Earthquakes

An earthquake is the shaking and vibration of the Earth's crust due to movement of the Earth's plates (plate tectonics). Earthquakes can happen along any type of plate boundary.

Earthquakes occur when tension is released from inside the crust. Plates do not always move smoothly alongside each other and sometimes get stuck. When this happens pressure builds up. When this pressure is eventually released, an earthquake tends to occur.

The point inside the crust where the pressure is released is called the focus. The point on the Earth's surface above the focus is called the epicentre.

Earthquake energy is released in seismic waves. These waves spread out from the focus. The waves are felt most strongly at the epicentre, becoming less strong as they travel further away. The most severe damage caused by an earthquake will happen close to the epicentre.

The power of an earthquake is measured using a seismometer. A seismometer detects the vibrations caused by an earthquake. It plots these vibrations on a seismograph.

The strength, or magnitude, of an earthquake is measured using the Richter scale. The Richter scale is numbered 0-10.

Earthquakes can destroy settlements and kill many people. Aftershocks can cause even more damage to an area. It is possible to classify the impacts of an earthquake, by taking the following factors into account:

• short-term (immediate) impacts
• long-term impacts
• social impacts (the impact on people)
• economic impacts (the impact on the wealth of an area)
• environmental impacts (the impact on the landscape)

Effects are often classified as primary and secondary impacts. Primary effects occur as a direct result of the ground shaking, eg buildings collapsing. Secondary effects occur as a result of the primary effects, eg tsunamis or fires due to ruptured gas mains.

Short term social impacts: People may be killed or injured. Homes may be destroyed. Transport and communication links may be disrupted. Water pipes may burst and water supplies may be contaminated.

Long term social impacts: Disease may spread. People may have to be re-housed, sometimes in refugee camps.

Short term economic impacts: Shops and business may be destroyed. Looting may take place. The damage to transport and communication links can make trade difficult.

Long term economic impacts: The cost of rebuilding a settlement is high. Investment in the area may be focused only on repairing the damage caused by the earthquake. Income could be lost.

Short term enviromental impacts: The built landscape may be destroyed. Fires can spread due to gas pipe explosions. Fires can damage areas of woodland. Landslides may occur. Tsunamis may cause flooding in coastal areas.

Long term enviromental impacts: Important natural and human landmarks may be lost.

• Distance from the epicentre - the effects of an earthquake are more severe at its centre.
• The higher on the Richter scale, the more severe the earthquake is.
• Level of development (MEDC or LEDC) - MEDCs are more likely to have the resources and technology for monitoring, prediction and response.
• Population density (rural or urban area). The more densely populated an area, the more likely there are to be deaths and casualties.
• Communication - accessibility for rescue teams.
• Time of day influences whether people are in their homes, at work or travelling. A severe earthquake at rush hour in a densely populated urban area could have devastating effects.
• The time of year and climate will influence survival rates and the rate at which disease can spread.

LEDCs often suffer more from the effects of volcanoes and earthquakes than MEDCs.

• Communication systems may be underdeveloped, so the population may not be well educated about what to do in the event of a volcanic eruption or an earthquake.
• Construction standards tend to be poor in LEDCs. Homes and other buildings may suffer serious damage when a disaster occurs.
• Buildings collapsing can cause high death tolls.
• Evacuation and other emergency plans can be difficult to put into action due to limited funds and resources.
• Clearing up can be difficult. There may not be enough money to rebuild homes quickly and safely. Many people could be forced to live in emergency housing or refugee camps.