NaturalHazards.org

earthquake image

Rockfall along a highway in Hawaii caused by a 6.0 magnitude earthquake.

perilous planet
for more earthquake photography.



Other Resources:

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Furious Earth: the Science and Nature of Earthquakes, Volcanoes, and Tsunamis (Ellen Prager)

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Earthshaking Science: What We Know, and Don't Know, about Earthquakes (Susan Elizabeth Hough)


A Dangerous Place (Marc Reisner)

WHAT are earthquakes?

An earthquake is a massive release of energy that usually occurs at a fault, which is a large break in the Earth's bedrock. Earthquakes are most common along the boundaries of the plates that make up the Earth's crust, but they can occur almost anywhere. Although usually lasting only seconds, they can result in extensive damage to buildings, gas and water pipes, power and communication lines, and roadways. Besides these immediate threats, earthquakes can also serve as triggers for several other natural hazards. In fact, the primary cause of damage in modern earthquakes in developed countries is fire, which is usually started by broken gas pipes and power lines. Slope failures, such as landslides, can easily be triggered by earthquakes. Among the slope failures caused by earthquakes, liquefaction is perhaps the least expected and most costly, especially in coastal areas. Here, normally stable soill becomes fluidized and can cause the collapse of structures over a large area. When earthquakes occur in an ocean or large lake, a tsunami may form and flood surrounding coastlines. Volcanic activity frequently accompanies earthquakes, espeically near the boundaries of the plates.


WHERE do earthquakes occur?

Wherever the plates that make up the Earth's crust subduct (one plate slides under another plate), crumple, or grind past each other, earthquakes occur. Along the western edge of North America, oceanic crust is sliding past and diving under continental crust, resulting in vast amounts of stress building up and being released in the form of earthquakes. In the last century, over 4600 earthquakes have been recorded along the western coast of North America. The western states, including California, Oregon, Washington, British Columbia, Yukon Territory, and Alaska are all prone to earthquakes. Baja California and western Mexico also have many earthquakes. Active faults are also found in the center of the continent, far from plate boundaries. The New Madrid fault in Missouri has produced several major earthquakes in recent history. Even New York City has several major fault lines underneath its skyscrapers. The east coast of Canada has many fault lines capable of producing moderate and high magnitude earthquakes.

Mountains are built by tectonic and volcanic activity. Everywhere in the world where these activities happen, earthquakes occur with them. Besides North America, two large regions in particular are at high risk:

  • The Pacific Rim (which outlines the Pacific Ocean) records more earthquakes than anywhere else on Earth. In the east, this includes the countries of Japan, the Philippines, Indonesia, New Guinea and New Zealand. In South America, all west coast countries (Chile, Peru, Ecuador, Columbia) are at risk.

  • The Mediterranean and Middle East regions including the countries of Italy, Greece, Turkey, Iran, and Pakistan have been plagued by substantial numbers of earthquakes.


WHEN do earthquakes occur?

In an average year, approximately 12 million earthquakes occur around the world. Fortunately, only about 100 of these have a signficant impact on human activities. Earthquakes have no seasonal time of arrival. Because they are geologic events, they can occur any time of the day or night, and in any season of the year.


HOW do we cope with earthquakes?

Earthquakes are very difficult to predict and are unpreventable. Therefore, mitigation of earthquakes requires designing structures (especially homes, office buildings, and roadways) that can withstand repeated shaking. However, since earthquakes usually cause other natural hazards, shaking isn't the only event for which a structure needs to be protected against. Toppling (resulting from liquefaction) and fire can cause at least as much destruction as shaking. Extinguishing fires in a city when water supply lines have been broken by the earthquake can become extremely difficult. Communities also need to be prepared for other hazards that can be caused by earthquakes, such as slope failures and flooding. Earthquake survival education is extremely important for understanding not only what to do when an earthquake is occurring, but what to do after an earthquake. The often unexpected consequences of an earthquake, such as water shortages, loss of electricity, and communication problems, can add considerable distress to already suffering communities.


WHO has more information on earthquakes?