Acupuncture | Alphorn | Astronomy | Aurora | Bamboo | Blackjack | Cannabis | Cavalier | Cellulitis | Cocktails | Didgeridoo | Earthquakes | Forex | Genealogy | Guatemala | Holidays | Isotopes | Jamaica | Mezcal | Paragliding | Penicillin | Sphinx | SwissCheese | Swooping | TrafficExchanges | Ukulele | Wakeboarding
Earthquakes / Seismology
Seismology is the scientific study of earthquakes and the movement of waves through the Earth. The field also includes studies of variants such as seaquakes, causes such as volcanoes and tectonic plates. Earthquakes (and other earth movements) produce different types of seismic waves. These waves travel through rock, and provide an effective way to see events and structures deep in the Earth. One of the earliest important discoveries was that the outer core of the Earth is liquid. Pressure waves pass through the core. Transverse or shear waves that shake side-to-side require rigid material so they do not pass through the core.
The process of mapping subsurface features is a specialty called seismography. Seismic waves produced by explosions have been used to map salt domes and other oil-bearing rocks, faults (cracks in deep rock), rock types, and long-buried giant meteor craters. For example, the Chicxulub impactor, which is believed to have killed the dinosaurs, was localized to Central America by analyzing ejecta in the cretaceous boundary, and then physically proven to exist using seismic maps from oil exploration. Using seismic tomography with earthquake waves, the interior of the Earth has been completely mapped to a resolution of several hundred kilometers. This process has enabled scientists to identify convection cells, mantle plumes and other large features of the inner Earth. Seismographs also effectively discover unusual, otherwise unobserved phenomena such as large meteors striking uninhabited ocean, or underground nuclear tests. Ocean meteor strikes as large as ten kilotons of TNT, (equivalent to about 4.2 × 1013 J of effective explosive force) have been reported. One of the first attempts at the scientific study of earthquakes followed the 1755 Lisbon earthquake.
An earthquake is a sudden and sometimes catastrophic movement of a part of the Earth's surface. Earthquakes result from the dynamic release of elastic strain energy that radiates seismic waves. Earthquakes typically result from the movement of faults, planar zones of deformation within the Earth's upper crust. The word earthquake is also widely used to indicate the source region itself. The Earth's lithosphere is a patch work of plates in slow but constant motion. Earthquakes occur where the stress resulting from the differential motion of these plates exceeds the strength of the crust. The highest stress (and possible weakest zones) are most often found at the boundaries of the tectonic plates and hence these locations are where the majority of earthquakes occur. Events located at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes. Earthquakes also occur in volcanic regions and as the result of a number of anthropogenic sources, such as reservoir induced seismicity, mining and the removal or injection of fluids into the crust. Seismic waves including some strong enough to be felt by humans can also be caused by explosions (chemical or nuclear), landslides, and collapse of old mine shafts, though these sources are not strictly earthquakes.
Most earthquakes are powered by the release of the elastic strain that accumulate over time, typically, at the boundaries of the plates that make up the Earth's lithosphere via a process called Elastic-rebound theory. The Earth is made up of tectonic plates driven by the heat in the Earth's core. these plates collide against each other all the time but sometimes the gaps between them are stressed. Eventually, the plates make way and all that energy is sent out in the form of seismic waves. Deep focus earthquakes, at depths of 100's km, are possibly generated as subducted lithospheric material catastrophically undergoes a phase transition since at the pressures and temperatures present at such depth elastic strain cannot be supported.
Some earthquakes are also caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. 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. Finally, ground shaking can also result from the detonation of explosives. Thus scientists have been able to monitor, using the tools of seismology, nuclear weapons tests performed by governments that were not disclosing information about these tests along normal channels. Earthquakes such as these, that are caused by human activity, are referred to by the term induced seismicity.
Most large earthquakes are accompanied by other, smaller ones, that can occur either before or after the principal quake — these are known as foreshocks or aftershocks, respectively. While almost all earthquakes have aftershocks, foreshocks are far less common occurring in only about 10% of events. The power of an earthquake is distributed over a significant area, but in the case of large earthquakes, it can spread over the entire planet. Ground motions caused by very distant earthquakes are called teleseisms. The Rayleigh waves from the Sumatra-Andaman Earthquake of 2004 caused ground motion of over 1 cm even at the seismometers that were located the greatest distance from it. Using such ground motion records from around the world it is possible to identify a point from which the earthquake's seismic waves appear to originate. That point is called its "focus" or "hypocenter" and usually proves to be the point at which the fault slip was initiated. The location on the surface directly above the hypocenter is known as the "epicenter". The total size of the fault that slips, the rupture zone, can be as large as 1000 km, for the biggest earthquakes. Just as a large loudspeaker can produce a greater volume of sound than a smaller one, large faults are capable of higher magnitude earthquakes than smaller faults are.
External Links
EarthQuake Information Network
The Earthquake Information Network (EQNET) Web site is sponsored by a consortium of state and national organizations in the United States that disseminate earthquake related information. The purpose of EQNET is to provide links to authoritative Internet resources about earthquakes and the issues surrounding them. The Earthquake Information Providers Group (EqIP) is responsible for overseeing the administration of EQNET, which is funded primarily by the Federal Emergency Management Agency (FEMA).
National Earthquake Information Center
The mission of the National Earthquake Information Center (NEIC) is to 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. As World Data Center for Seismology, Denver, the NEIC compiles and maintains an extensive, global seismic database on earthquake parameters and their effects that serves as a solid foundation for basic and applied earth science research.
The GeoWall Consortium
A combination of new projection technology, fast graphics cards and inexpensive computers have made it possible to provide a stereo projection system that is much more affordable than previous commercial solutions. The GEOWALL project makes use of these projection systems to visualize structure and dynamics of the Earth in stereo to aid the understanding of spatial relationships.