Earthquakes

A severe earthquake is one of the most frightening phenomena of nature. Earthquakes are the result of sudden movements of the Earth, caused by the release of strain that has accumulated over a long time. If the earthquake occurs in a populated area, it may cause deaths, injuries and extensive property damage.

The Earth is formed of several layers that have different physical and chemical properties. The outer layer consists of several large, irregularly shaped plates that slide over, under and past each other on top of the partly molten inner layer. Sometimes the movement of the plates is gradual. If the plates are locked together, the energy accumulates until it grows strong enough and the plates break free.

An apartment building is torn apart by an earthquake. Learn more...

Ninety percent of all earthquakes are found at crustal plate boundaries such as the Pacific Plate. Earthquakes can also occur within plates, such as the New Madrid earthquakes of 1811–1812 and the 1886 Charleston earthquake which occurred within the North American plate.

The vibrations produced by earthquakes are detected, recorded, and measured by instruments called seismographs. Seismographs record the motion of the ground during an earthquake. They are installed in the ground throughout the world and operate as seismographic networks. The first seismograph was developed in 1890.

There are many different ways to measure earthquakes. Magnitude is the most common measure of an earthquake's size. It is a measure of the size of the earthquake source. Intensity is a measure of the shaking and damage caused by the earthquake.

Earthquakes beneath the ocean floor sometimes generate tsunamis. Tsunami waves can travel across the ocean and cause death and destruction among coastal communities. Since NOAA has the primary responsibility for providing tsunami warnings to the Nation, NOAA also collects information on significant earthquakes, and those that generate tsunamis. For more information about significant earthquakes, visit NOAA's Earthquake Website.