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Intraplate Earthquake

Intraplate earthquake

Although the theory of plate tectonics well describes the mechanisms for interplate earthquakes (earthquakes at plate boundaries), very large intraplate earthquakes (earthquake within plates) can inflict heavy damage on towns and cities. With plate tectonics the world is modeled as a collection of 'dinner plates' sliding past each other on the giant table of the earth, these are, in fact, cracked dinner plates, under high stress. Nearly all the relative motion takes place at the edges of the plates, but there are still the 'creaks and groans of an ancient crust'. At times, motions along these interior weak zones produce rather large earthquakes. A series of famous intraplate earthquakes occurred on the New Madrid fault zone in 1812 that were above magnitude 8 and were felt for hundreds of miles. A similar large earthquake devastated the region of Gujarat, India, in 2001, resulting in a large loss of life. Many cities in North America and elsewhere live with the seismic risk of a rare, large intraplate earthquake. Historic examples of this occurred in Boston in 1755 (the largest U.S. earthquake ever recorded east of the New Madrid fault zone; some estimates put its magnitude as high as 7.0), New York City in 1737 and 1884 (both quakes estimated at about 5.5 magnitude) and Charleston, SC earthquake in 1886 (estimated magnitude 6.5 to 7.3). The Charleston quake was particularly surprising because unlike Boston and New York the area had almost no history of even minor earthquakes (to put in perspective, in addition to the three northeastern U.S. events previously mentioned, a more moderate magnitude 4 earthquake was recorded just north of New York City in 1985). Nobody is exactly sure what causes these earthquakes. In many cases, the causative fault is deeply buried, and sometimes cannot even be found. Under these circumstances it is difficult to calculate the exact seismic hazard for a given city, especially if there was only one earthquake in historical times. An especially dangerous form of earthquake, which has been involved in many deaths is the blind thrust earthquake, although this is more associated with interplate earthquakes. Some progress is being made in understanding the fault mechanics driving these earthquakes. Scientists continue to search for the causes of these earthquakes, and especially for some indication of when they will strike next. The best success has come with detailed micro-seismic monitoring, involving dense arrays of seismometers. In this manner, very small earthquakes associated with a causative fault can be located with great accuracy, and in most cases these line up in patterns consistent with faulting.

Compare

- Interplate earthquake

External link

A very nice explanation of Intraplate earthquakes http://www2.bc.edu/~kafka/Why_Quakes/why_quakes.html Category:Seismology

Plate tectonics

Plate tectonics (from the Greek word for "one who constructs", τεκτων, tekton) is a theory of geology developed to explain the phenomenon of continental drift, and is currently the theory accepted by the vast majority of scientists working in this area. In the theory of plate tectonics the outermost part of the Earth's interior is made up of two layers, the outer lithosphere and the inner asthenosphere. The lithosphere essentially "floats" on the asthenosphere and is broken-up into ten major plates: African, Antarctic, Australian, Eurasian, North American, South American, Pacific, Cocos, Nazca, and the Indian plates. These plates (and the more numerous minor plates) move in relation to one another at one of three types of plate boundaries: convergent (two plates push against one another), divergent (two plates move away from each other), and transform (two plates slide past one another). Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries (most notably around the so-called "Pacific Ring of Fire"). Plate tectonic theory arose out of two separate geological observations: continental drift, noticed in the early 20th century, and seafloor spreading, noticed in the 1960s. The theory itself was developed during the late 1960s and has since almost universally been accepted by scientists and has revolutionized the Earth sciences (akin to the development of the periodic table for chemistry, the discovery of the genetic code for genetics, or evolution in biology). biology

Key principles

The division of the Earth's interior into lithospheric and asthenospheric components is based on their mechanical differences. The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker. This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which "float" on the fluid-like asthenosphere. The relative fluidity of the asthenosphere allows the tectonic plates to undergo motion in different directions. One plate meets another along a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features like mountains, volcanoes and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and famous. These boundaries are discussed in further detail below. Tectonic plates are comprised of two types of lithosphere: continental and oceanic lithospheres; for example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction is based on the density of constituent materials; oceanic lithospheres are denser than continental ones due to their greater mafic mineral content. As a result, the oceanic lithospheres generally lie below sea level (for example the entire Pacific Plate, which carries no continent), while the continental ones project above sea level (see isostasy for explanation of this principle, which is essentially a largescale version of Archimedes' Bath).

Types of plate boundaries

Archimedes There are three types of plate boundaries, characterised by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are: # Transform boundaries occur where plates slide, or perhaps more accurately grind, past each other along transform faults. The relative motion of the two plates is either sinistral or dextral. # Divergent boundaries occur where two plates slide apart from each other. # Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or an orogenic belt (if the two simply collide and compress). Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three boundary types.

Transform (conservative) boundaries

The left- or right-lateral motion of one plate against another along transformstrike slip faults can cause highly visible surface effects. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the slipping-point of rocks on either side of the transform-faults the accumulated potential energy is released as strain, or motion along the fault. The massive amounts of energy that are released are the cause of earthquakes, a common phenomenon along transform boundaries. A good example of this type of plate boundary is the San Andreas Fault complex, which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving north with respect to North America.

Divergent (constructive) boundaries

At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimetres per century. Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by [http://pubs.usgs.gov/publications/text/baseball.html linear features] perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (due to thermal contraction and subsidence). It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centres. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

Convergent (destructive) boundaries

The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water). As this water rises into the mantle of the overriding plate, it lowers its melting temperature, resulting in the formation of magma with large amounts of dissolved gases. This can erupt to the surface, forming long chains of volcanoes inland from the continental shelf and parallel to it. The continental spine of South America is dense with this type of volcano. In North America the Cascade mountain range, extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with hot magma. The entire Pacific ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire. Where two continental plates collide the plates either crumple and compress or one plate burrows under or (potentially) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margins of the Indian subcontinental plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalaya. When two oceanic plates converge they form an island arc as one oceanic plate is subducted below the other. A good example of this type of plate convergence would be Japan.

Sources of plate motion

As noted above, the plates are able to move because of the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the source of energy driving plate tectonics. Somehow, this energy must be converted into force in order for the plates to move. There are essentially two forces that could be driving plate motion: friction and gravity. These are further subdivided below.

Friction

;Mantle drag : Convection currents in the mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere. ;Trench suction : Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches.

Gravity

;Ridge-push : Plate motion is driven by the higher elevation of plates at mid-ocean ridges. Essentially stuff slides downhill. The higher elevation is caused by the relatively low density of hot material upwelling in the mantle. The real motion producing force is the upwelling and the energy source that runs it. This is a misnomer as nothing is pushing and tensional features are dominant along ridges. Also, it is difficult to explain continental break-up with this. ;Slab-pull : Plate motion is driven by the weight of cold, dense plates sinking into the mantle at trenches. There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by friction. Slab pull is widely believed to be the strongest force directly operating on plates. Recent models indicate that trench suction plays an important role as well. The over-all driving force and its energy source are still debatable subjects of on-going research.

Major plates

Convection The main plates are
- African Plate, covering Africa
- Antarctic Plate, covering Antarctica
- Australian Plate, covering Australia
- Eurasian Plate covering Eurasia
- North American Plate covering North America and north-east Siberia
- South American Plate covering South America
- Pacific Plate, covering the Pacific Ocean Notable minor plates include the Indian Plate and the Arabian Plate. The movement of plates has caused the formation and breakup of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Rodinia is thought to have formed about 1000 million years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea; Pangea eventually broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents). ;Related article
- List of tectonic plates

History and impact

Continental drift

Continental drift was one of many ideas about tectonics proposed in the late 19th and early 20th century. The theory has been superseded by and the concepts and data have been incorporated within plate tectonics. By 1915 Alfred Wegener was making serious arguments for the idea with the first edition of The Origin of Continents and Oceans. In that book he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener wasn't the first to note this (Francis Bacon, Benjamin Franklin and Snider-Pellegrini preceded him), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation. However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. In the early 1940s, Maurice Ewing seismically tested the Atlantic edge of the North American continental shelf, and found a granitic layer dropped down to the basaltic ocean floor. If the continent had been torn from Europe and was plowing through the ocean bottom, the edge of the continental shelf should have marked the end of granitic rocks. Later studies aboard the Atlantis found that ocean bottom was not smooth, which suggested it was much stronger than if continents could push it aside. Beginning in the 1950s, scientists, using magnetic instruments (magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt -- the iron-rich, volcanic rock making up the ocean floor-- contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time. As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping. When the rock strata of the tips of separate continents are very similar it suggests that these rocks were formed in the same way implying that they were joined initially. For instance, some parts of Scotland contain rocks very similar to those found in eastern North America. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology.

Floating continents

The prevailing concept was that there were static shells of strata under the continents. It was early observed that although granite existed on continents, seafloor seemed to be composed of denser basalt. It was apparent that a layer of basalt underlies continental rocks. However, based upon abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later during study of Himalayan gravitation, and seismic studies detected corresponding density variations. By the mid-1950s the question remained unresolved of whether mountain roots were clenched in surrounding basalt or were floating like an iceberg.

Plate tectonic theory

Significant progress was made in the 1960s, and was prompted by a number of discoveries, most notably the Mid-Atlantic ridge. The most notable was the 1962 publication of a paper by American geologist Harry Hess. Hess suggested that instead of continents moving through oceanic crust (as was suggested by continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate. In 1967, Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, in 1968, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions.

Explanation of magnetic striping

Xavier Le Pichon The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists began to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process, later called seafloor spreading, operating over many millions of years has built the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence: # at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest; # the youngest rocks at the ridge crest always have present-day (normal) polarity; # stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times. By explaining both the zebralike magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the Earth's magnetic field.

Subduction discovered

A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. This idea found great favor with some scientists who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "expanding Earth" hypothesis was unsatisfactory because its supporters could offer no convincing geologic mechanism to produce such a huge, sudden expansion. Most geologists believe that the Earth has changed little, if at all, in size since its formation 4.6 billion years ago, raising a key question: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth? This question particularly intrigued Harry Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the term seafloor spreading. Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spread away from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the oceanic trenches -- very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust was consumed in the trenches, new magma rose and erupted along the spreading ridges to form new crust. In effect, the ocean basins were perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.

Mapping with earthquakes

During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording instruments (seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40-60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide.

Geological paradigm shift

The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) can be compared to the Copernican revolution in astronomy (see Nicolaus Copernicus). Within a matter of only several years geophysics and geology in particular were revolutionized. The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?". Before acceptance of plate tectonics, geology in particular was trapped in a "pre-Copernican" box. However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable scientific journal was eagerly accepted within a few short years in the 1960s and 1970s. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy. One simply has to read the pre-plate descriptions of why the Alps or Himalaya exist to see the difference. In an attempt to answer "how" questions like "How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the Dolomites?", or "How did the convex and concave margins of the Alpine chain form?", any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics. With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift sea floor into thin atmospheres. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood. Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre-tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics. A great rift, similar to the Great Rift Valley in northeastern Africa, had split apart a single continent, eventually forming the Atlantic Ocean, and the forces were still at work in the Mid-Atlantic Ridge. We have inherited some of the old terminology, but the underlying concept is as radical and simple as "The Earth moves" was in astronomy.

Plate tectonics on Mars

As a result of 1999 observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft, it has been proposed that the mechanisms of plate tectonics may once have been active on the planet - see Geology of Mars.

References

- Earth System History, Steven M. Stanley, (W.H. Freeman and Company; 1999) pages 211-228 ISBN 0-7167-2882-6
- Geographica: The complete illustrated Atlas of the world, Editors of James Mills-Hicks (Barnes and Noble Books; New York; 2004) ISBN 0-7607-5974-X
- Plate Tectonics : An Insider's History of the Modern Theory of the Earth, Oreskes, Naomi ed., Westview Press, 2003, ISBN 0813341329
- Krakatoa: The Day the World Exploded: August 27, 1883, Simon Winchester, (HarperCollins; 2003) Part 2-4 ISBN 0-0662-1285-5

External links

- U.S. Geological Survey Web Page Links
- [http://pubs.usgs.gov/publications/text/dynamic.html This Dynamic Earth] provides an excellent overview of the subject.
- [http://pubs.usgs.gov/publications/text/understanding.html Understanding plate motions]
- [http://pubs.usgs.gov/publications/text/slabs.html plate map]
- [http://pubs.usgs.gov/publications/text/Vigil.html Artist's cross section illustrating the main types of plate boundaries]
- [http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/description_plate_tectonics.html "Ring of Fire", Plate Tectonics, Sea-Floor Spreading, Subduction Zones, "Hot Spots"]
- [http://pubs.usgs.gov/publications/text/Wilson.html J. Tuzo Wilson: Discovering transforms and hotspots]
- [http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_world.html Active volcanoes]
- [http://sepwww.stanford.edu/oldsep/joe/fault_images/BayAreaSanAndreasFault.html San Andreas fault information]
- [http://www-sst.unil.ch/research/plate_tecto/links.htm Academic research] sites.
- [http://www.ucmp.berkeley.edu/geology/tecall1_4.mov Interactive movie] showing 750 myr (million years) of global tectonic activity.
- [http://www.ucmp.berkeley.edu/geology/tectonics.html More movies] over smaller regions and smaller time scales.
- [http://www.scotese.com/ The Paleomap Project:] numerous maps and movies.
- [http://www.uky.edu/ArtsSciences/Geology/webdogs/plates/reconstructions.html Web Dogs] tectonic reconstructions and interactive movies.
- [http://my.execpc.com/~acmelasr/mountains/wghnf.html Exceptionally detailed tectonic history] of Wisconsin.
- [http://www.windows.ucar.edu/tour/link=/earth/interior/how_plates_move.html Illustration of ridge-push and slab-pull].
- [http://www2.nature.nps.gov/geology/usgsnps/pltec/scplseqai.html Maps of the Earth back to 620 million years ago]
- [http://www.pbs.org/wgbh/aso/tryit/tectonics/ See what happens when you move tectonic plates] - An interactive guide
- [http://www.tectonic-forces.org The Origin and the Mechanics of the Forces Responsible for Tectonic Plate Movements]
- [http://www.sciencenews.org/pages/sn_arc99/5_1_99/bob2.htm Evidence (but not proof) for tectonics on Mars]
- [https://www3.imperial.ac.uk/portal/page?_pageid=46,73409&_dad=portallive&_schema=PORTALLIVE Plate Tectonics on Venus]"The mapping and interpretion of the regional tectonic features of Venus over the past ten years or so has led to a qualitative picture of buoyant plate tectonics."
- [http://www.platetectonics.com/ The Story of Plate Tectonics]
- [http://www.djburnette.com/projects/climate.html Plate Tectonics and Climate]
- [http://www.agiweb.org/earthcomm/resources/platetectonics.html Plate Tectonics...and Your Community] Category:Geophysics
- Category:Evolution Category:Geology ko:판구조론 ms:Plat tektonik ja:プレートテクトニクス

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 (see plate tectonics). 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 (see New Madrid Seismic Zone). 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.

Characteristics

Large numbers of earthquakes occur on a daily basis on Earth, but the majority of them are detected only by seismometers and cause no damage ([http://neic.usgs.gov/neis/general/magnitude_intensity.html magnitude] 5). Most earthquakes occur in narrow regions around plate boundaries down to depths of a few tens of kilometres where the crust is rigid enough to support the elastic strain. Where the crust is thicker and colder they will occur at greater depths and the opposite in areas that are hot. At subduction zones where plates descend into the mantle earthquakes have been recorded to a depth of 600 km. Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or a release of hazardous materials. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most of the earthquakes shaking is the dominant and most widespread cause of damage. There are four types of seismic waves that are all generated simultaneously and can be felt on the ground. S-waves (secondary or shear waves) and the two types of surfaces waves (Love waves and Rayleigh waves) are responsible for the shaking hazard. Rayleigh waves Rayleigh waves 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. Earthquakes, especially those that occur beneath oceans or seas (also called seaquake) and have large vertical displacements, can give rise to tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake, or as a result of submarine landslips or "slides" indirectly triggered by it.

Earthquake Size

The first method of quantifying earthquakes was intensity scales. In the United States the Mercalli (or Modified Mercalli, MM) scale, is commonly used while Japan (shindo) and the EU (European Macroseismic Scale) each have their own scales. These assign a numeric value (different for each scale) to a location based on the size of the shaking experienced there. The values 6 (normally denoted ‘’VI’’) in the MM scale for example is: Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. The problem with these scales is the measurement is subjective, often based on the worst damage in an area and influenced by local effects like site conditions that make it a poor measure for the relative size of different events in different places. For some tasks related to engineering and local planning it is still useful for the very same reasons and thus still collected. If you feel an earthquake in the US you can report the effects to the USGS here: [http://pasadena.wr.usgs.gov/shake/ Did you feel it?] The first attempt to qualitatively define one value to describe the size of earthquakes was the magnitude scale (the name being taking from similar formed scales used on the brightness of stars). In the 1930s, a California seismologist named Charles F. Richter devised a simple numerical scale (which he called the magnitude) to describe the relative sizes of earthquakes in Southern California. This is known as the “Richter scale”, “Richter Magnitude” or “Local Magnitude” (M_L). It is obtained by measuring the maximum amplitude of a recording on a Wood-Anderson torsion seismometer (or one calibrated to it) at a distance of 600km from the earthquake. Other more recent Magnitude measurements include: body wave magnitude (m_b), surface wave magnitude (M_s) and duration magnitude (M_D). Each of these is scaled to gives values similar to the values given by the Richter scale. However as each is also based on the measurement of one part of the seismogram they do not measure the overall power of the source and can suffer from saturation at higher magnitude values (larger events fail to produce higher magnitude values).These scales are also empirical and as such there is no physical meaning to the values. They are still useful however as they can be rapidly calculated, there are catalogues of them dating back many years and are they are familiar to the public. Seismologists now favor a measure called the seismic moment, related to the concept of moment in physics, to measure the size of a seismic source. The seismic moment is calculated from seismograms but can also by obtained from geologic estimates of the size of the fault rupture and the displacement. The values of moments for different earthquakes ranges over several order of magnitude. As a result the moment magnitude (M_W) scale was introduced by Hiroo Kanamori, which is comparable to the other magnitude scales but will not saturate at higher values. seismogram on February 28 2001.]] 2001 of the shaking of the Nisqually earthquake on February 28 2001.]]

Causes

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. Another type of movement of the Earth is observed by terrestrial spectroscopy. These oscillations of the earth are either due to the deformation of the Earth by tide caused by the Moon or the Sun, or other phenomena.

Preparation for earthquakes

- Emergency preparedness
- Household seismic safety
- Seismic retrofit
- Earthquake prediction

Specific fault articles

- Alpine Fault
- Calaveras Fault
- Hayward Fault Zone
- North Anatolian Fault Zone
- New Madrid Fault Zone
- San Andreas Fault

Specific earthquake articles

- Shaanxi Earthquake (1556). Deadliest known earthquake in history, estimated to have killed 830,000 in China.
- Cascadia Earthquake (1700).
- Kamchatka earthquakes (1737 and 1952).
- Lisbon earthquake (1755).
- New Madrid Earthquake (1811).
- Fort Tejon Earthquake (1857).
- Charleston earthquake (1886). Largest earthquake in the Southeast and killed 100.
- San Francisco Earthquake (1906).
- Great Kantō earthquake (1923). On the Japanese island of Honshu, killing over 140,000 in Tokyo and environs.
- Kamchatka earthquakes (1952 and 1737).
- Great Chilean Earthquake (1960). Biggest earthquake ever recorded, 9.5 on Moment magnitude scale.
- Good Friday Earthquake (1964) Alaskan earthquake.
- Ancash earthquake (1970). Caused a landslide that buried the town of Yungay, Peru; killed over 40,000 people.
- Sylmar earthquake (1971). Caused great and unexpected destruction of freeway bridges and flyways in the San Fernando Valley, leading to the first major seismic retrofits of these types of structures, but not at a sufficient pace to avoid the next California freeway collapse in 1989.
- Tangshan earthquake (1976). The most destructive earthquake of modern times. The official death toll was 255,000, but many experts believe that two or three times that number died.
- Great Mexican Earthquake (1985). 8.1 on the Ritcher Scale, killed over 6,500 people (though it is believed as many as 30,000 may have died, due to missing people never reappearing.)
- Whittier Narrows earthquake (1987).
- Armenian earthquake (1988). Killed over 25,000.
- Loma Prieta earthquake (1989). Severely affecting Santa Cruz, San Francisco and Oakland in California. Revealed necessity of accelerated seismic retrofit of road and bridge structures.
- Northridge, California earthquake (1994). Damage showed seismic resistance deficiencies in modern low-rise apartment construction.
- Great Hanshin earthquake (1995). Killed over 6,400 people in and around Kobe, Japan.
- İzmit earthquake (1999) Killed over 17,000 in northwestern Turkey.
- Düzce earthquake (1999)
- Chi-Chi earthquake (1999).
- Nisqually Earthquake (2001).
- Gujarat Earthquake (2001).
- Dudley Earthquake (2002).
- Bam Earthquake (2003).
- Parkfield, California earthquake (2004). Not large (6.0), but the most anticipated and intensely instrumented earthquake ever recorded and likely to offer insights into predicting future earthquakes elsewhere on similar slip-strike fault structures.
- Chuetsu Earthquake (2004).
- Indian Ocean Earthquake (2004). One of the largest earthquakes ever recorded at 9.0. Epicenter off the coast of the Indonesian island Sumatra. Triggered a tsunami which caused nearly 300,000 deaths spanning several countries.
- Sumatran Earthquake (2005).
- Fukuoka earthquake (2005).
- Kashmir earthquake (2005). Killed over 79,000 people. Many more at risk from the Kashmiri winter.
- Lake Tanganyika earthquake (2005). See also List of earthquakes

External links

- [http://www.eqnet.org/ EQNET: Earthquake Information Network]
- [http://neic.usgs.gov/ The U.S. National Earthquake Information Center]
- [http://earthquake.usgs.gov/faq/ USGS Earthquake FAQs]
- [http://www.ssn.unam.mx/ Mexican Sismological Service] Reports earthquakes in Mexico. Updated regularly.
- [http://wapi.isu.edu/envgeo/EG5_earthqks/eg_mod5.htm Environmental Geology - GEOL 406/506 (Earthquakes)]
- [http://www.quakes.bgs.ac.uk/hazard/ems1.htm The European Macroseismic Scale]
- [http://simscience.org/crackling/Advanced/Earthquakes/GutenbergRichter.html Gutenberg-Richter] power law of earthquake frequency against magnitude
- [http://www.guardian.co.uk/flash/0,5860,1121610,00.html Interactive guide: Earthquakes] an educational presentation on why earthquakes happen by Guardian Unlimited
- [http://www.geowall.org Geowall]- an educational 3d presentation system for looking at and understanding earthquake data
- [http://www.sciencecourseware.com/VirtualEarthquake/ Virtual Earthquake] educational site explaining how epicenters are located and magnitude is determined
- [http://www.pbs.org/newshour/science/earthquake/ PBS NewsHour - Predicting Earthquakes]
- [http://www.lamit.ro/earthquake-early-warning-system.htm Earthquake Warning System] Personal Earthquake warning system. Highly advanced detector, featuring sos signals and carrying strip.
- [http://www.data.scec.org/ Southern California Earthquake Data Center]
- [http://www.emsc-csem.org/ European-Mediterranean Seismological Centre (EMSC)]
- [http://www.gfz-potsdam.de/geofon/seismon/globmon.html Global Seismic Monitor at GFZ Potsdam]
- [http://earthquake.usgs.gov/bytopic/eqmonitoring/history/part09.php USGS Earthquake Monitoring History]
- [http://tsunami.geo.ed.ac.uk/local-bin/quakes/mapscript/demo_run.pl Global Earthquake Report – chart updated with each new earthquake or aftershock]
- [http://hraun.vedur.is/ja/englishweb/index.html Earthquakes in Iceland during the last 48 hours], updated automatically once every 2 minutes.
- [http://www.data.scec.org/recenteqs/Quakes/quakes0.html Recent earthquakes in California and Nevada ]
- [http://neic.usgs.gov/neis/eqlists/10maps_world.html USGS – Largest earthquakes in the world since 1900]
- [http://www.armageddononline.org/earthquake.php The Destruction of Earthquakes - and a List of the Worst ever recorded]
- [http://www.losangelesearthquakes.com Los Angeles Earthquakes plotted on a Google map]
- [http://rev.seis.sc.edu Seismograms for recent earthquakes via REV, the Rapid Earthquake Viewer]
- [http://www.iris.edu Incorporated Research Institutions for Seismology (IRIS)], earthquake database and software
- [http://www.iris.edu/seismon/ IRIS Seismic Monitor], world map of recent earthquakes
- [http://www.iris.edu/seismo/ SeismoArchives], Seismogram Archives of Significant Earthquakes of the World Category:Seismology Category:Geological hazards ko:지진 ms:Gempa bumi ja:地震 simple:Earthquake th:แผ่นดินไหว

New Madrid fault zone

The New Madrid Seismic Zone, also known as the Reelfoot Rift or the New Madrid Fault Line, is a major seismic zone located in the Midwestern United States. Largely inactive during the 20th century, the New Madrid fault system was responsible for the 1812 New Madrid Earthquake and has the potential to produce damaging earthquakes in coming decades.

Geology

The New Madrid Seismic Zone is made up of reactivated faults that formed when North America began to split or rift apart during the breakup of the supercontinent Rodinia in the Neoproterozoic Era (about 750 million years ago). The rift failed, but remained as a scar or zone of weakness. During the Mesozoic Era (about 200 million years ago), as the Atlantic Ocean was opening in the east, rifting was once again re-activated and intrusive igneous rocks were emplaced. But again the rifting failed and the continent remained intact, although with a significant zone of weakness. This rift is known as the Reelfoot Rift and coincides with the northernmost portion of the Mississippi embayment. Most of the seismicity is located from 5 to 25 km beneath the Earth's surface. The 150-mile long fault system, which extends into five states, stretches southward from Cairo, Illinois, through Hayti-Caruthersville and New Madrid, Missouri, through Blytheville, to Marked Tree, Arkansas. It also covers a part of Tennessee, near Reelfoot Lake, extending southeast into Dyersburg. The red zones on the map above indicate the epicenter locations of hundreds of minor earthquakes recorded since the 1970s. Two trends are apparent. First is the general NE-SW trend paralleling the trend of the Reelfoot Rift. The second is the intense cross trend, NW-SE, that occurs just southwest of New Madrid. This second trend coincides with an intrusive igneous body which lies deeply buried beneath the sediments of the rift zone. Several other bodies of deeply buried intrusive rock are known to exist within the seismic zone. The depths of these igneous rock bodies closely corresponds to the depth of the seismic activity.

History

1970s The zone has seen four of the largest North American earthquakes in recorded history, with magnitude estimates greater than 7.0 on the Richter scale, all within a 3 month period. Many of the published accounts describe the cumulative effects of all the earthquakes, thus finding the individual effects of each quake can be difficult.
- First earthquake of December 16, 1811, 0815 UTC (2:15 a.m.); 7.7 magnitude; epicenter in northeast Arkansas; Mercalli XI. It caused only slight damage to man-made structures, mainly because of the sparse population in the epicentral area. However, landslides and geological changes occurred along the Mississippi River, and large localized waves occurred due to fissures opening and closing below the Earth's surface.
- Second earthquake of December 16, 1811, 1415 UTC (8:15 a.m.); 7.0 magnitude; epicenter in northeast Arkansas; Mercalli X-XI. This shock followed the first earthquake by six hours.
- Earthquake of January 23, 1812, 1500 UTC (9 a.m.); 7.6 magnitude; epicenter in Missouri Bootheel. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks.
- Earthquake of February 7, 1812 (the New Madrid Earthquake), 0945 UTC (4:45 a.m.); 7.9 magnitude; epicenter near New Madrid, Missouri. New Madrid was destroyed. At St. Louis, many houses were damaged severely and their chimneys were thrown down. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks. These catastrophic earthquakes occurred during a three-month period in December 1811 and early 1812. They caused permanent changes in the course of the Mississippi River, which flowed backwards temporarily, and were felt as far away as New York City and Boston, Massachusetts where churchbells rang. Large areas sank into the earth, fissures opened, lakes permanently drained, new lakes were formed, and forests were destroyed over an area of 150,000 acres (600 km²). Many houses at New Madrid were thrown down. "Houses, gardens, and fields were swallowed up" one source notes. But fatalities and damage were low, because the area was sparsely settled. Hundreds of aftershocks followed over a period of several years. All three major quakes are generally believed to have exceeded 8.0 on the Richter Scale, and some seismologists believe the largest was 9.0 or larger. From what is known about the present seismicity of the area, it can be inferred that their focal depths were probably between 5 and 20 kilometers. The fault plane -- or planes -- on which the Earth rupture occurred are inferred to have had a NNE - SSW strike direction, more or less parallel to the Mississippi River. Aftershocks strong enough to be felt occurred until the year 1817. The largest earthquakes to have occurred since then were on January 4, 1843 and October 31, 1895 with magnitude estimates of 6.0 and 6.2 respectively. In addition to these events, seven events of magnitude >= 5.0 have occurred in the area. From the early years of the Nineteenth Century until well after the American Civil War, the citizens of Union City, Tennessee would gather every February 7th for an all-night "vigil and fish fry" on the site currently occupied by the Cumberland Presbyterian Church, praying, singing and beseeching the Almighty to "spare the land over" for another year. Instruments were installed in and around this area in 1974 to closely monitor seismic activity. Since then, more than 4000 earthquakes have been located, most of which are too small to be felt. On average one earthquake per year will be large enough to be felt in the area.

More quakes predicted

1974 The potential for the recurrence of large earthquakes and their impact today on densely populated cities in and around the seismic zone has generated much research devoted to understanding earthquakes. Establishing the probability for an earthquake of a given magnitude is an inexact science. By studying evidence of past quakes and closely monitoring ground motion and current earthquake activity, scientists attempt to understand their causes, recurrence rates, ground motion and disaster mitigation. The probability of magnitude 6.0 or greater in the near future is considered significant; a 90% chance of such an earthquake by the year 2040 has been given. In the June 23, 2005 issue of the journal Nature, the odds of another 8.0 event within 50 years were estimated to be between 7 and 10 percent.[http://www.livescience.com/forcesofnature/050622_new_madrid.html] Because of the unconsolidated sediments which are a major part of the underlying geology of the Mississippi embayment as well as the river sediments along the Mississippi and Ohio River valleys to the north and east (note the red fingers extending up these valleys in the image above), large quakes here have the potential for more widespread damage than major quakes on the west coast.

References

- United States Geological Survey (October 15, 2003). [http://wwwneic.cr.usgs.gov/neis/eq_depot/usa/1811-1812.html "USGS Earthquake Hazards Program: 1811 - 1812 Earthquakes in the New Madrid Seismic Zone"]. Retrieved May 3, 2005.
- [http://wwwneic.cr.usgs.gov/neis/new_madrid/new_madrid.html USGS New Madrid]
- [http://quake.wr.usgs.gov/prepare/factsheets/NewMadrid/ U.S. Geologic Survey site discusses the New Madrid seismic zone.]
- [http://quake.wr.usgs.gov/prepare/factsheets/HiddenHazs/ Uncovering Hidden Hazards in the Mississippi Valley]
- [http://www.cusec.org Central US Earthquake Consortium]
- [http://www.ceri.memphis.edu/compendium/ U. Memphis, TN, Center for Earthquake Research and Information (CERI) comprehensive references for the 1811–1812 earthquakes]
- [http://hsv.com/genlintr/newmadrd/ Links to eyewitness descriptions of the three 1811-12 earthquakes, and results of the earthquakes as seen in photographs of 1904.]
- [http://rockhoundingar.com/geology/fault.html Your Fault, My Fault, and the New Madrid Fault] Category:Earthquakes Category:Faults Category:Geography of Arkansas Category:Geography of Missouri Category:Plate tectonics Category:Rift valleys Category:Seismic faults Category:Seismology

Seismic risk

Seismic risk takes the results of seismic hazard analysis, and calculates the 'follies of man'. Your safety depends on what you build. You can locate in a region of high seismic hazard, but still sleep soundly at night if you have built to sound engineering principles. On the other hand, you can be located in a low seismic hazard district, such as New York City, in an ancient, poorly maintained brick building, suffering settlement problems on a filled swamp. This is the epitome of high seismic risk! Seismic risk calculations are mainly used for insurance purposes. The computer programs take all the seismic hazard inputs, and combine them with the known susceptibilities of structures and facilities, such as electrical power switching stations. The result gives probabilities for economic damage or casualties. Seismic risk can be reduced by active social programs that improve emergency response, and improve basic infrastructure. You can improve your own situation through earthquake preparedness. Building codes are a classic way to improve seismic risk, but it takes a long time for any significant effect. A special subset is urban seismic risk which looks at the specific issues of cities. Category:Seismology

New Madrid fault zone

Geology

History

More quakes predicted

References

Charleston earthquake

The Charleston Earthquake of 1886 was the largest quake to hit the Southeastern United States. It occurred at 9:50 PM on August 31, 1886. The earthquake caused severe damage in Charleston, South Carolina, damaging 2,000 buildings and causing $6 million worth in damages, while in the whole city the buildings were only valued at approximately $24 million. Between 60 and 110 lives were lost. Major damage occurred as far away as Tybee Island, Georgia (over 60 miles away) and structural damage was reported several hundred miles from Charleston (including central Alabama, central Ohio, eastern Kentucky, southern Virginia, and western West Virginia). It was felt as far away as Boston to the North, Chicago and Milwaukee to the Northwest, as far West as New Orleans, as far South as Cuba, and as far East as Bermuda. Bermuda The earthquake is estimated to be between 6.6 to 7.3 on the Richter scale with an Modified Mercalli Intensity Scale of X. Sandblows were common throughout the affected area due to liquefaction of the soil. More than 300 aftershocks of the 1886 earthquake occurred within thirty-five years. Minor earthquake activity that still continues in the area today may be a continuation of aftershocks. Very little to no historical earthquake activity occurred in the Charleston area prior to the 1886 event, which is unusual for any seismic area. This may have contributed to the severity of the tremor. The 1886 earthquake is a heavily studied example of an intraplate earthquake. The earthquake is thought to have occurred on faults formed during a Jurassic-aged rifting process during the break-up of Pangea. Similar faults are found along the east coast of North America. Category:Earthquakes in the nineteenth century Category:South Carolina history Category:Charleston, South Carolina

References

- [http://scsn.seis.sc.edu/html/eqchas.html from SCU]
- [http://www.eas.slu.edu/Earthquake_Center/1886EQ/ photos of damage]
- [http://www.eas.slu.edu/Earthquake_Center/1886EQ/sc37.html isoseismal map]
- [http://neic.usgs.gov/neis/eq_depot/usa/1886_09_01.html USGS]

Blind thrust earthquake

A blind thrust earthquake is an earthquake along a thrust fault that has not been mapped by standard surface geological mapping, hence the designation "blind". Although such earthquakes are not amongst the most violent, they sometimes constitute the most deadly, as conditions combine to form a classic urban earthquake which greatly affects urban seismic risk. Blind thrust faults generally exist near plate margins (plate tectonics), in the broad disturbance zone. They form when a section of the earth's crust is under high compressive stresses, due to plate margin collision, or the general geometry of how the plates are sliding past each other.

Image:Blindthrust2.png
Blind Thrust Faulting

As shown in the diagram, a weak plate under compression generally forms thrusting sheets, or overlapping sliding sections. This can form a hill and valley landform, with the hills being the strong sections, and the valleys being the highly disturbed thrust faulted and folded sections. The valley rock is very weak and usually highly weathered, presenting deep, fertile soil. Naturally, this is the area that becomes populated. If the region is under active compression, these faults are constantly rupturing, but any given valley might only experience a large earthquake every few hundred years. Although usually only a Magnitude 6 or 7 earthquake, it is especially deadly because the seismic waves are highly directed, and the soft basin soil of the valley can amplify the ground motions by over a factor of 10. It is said that these types of 'urban earthquakes' contribute more to urban seismic risk than the 'big ones', of Magnitude 8 or more. All the more reason to better prepare for earthquakes wherever people live under this threat. Category:Seismology

Fault mechanics

Fault mechanics is a field of study that investigates the behavior of geologic faults. Behind every good earthquake is some weak rock. Whether the rock remains weak becomes an important point in determining the potential for bigger earthquakes. On a small scale, fractured rock behaves essentially the same throughout the world, in that the angle of friction is more or less uniform (see Fault friction). A small element of rock in a larger mass responds to stress changes in a well defined manner: if it is squeezed by differential stresses greater than its strength, it is capable of large deformations. A band of weak, fractured rock in a competent mass, can deform to resemble a classic geologic fault. Using seismometers and earthquake location, the requisite pattern of micro-earthquakes can be observed. earthquake location For earthquakes, it all starts with an embedded penny-shaped crack as first envisioned by Brune [http://www.garfield.library.upenn.edu/classics1987/A1987J040600001.pdf] . As illustrated, an earthquake zone may start as a single crack, and grow to form many individual cracks, and collections of cracks along a fault. The key to fault growth is the concept of a ‘following force’, as conveniently provided for interplate earthquakes, by the motion of tectonic plates. Under a following force, the seismic displacements eventually form a topographic feature, such as a mountain range. interplate earthquake Intraplate earthquakes do not have a following force, and are not associated with mountain building. Thus, there is the puzzling question of how long any interior active zone has to live. For, in a solid stressed plate, every seismic displacement acts to relieve (reduce) stress; the fault zone should come to equilibrium; and all seismic activity cease. You can see this type of arching ‘lockup’ in many natural processes [http://www.exploratorium.edu/ronh/adventure/arches.html]. In fact, the seismic zone (such as the New Madrid Fault Zone) is ensured eternal life, by the action of water. As shown, if we add the equivalent of a giant funnel to the crack, it becomes the beneficiary of stress corrosion (the progressive weakening of the crack edge by water) [http://www.nire.go.jp/annual/1998/28.htm] . If there is a continuing supply of new water, the system does not come to equilibrium, but continues to grow, ever relieving stress from a larger and larger volume. New Madrid Fault Zone Thus, the prerequisite for a continuing seismically active interior zone is the presence of water, the ability of the water to get down to the fault source (high permeability), and the usual high horizontal interior stresses of the rock mass. All small earthquake zones eventually want to grow up to be like New Madrid or Charlevoix .[http://www.seismo.nrcan.gc.ca/historic_eq/charpage_e.php].

References

# James N. Brune, Tectonic stress and the spectra of seismic shear waves from earthquakes, J. Geophys. Res. 75:4997-5009, 1970., review written in 1987. http://www.garfield.library.upenn.edu/classics1987/A1987J040600001.pdf retrieved Aug. 01, 2005 # Arches National Park, http://www.exploratorium.edu/ronh/adventure/arches.html retrieved Aug. 01,2005 # Stress Corrosion Cracking of Rock in a Chemical Environment, http://www.nire.go.jp/annual/1998/28.htm retrieved Dec. 9, 2005 # Maurice Lamontagne, last modified 2003-12-22, The Charlevoix-Kamouraska
- Seismic Zone, Canada - Natural Resources, http://www.seismo.nrcan.gc.ca/historic_eq/charpage_e.php Retrieved Aug. 01,2005 Category: Seismology Category:Structural geology

Interplate earthquake

An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates. If one plate is trying to move past the other, they will be locked until sufficient stress builds up to cause the plates to slip relative to each other. The slipping process creates an earthquake with land deformations and resulting seismic waves which travel through the Earth and along the Earth's surface. Relative plate motion can be lateral as along a transform fault boundary or vertical if along a convergent subduction boundary or a rift at a divergent boundary. At a subduction boundary the motion is due to one plate slipping beneath the other plate resulting in an interplate thrust or megathrust earthquake. Some areas of the world that are particularly prone to such events include the west coast of North America (especially California and Alaska), the northeastern Mediterranean region (Greece, Italy, and Turkey in particular), Iran, New Zealand, Indonesia, Japan, and parts of China. The frequency (how often) of these large events may be controlled by the Gap Theory and Characteristic earthquakes.

Eutektisch

Eine Legierung oder Lösung wird eutektisch oder Eutektikum (griech.: ευ=gut τεκτειν=bauen; Plural: Eutektika) genannt, wenn ihre Bestandteile in einem solchen Verhältnis zueinander stehen, dass sie als Ganzes bei einer bestimmten Temperatur (Liquidustemperatur bei vollständiger Verflüssigung, auch: Schmelzpunkt und der Solidustemperatur bei vollständigem Erstarren) flüssig bzw. fest wird. Andere Mischungsverhältnisse weisen einen Schmelz- bzw. Erstarrungsbereich auf, in dem neben der Schmelze auch eine feste Phase vorliegt. Die Liquidustemperatur kennzeichnet die vollständige Verflüssigung, die Solidustemperatur das vollständige Erstarren der Legierung. Einige Weichlote haben eine zusammenfallende Liquidus- und Solidustemperatur, z.B. ISO BSn63Pb 178 (L-Sn63PbAg) bei 178 °C. Ein Eutektikum hat den niedrigsten Schmelzpunkt aller Mischungen aus denselben Bestandteilen. Beim Erstarren scheiden sich gleichzeitig alle Bestandteile in sehr feinen Kristallen ab, das Gefüge erscheint gleichmäßig und weist eine charakteristische lamellare Struktur auf. Beispiele für Eutektika sind das System Sn-Pb ("Lötzinn") z.B. mit einer Zusammensetzung von 62/38, das System Ag-Cu (Silber-Kupfer-Legierung) mit einer Zusammensetzung von 72/28, Roses Metall, das Woodsche Metall, bestimmte Quarzporphyre oder eine Lösung von 30,9 g Kochsalz auf 100 g Wasser. Verwendung finden diese gut schmelzenden Legierungen in Sprinkleranlagen, als Lötlegierungen oder in Scherzartikeln. Quecksilberfreie analoge Fieberthermometer enthalten ein Eutektikum, z.B. aus Gallium, Indium und Zinn. Diese Legierung ist ungifitg, umweltunbedenklich und kann in den Hausmüll gegeben werden. Ein eutektisches System bezeichnet ein Mehrstoffsystem, bei dem ein Eutektikum auftreten kann. Voraussetzung hierfür ist eine Schmelztemperatur des Mischsystems unterhalb der Schmelztemperaturen der Einzelkomponenten und eine Mischungslücke im Festen, die bis zum Erreichen der Schmelztemperatur des Eutektikums bestehen bleibt. Der eutektische Punkt bezeichnet im Phasendiagramm eines Mehrstoffsystems den Punkt, der durch das Konzentrationsverhältnis des Eutektikums und durch dessen Schmelztemperatur (eutektische Temperatur) gekennzeichnet ist. Siehe auch: Peritektisch, Monotektisch. Kategorie:Legierung

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Intraplate earthquake

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Plate tectonics

Key principles

Types of plate boundaries

Transform (conservative) boundaries

Divergent (constructive) boundaries

Convergent (destructive) boundaries

Sources of plate motion

Friction

Gravity

Major plates

History and impact

Continental drift

Floating continents

Plate tectonic theory

Explanation of magnetic striping

Subduction discovered

Mapping with earthquakes

Geological paradigm shift

Plate tectonics on Mars

See also

References

External links

Earthquake

Characteristics

Earthquake Size

Causes

Preparation for earthquakes

Specific fault articles

Specific earthquake articles

See also

External links

New Madrid fault zone

Geology

History

More quakes predicted

See also

References

Seismic risk

New Madrid fault zone

Geology

History

More quakes predicted

See also

References

Charleston earthquake

References

Blind thrust earthquake

Fault mechanics

References

Interplate earthquake

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Eutektisch