle, accomodating these movements causes breaks in the crust, called faults. Movement along faults is rarely smooth, and often, faults will remain "locked" for a period of time due to frictional resistance. When the driving force overcomes the friction, the material on each side of the fault will suddenly move relative to the other side. This movement causes shaking which is felt most strongly close to the fault and dissipates with distance.

The shaking cause by an earthquake can be strong enough to knock down people, animals, and buildings, and can trigger landslides.

Earthquakes are very frequent, though the frequency varies inversely with the size of the earthquake - earthquakes as large as the Indonesia Earthquake of 2004 occur once or twice a century worldwide, while earthquakes barely perceptible by humans occur several times a day throughout the world.

Earthquake terms

hypocenter - the location in the earth where the earthquake movement began.

epicenter - the location on the surface of the earth directly above the hypocenter.

Measurement of earthquakes

Earthquakes are measured by geologists and seismologists in two main ways, magnitude and intensity. The magnitude of an earthquake is a property of the earthquake as a whole, and measures the energy released during the earthquake. The intensity of an earthquake is a property of both the earthquake and a specific location, and measures the shaking felt at any given location. An earthquake with a specific magnitude will have a range of intensities, dropping with distance from the hypocenter.

Magnitude was originally measured by the Richter magnitude scale, which is a logarithmic scale based on the movements an earthquake caused on a Wood-Anderson seismograph at a distance of 100km. The Richter magnitude scale, often called the "Richter scale", was developed by Charles Richter for use in California, and does not produce accurate results for the largest earthquakes. Several other scales have been proposed to supplement or modify the Richter scale, the most generally used one now being the moment magnitude scale, which is also a logarithmic scale, based on the seismic moment of an earthquake; the moment magnitude scale has been calibrated to yield comparable numbers to the Richter scale.

Intensity is measured by the Modified Mercalli scale, the Japan Meteorological Agency seismic intensity scale, or peak ground acceleration. The three scales (and some historic ones) can be roughly calibrated to each other. After an earthquake, based on reports or instrument readings, a map showing the variation of intensity across the area where the earthquake was felt can be produced. Maps predicting the maximum intensity expected at given locations are prepared for planning purposes.