Earthquake

An earthquake is a trembling or a shaking movement of the Earth's surface. Earthquakes typically result from the movement of faults, quasi-planar zones of deformation within its uppermost layers. The word earthquake is also widely used to indicate the source region itself. The solid earth is in slow but constant motion (see plate tectonics) and earthquakes occur where the resulting stress exceeds the capacity of Earth materials to support it. This condition is most often found at (and the resulting frequent occurrence of earthquakes is used to define) the boundaries of the tectonic plates into which the Earth's lithosphere is divided. Events that occur at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes.

Types of Earthquakes

Naturally Occurring Earthquakes

Most naturally occurring earthquakes are related to the tectonic nature of the Earth. Such earthquakes are called tectonic earthquakes. The Earth's lithosphere is a patch work of plates (see plate tectonics) in slow but constant motion caused by the heat in the Earth's mantle and core. Plate boundaries glide past each other, creating frictional stress. When the frictional stress exceeds a critical value, called local strength, a sudden failure occurs. The boundary of tectonic plates along which failure occurs is called the fault plane. When the failure at the fault plane results in a violent displacement of the Earth's crust, the elastic strain energy is released and elastic waves are radiated, thus causing an earthquake. It is estimated that only 10 percent or less of an earthquake's total energy is ultimately radiated as seismic energy, while most of the earthquake's energy is used to power the earthquake fracture growth and is eventually converted into heat. Therefore, earthquakes lower the Earth's available potential energy and thermal energy, though these losses are negligible. To describe the physical process of occurrence of an earthquake, seismologists use the Elastic-rebound theory.

The majority of tectonic earthquakes originate at depths not exceeding a few tens of kilometers. Earthquakes occurring at boundaries of tectonic plates are called interplate earthquakes, while the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes.

Where the crust is thicker and colder, earthquakes occur at greater depths of hundreds of kilometers along subduction zones where plates descend into the Earth's mantle. These types of earthquakes are called deep focus earthquakes. They are possibly generated when subducted lithospheric material catastrophically undergoes a phase transition (e.g., olivine to spinel), releasing stored energy such as elastic strain, chemical energy or gravitational energy that cannot be supported at the pressures and temperatures present at such depths.

Earthquakes may also occur in volcanic regions and are caused by the movement of magma in volcanoes. Such quakes can be an early warning of volcanic eruptions.

A recently proposed theory suggests that some earthquakes may occur in a sort of earthquake storm, where one earthquake will trigger a series of earthquakes each triggered by the previous shifts on the fault lines, similar to aftershocks, but occurring years later, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the Anatolian Fault in Turkey in the 20th Century, the half dozen large earthquakes in New Madrid in 1811-1812, and has been inferred for older anomalous clusters of large earthquakes in the Middle East and in the Mojave Desert.

Induced Earthquakes

Some earthquakes have anthropogenic sources, such as extraction of minerals and fossil fuel from the Earth's crust, the removal or injection of fluids into the crust, reservoir-induced seismicity, massive explosions, and collapse of large buildings. Seismic events caused by human activity are referred to by the term induced seismicity. They however are not strictly earthquakes and usually show a different seismogram than earthquakes that occur naturally.

A rare few earthquakes have been associated with the build-up of large masses of water behind dams, such as the Kariba Dam in Zambia, Africa, and with the injection or extraction of fluids into the Earth's crust (e.g. at certain geothermal power plants and at the Rocky Mountain Arsenal). Such earthquakes occur because the strength of the Earth's crust can be modified by fluid pressure. Earthquakes have also been known to be caused by the removal of natural gas from subsurface deposits, for instance in the northern Netherlands. The world’s largest reservoir-induced earthquake occurred on December 10, 1967 in the Koyna region of western Maharashtra in India. It had a mgnitude of 6.3 on the Richter scale. However, the U.S. geological survey reported the magnitude of 6.8.

The detonation of powerful explosives, such as nuclear explosions, can cause low-magnitude ground shaking. Thus, the 50-megaton nuclear bomb code-named Ivan detonated by the Soviet Union in 1961 created a seismic event comparable to a magnitude 7 earthquake, producing the seismic shock so powerful that it was measurable even on its third passage around the Earth. In an effort to promote nuclear non-proliferation, the International Atomic Energy Agency uses the tools of seismology to detect illicit activities such as nuclear weapons tests. The nuclear nations routinely monitor each others activities through networks of interconnected seismometers, which allow to precisely locate the source of an explosion.

Earthquake Resources Links

About Earthquakes
List of frequently asked questions by school kids and the general public.

National Earthquake Information Center
Rapidly determine location and size of all destructive earthquakes worldwide and to immediately disseminate this information to concerned national and international agencies, scientists, and the general public.

Earthquakes by Kaye M. Shedlock & Louis C. Pakiser
One of the most frightening and destructive phenomena of nature is a severe earthquake and its terrible aftereffects. An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has accumulated over a long time...

How to Survive Earthquakes and Tsunamis on the North Coast
Preparing for earthquakes makes good sense. The responsibility and the choice are ours.

Earthquake Effects
The effects of any earthquake depend on a number of videly varying factors. These factors are all of: intrinsic to the earthquake, geologic conditions where effects are felt, societal conditions reacting to the earthquake...

World-Wide Earthquake Locator
Aims to provide up-to-date information and detailed dynamic maps of earthquakes across the world within a maximum of 24 hours of their occurence. This web site also includes a database of past earthquakes, together with statistical earthquake prediction.

Hotspots [This Dynamic Earth, USGS]
The vast majority of earthquakes and volcanic eruptions occur near plate boundaries, but there are some exceptions. For example, the Hawaiian Islands, which are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more than...

Pacific Northwest Seismograph Network
All about earthquakes and geologic hazards of the Pacific Northwest.

New Mexico Earthquake Education and Resources
Our terminally cool and uncommonly friendly experts have sorted through virtual truckloads of information to assemble some earth-shattering resources about earthquakes, particularly those in New Mexico.

Northern California Earthquake Data Center
The NCEDC is a long-term archive and distribution center for seismological and geodetic data for Northern and Central California.

Induced Earthquake Bibliography
This bibliography contains references to publications concerning earthquakes and other seismicity induced by human activity.

Earthquakes & Historical Facts
Are there more earthquakes this century? Is this a sign that Jesus is to return?

Enviromental Geology: Earthquakes
are probably the most frightening naturally occurring hazard encountered. Why? Earthquakes typically occur with little warning. There is no escape from an earthquake! Earthquakes have devastating effects, resulting in hundreds to thousands of deaths and injuries, and millions to billions of dollars worth of property damage.

Southern California Earthquake Data Center
To maintain an easily-accessible, well-organized, high-quality, searchable archive of earthquake data for research in seismology and earthquake engineering.

Prediction of Earthquakes with AE/MS? Why Not
Earthquakes account for more loss of life and property than any other natural phenomena. In spite of this fact, and the fact that we know why and how earthquakes occur, there is a great deal of pessimism from both the scientific community and Government agencies concerning one's ability to accurately predict earthquakes.

Canadian Earthquake Engineering Research Database
The primary objective of CAEE/ACGP is to foster earthquake engineering practiceStanding committee on Design plays a key role in this regard. Information about the Standing committee on design can be found at the main CAEE website. and researchStanding committee on Reseach is the key player in this regard in Canada.

Earthquake Advisor
Earthquake information, photos, stories, safety and construction how-to's.

The BGS Earthquakes Page
The activities of the British Geological Survey's (BGS) Earthquake and Forensic Seismology and Geomagnetism Programme cover a broad spectrum of research and information services concerned with earthquakes and man-made seismic disturbances.

National Information Service for Earthquake Engineering, University of California
Including an open access archive, slides, images and photographs, selected full-text papers, related websites, earthquake engineering software archive and data.

The Earthquake Engineering Research Institute
A national, nonprofit, technical society of engineers, geoscientists, architects, planners, public officials, and social scientists. EERI members include researchers, practicing professionals, educators, government officials, and building code regulators.

The Southern California Earthquake Center (SCEC)
To gather new information about earthquakes in Southern California, integrate this information into a comprehensive and predictive understanding of earthquake phenomena, and communicate this understanding to end-users and the general public in order to increase earthquake awareness, reduce economic losses, and save lives.