The San Andreas Fault. Image from USGS   Fault diagram. Image from USGS


The San Andreas Fault is perhaps the most famous geological fault and runs a length of roughly 800 miles (1300 kilometres) through western and southern California in the United States. The fault, marks a transform (or sliding) boundary between the Pacific Plate and the North American Plate

What causes an earthquake?

An earthquake begins miles down in the earth where there is a fault in the earth’s crust (see the Plate Tectonics page). A fault is a break, crack or fracture between two blocks of rock in the outside layer of the earth’s surface. If one block of rock suddenly slips and rubs up against the other it causes energy to be released that can vibrate the rock causing an earthquake. This energy and vibration moves out from the centre or slip in a wave causing the area all around to shake and move. An earthquake first begins with a slight shaking motion, which stops briefly and then begins a little stronger. The bigger the earthquake the more intense the shaking and the further it will travel. The further away from the fault or origin of the earthquake the less intense the shaking. Earthquakes can last different lengths of time from a few seconds to a few minutes. There are three main types of earthquakes. A “foreshock” is an earthquake that occurs just before a larger earthquake (or “mainshock”) in the same area.

An “aftershock” is an earthquake that occurs in the same area after a mainshock and can occur days to years after the mainshock. Because earthquakes are a result of geological process up to 800 kilometres below the surface neither the weather nor the time of year can cause or have an effect on them and we cannot prevent earthquakes but we can learn more about them and how to be safe during one.

How are earthquakes measured?

A seismograph reading of the Sumatra Earthquake that triggered the 2004 Tsunami in South East Asia. Image from

A seismograph measures the movement of the ground in a particular area and the Richter scale is then used to measure the size or magnitude of that earthquake. This is not a machine but a mathematical formula internationally agreed as the best measurement scale today. Earthquakes are divided into classes of magnitude that show how strong an earthquake is. There are seven different classes with a magnitude less that three considered a micro earthquake, a magnitude of less than five a moderate one and a magnitude of more than 8 a great earthquake.

Earthquake facts

  • There are roughly 500,000 earthquakes each year, 100,000 of these can be felt and 100 of them cause damage.
  • The largest earthquake in the world was in Chile in 1960 had a magnitude of 9.5.
  • Earthquakes occur on the moon but are called moonquakes.
  • Earthquakes do not cause volcanic eruptions, as there are different processes involved. An earthquake may occur in the area of a volcano eruption as it is an area where the earth’s crust is not at rest.
  • Humans have caused small earthquakes when liquids have been injected into deep wells in the ground, either for disposing of waste, when recovering oil or for water supplies.

What damage can an earthquake cause?

  • During an earthquake the ground shakes which causes buildings to shake. These buildings will be damaged if they cannot withstand the vibrations.
  • When faults or fractures are created in the earth’s surface damage can be caused to buildings, bridges, tunnels, railways and pipelines.
  • Landslides have been known to occur during earthquakes. See section on landslides. Underwater earthquakes or landslides cause large waves or tsunamis. See section on Tsunamis.

Can we prevent earthquakes, or protect ourselves from damage?

Ireland is a very stable part of the Earth's crust, but in many parts of the world, especially tectonically active zones at crustal plate margins, earthquakes are a major hazard and can kill thousands of people in one go. Whilst California, for example, can afford to design and engineer cities to absorb the impact of many quakes, in Third World countries the results of earthquakes can be devastating, with homelessness, exposure, disease and lack of services killing as many as the actual earthquake itself.

Northridge, California, 17 January 1994

Photo: Northridge earthquake, California, 4.31am, 17Jan 1994,
Photo: Kobe Earthquake, Japan, 17 Jan 1995 © USGS
.Kobe Earthquake

Read about the June 6th, 2012 earthquake off the Mayo coast

To go further:/
U.S. Geological Survey Earthquakes for kids :

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