125-421: A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea . Much of these shelves were exposed by drops in sea level during glacial periods . The shelf surrounding an island is known as an insular shelf . The continental margin , between the continental shelf and the abyssal plain , comprises a steep continental slope, surrounded by
250-489: A quake , tremor , or temblor – is the shaking of the Earth 's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves . Earthquakes can range in intensity , from those so weak they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities. The seismic activity of an area
375-729: A tsunami . Earthquakes can trigger landslides . Earthquakes' occurrence is influenced by tectonic movements along faults, including normal, reverse (thrust), and strike-slip faults, with energy release and rupture dynamics governed by the elastic-rebound theory . Efforts to manage earthquake risks involve prediction, forecasting, and preparedness, including seismic retrofitting and earthquake engineering to design structures that withstand shaking. The cultural impact of earthquakes spans myths, religious beliefs, and modern media, reflecting their profound influence on human societies. Similar seismic phenomena, known as marsquakes and moonquakes , have been observed on other celestial bodies, indicating
500-574: A continent as "a large space of dry land comprehending many countries all joined together, without any separation by water. Thus Europe, Asia, and Africa is one great continent, as America is another." However, the old idea of Europe, Asia and Africa as "parts" of the world ultimately persisted with these being regarded as separate continents. By convention, continents "are understood to be large, continuous, discrete masses of land, ideally separated by expanses of water". In modern schemes with five or more recognized continents, at least one pair of continents
625-520: A continent. This divides the entire land surface of Earth into continents, regions, or quasi-continents. The criterion that each continent is a discrete landmass is commonly relaxed due to historical conventions and practical use. Of the seven most globally recognized continents, only Antarctica and Australia are completely separated from other continents by the ocean. Several continents are defined not as absolutely distinct bodies but as " more or less discrete masses of land". Africa and Asia are joined by
750-455: A continental shelf differs significantly from the geological definition. UNCLOS states that the shelf extends to the limit of the continental margin , but no less than 200 nmi (370 km; 230 mi) and no more than 350 nmi (650 km; 400 mi) from the baseline . Thus inhabited volcanic islands such as the Canaries , which have no actual continental shelf, nonetheless have
875-571: A depth of less than 70 km (43 mi) are classified as "shallow-focus" earthquakes, while those with a focal depth between 70 and 300 km (43 and 186 mi) are commonly termed "mid-focus" or "intermediate-depth" earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 to 700 km (190 to 430 mi)). These seismically active areas of subduction are known as Wadati–Benioff zones . Deep-focus earthquakes occur at
1000-488: A depth where the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure. Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes . Such earthquakes can serve as an early warning of volcanic eruptions, as during
1125-615: A discrete landmass is completely disregarded if the continuous landmass of Eurasia is classified as two separate continents (Asia and Europe). Physiographically , Europe and the Indian subcontinent are large peninsulas of the Eurasian landmass. However, Europe is considered a continent with its comparatively large land area of 10,180,000 square kilometres (3,930,000 sq mi), while the Indian subcontinent, with less than half that area,
1250-475: A few exceptions to this: Supershear earthquake ruptures are known to have propagated at speeds greater than the S wave velocity. These have so far all been observed during large strike-slip events. The unusually wide zone of damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic boom developed in such earthquakes. Slow earthquake ruptures travel at unusually low velocities. A particularly dangerous form of slow earthquake
1375-461: A few exceptions, the shelf break is located at a remarkably uniform depth of roughly 140 m (460 ft); this is likely a hallmark of past ice ages, when sea level was lower than it is now. The continental slope is much steeper than the shelf; the average angle is 3°, but it can be as low as 1° or as high as 10°. The slope is often cut with submarine canyons . The physical mechanisms involved in forming these canyons were not well understood until
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#17327651778591500-425: A geological continent has been disputed by some geologists. However, a study conducted in 2021 found that part of the submerged continent is indeed Precambrian , twice as old as geologists had previously thought, which is further evidence that supports the idea of Zealandia being a geological continent. All seven geological continents are spatially isolated by geologic features. The term "continent" translates
1625-998: A legal continental shelf, whereas uninhabitable islands have no shelf. Continent A continent is any of several large geographical regions. Continents are generally identified by convention rather than any strict criteria. A continent could be a single landmass or a part of a very large landmass, as in the case of Asia or Europe. Due to this, the number of continents varies; up to seven or as few as four geographical regions are commonly regarded as continents. Most English-speaking countries recognize seven regions as continents. In order from largest to smallest in area, these seven regions are Asia , Africa , North America , South America , Antarctica , Europe , and Australia . Different variations with fewer continents merge some of these regions; examples of this are merging North America and South America into America , Asia and Europe into Eurasia , and Africa, Asia, and Europe into Afro-Eurasia . Oceanic islands are occasionally grouped with
1750-571: A lesser extent the wind. Indian Ocean shelf seas are dominated by major river systems, including the Ganges and Indus rivers. The shelf seas around New Zealand are complicated because the submerged continent of Zealandia creates wide plateaus. Shelf seas around Antarctica and the shores of the Arctic Ocean are influenced by sea ice production and polynya . There is evidence that changing wind, rainfall, and regional ocean currents in
1875-550: A nearby continent to divide all the world's land into geographical regions. Under this scheme, most of the island countries and territories in the Pacific Ocean are grouped together with the continent of Australia to form the geographical region Oceania . In geology , a continent is defined as "one of Earth's major landmasses, including both dry land and continental shelves ". The geological continents correspond to seven large areas of continental crust that are found on
2000-468: A part of South America. Apart from the current continents, the scope and meaning of the term continent includes past geological ones. Supercontinents , largely in evidence earlier in the geological record, are landmasses that comprise most of the world's cratons or continental cores. These have included Vaalbara , Kenorland , Columbia , Rodinia , Pannotia , and Pangaea . Over time, these supercontinents broke apart into large landmasses which formed
2125-612: A particular location in the Earth is the average rate of seismic energy release per unit volume. One of the most devastating earthquakes in recorded history was the 1556 Shaanxi earthquake , which occurred on 23 January 1556 in Shaanxi , China. More than 830,000 people died. Most houses in the area were yaodongs —dwellings carved out of loess hillsides—and many victims were killed when these structures collapsed. The 1976 Tangshan earthquake , which killed between 240,000 and 655,000 people,
2250-523: A separate land. In 1507 Martin Waldseemüller published a world map, Universalis Cosmographia , which was the first to show North and South America as separate from Asia and surrounded by water. A small inset map above the main map explicitly showed for the first time the Americas being east of Asia and separated from Asia by an ocean, as opposed to just placing the Americas on the left end of
2375-765: A single rupture) are approximately 1,000 km (620 mi). Examples are the earthquakes in Alaska (1957) , Chile (1960) , and Sumatra (2004) , all in subduction zones. The longest earthquake ruptures on strike-slip faults, like the San Andreas Fault ( 1857 , 1906 ), the North Anatolian Fault in Turkey ( 1939 ), and the Denali Fault in Alaska ( 2002 ), are about half to one third as long as
2500-821: A third part of the world. Division into three parts eventually came to predominate. From the Greek viewpoint, the Aegean Sea was the center of the world; Asia lay to the east, Europe to the north and west, and Africa to the south. The boundaries between the continents were not fixed. Early on, the Europe–Asia boundary was taken to run from the Black Sea along the Rioni River (known then as the Phasis ) in Georgia . Later it
2625-502: A very large peninsula on the fringe of a continent. Where the Americas are viewed as a single continent (America), it is divided into two subcontinents ( North America and South America ) or three ( Central America being the third). When Eurasia is regarded as a single continent, Asia and Europe are treated as subcontinents. Some areas of continental crust are largely covered by the ocean and may be considered submerged continents . Notable examples are Zealandia , emerging from
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#17327651778592750-428: A warming ocean are having an effect on some shelf seas. Improved data collection via Integrated Ocean Observing Systems in shelf sea regions is making identification of these changes possible. Continental shelves teem with life because of the sunlight available in shallow waters, in contrast to the biotic desert of the oceans' abyssal plain . The pelagic (water column) environment of the continental shelf constitutes
2875-525: Is a theory that earthquakes can recur in a regular pattern. Earthquake clustering has been observed, for example, in Parkfield, California where a long-term research study is being conducted around the Parkfield earthquake cluster. An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. Rapid changes of stress between rocks, and the stress from the original earthquake are
3000-410: Is called the hypocenter or focus, while the ground level directly above it is the epicenter . Earthquakes are primarily caused by geological faults , but also by volcanic activity , landslides, and other seismic events. The frequency, type, and size of earthquakes in an area define its seismic activity, reflecting the average rate of seismic energy release. Significant historical earthquakes include
3125-451: Is considered a subcontinent. The alternative view—in geology and geography—that Eurasia is a single continent results in a six-continent view of the world. Some view the separation of Eurasia into Asia and Europe as a residue of Eurocentrism : "In physical, cultural and historical diversity, China and India are comparable to the entire European landmass, not to a single European country. [...]." However, for historical and cultural reasons,
3250-473: Is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior. There are three main types of fault, all of which may cause an interplate earthquake : normal, reverse (thrust), and strike-slip. Normal and reverse faulting are examples of dip-slip, where
3375-400: Is divided into 754 Flinn–Engdahl regions (F-E regions), which are based on political and geographical boundaries as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones belong to larger F-E regions. Standard reporting of earthquakes includes its magnitude , date and time of occurrence, geographic coordinates of its epicenter , depth of
3500-435: Is divided into several principal oceanic components by the continents and various geographic criteria. The geological definition of a continent has four criteria: high elevation relative to the ocean floor; a wide range of igneous, metamorphic and sedimentary rocks rich in silica; a crust thicker than the surrounding oceanic crust; and well-defined limits around a large enough area. The most restricted meaning of continent
3625-507: Is inhabited by more numerous people and animals than our Europe, or Asia or Africa", the first known explicit identification of part of the Americas as a continent like the other three. Within a few years, the name "New World" began appearing as a name for South America on world maps, such as the Oliveriana (Pesaro) map of around 1504–1505. Maps of this time, though, still showed North America connected to Asia and showed South America as
3750-430: Is joined by land in some fashion. The criterion "large" leads to arbitrary classification: Greenland , with a surface area of 2,166,086 square kilometres (836,330 sq mi), is only considered the world's largest island, while Australia , at 7,617,930 square kilometres (2,941,300 sq mi), is deemed the smallest continent. Earth's major landmasses all have coasts on a single, continuous World Ocean , which
3875-545: Is probably a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the United States Geological Survey. A recent increase in the number of major earthquakes has been noted, which could be explained by a cyclical pattern of periods of intense tectonic activity, interspersed with longer periods of low intensity. However, accurate recordings of earthquakes only began in
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4000-423: Is proportional to the area of the fault that ruptures and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The most important parameter controlling the maximum earthquake magnitude on a fault, however, is not the maximum available length, but the available width because the latter varies by a factor of 20. Along converging plate margins,
4125-612: Is that of a continuous area of land or mainland, with the coastline and any land boundaries forming the edge of the continent. In this sense, the term continental Europe (sometimes referred to in Britain as "the Continent") is used to refer to mainland Europe, excluding islands such as Great Britain , Iceland , Ireland , and Malta , while the term continent of Australia may refer to the mainland of Australia , excluding New Guinea , Tasmania , and other nearby islands. Similarly,
4250-410: Is the tsunami earthquake , observed where the relatively low felt intensities, caused by the slow propagation speed of some great earthquakes, fail to alert the population of the neighboring coast, as in the 1896 Sanriku earthquake . During an earthquake, high temperatures can develop at the fault plane, increasing pore pressure and consequently vaporization of the groundwater already contained within
4375-421: Is the best understood part of the ocean floor, as it is relatively accessible. Most commercial exploitation of the sea, such as extraction of metallic ore, non-metallic ore, and hydrocarbons , takes place on the continental shelf. Sovereign rights over their continental shelves down to a depth of 100 m (330 ft) or to a distance where the depth of waters admitted of resource exploitation were claimed by
4500-467: Is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume. In its most general sense, the word earthquake is used to describe any seismic event that generates seismic waves. Earthquakes can occur naturally or be induced by human activities, such as mining , fracking , and nuclear tests . The initial point of rupture
4625-407: Is usually considered an island of Africa, but its divergent evolution has caused it to be referred to as "the eighth continent" from a biological perspective. Geologists use four key attributes to define a continent: With the addition of Zealandia in 2017, Earth currently has seven recognized geological continents: Due to a seeming lack of Precambrian cratonic rocks, Zealandia's status as
4750-458: The continental United States refers to "the 49 States (including Alaska but excluding Hawaii ) located on the continent of North America, and the District of Columbia ." From the perspective of geology or physical geography , continent may be extended beyond the confines of continuous dry land to include the shallow, submerged adjacent area (the continental shelf ) and the islands on
4875-509: The 1556 Shaanxi earthquake in China, with over 830,000 fatalities, and the 1960 Valdivia earthquake in Chile, the largest ever recorded at 9.5 magnitude. Earthquakes result in various effects, such as ground shaking and soil liquefaction , leading to significant damage and loss of life. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause
5000-400: The 1980 eruption of Mount St. Helens . Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltmeters (a device that measures ground slope) and used as sensors to predict imminent or upcoming eruptions. A tectonic earthquake begins as an area of initial slip on the fault surface that forms
5125-587: The 2004 Indian Ocean earthquake is simultaneously one of the deadliest earthquakes in history. Earthquakes that caused the greatest loss of life, while powerful, were deadly because of their proximity to either heavily populated areas or the ocean, where earthquakes often create tsunamis that can devastate communities thousands of kilometers away. Regions most at risk for great loss of life include those where earthquakes are relatively rare but powerful, and poor regions with lax, unenforced, or nonexistent seismic building codes. Tectonic earthquakes occur anywhere on
Continental shelf - Misplaced Pages Continue
5250-666: The Black Sea . The names were first applied just to lands near the coast and only later extended to include the hinterlands . But the division was only carried through to the end of navigable waterways and "... beyond that point the Hellenic geographers never succeeded in laying their finger on any inland feature in the physical landscape that could offer any convincing line for partitioning an indivisible Eurasia ..." Ancient Greek thinkers subsequently debated whether Africa (then called Libya ) should be considered part of Asia or
5375-499: The Himalayan Mountains . With the rapid growth of mega-cities such as Mexico City, Tokyo, and Tehran in areas of high seismic risk , some seismologists are warning that a single earthquake may claim the lives of up to three million people. While most earthquakes are caused by the movement of the Earth's tectonic plates , human activity can also produce earthquakes. Activities both above ground and below may change
5500-613: The Isthmus of Suez , and North America and South America by the Isthmus of Panama . In both cases, there is no complete separation of these landmasses by water (disregarding the Suez Canal and the Panama Canal , which are both narrow and shallow, as well as human-made). Both of these isthmuses are very narrow compared to the bulk of the landmasses they unite. North America and South America are treated as separate continents in
5625-456: The Pacific Ocean , as well as the continent of Australia . Zealandia (a submerged continent ) has been called the eighth continent. The following table provides areas given by Encyclopædia Britannica for each continent in accordance with the seven-continent model, including Australasia along with Melanesia , Micronesia , and Polynesia as parts of Oceania. It also provides populations of continents according to 2021 estimates by
5750-480: The Persian Gulf . The average width of continental shelves is about 80 km (50 mi). The depth of the shelf also varies, but is generally limited to water shallower than 100 m (330 ft). The slope of the shelf is usually quite low, on the order of 0.5°; vertical relief is also minimal, at less than 20 m (66 ft). Though the continental shelf is treated as a physiographic province of
5875-584: The Pleistocene ice ages , greater areas of the continental shelf were exposed as dry land, forming land bridges between Tasmania and the Australian mainland . At those times, Australia and New Guinea were a single, continuous continent known as Sahul . Likewise, Afro-Eurasia and the Americas were joined by the Bering Land Bridge . Other islands, such as Great Britain , were joined to
6000-782: The United Nations Statistics Division based on the United Nations geoscheme , which includes all of Egypt (including the Isthmus of Suez and the Sinai Peninsula ) as a part of Africa, all of Armenia, Azerbaijan, Cyprus, Georgia, Indonesia, Kazakhstan, and Turkey (including East Thrace ) as parts of Asia, all of Russia (including Siberia ) as a part of Europe, all of Panama and the United States (including Hawaii ) as parts of North America, and all of Chile (including Easter Island ) as
6125-426: The brittle-ductile transition zone and upwards by the ground surface. The mechanics of this process are poorly understood because it is difficult either to recreate such rapid movements in a laboratory or to record seismic waves close to a nucleation zone due to strong ground motion. In most cases, the rupture speed approaches, but does not exceed, the shear wave (S wave) velocity of the surrounding rock. There are
6250-413: The least principal stress. Strike-slip faulting is intermediate between the other two types described above. This difference in stress regime in the three faulting environments can contribute to differences in stress drop during faulting, which contributes to differences in the radiated energy, regardless of fault dimensions. For every unit increase in magnitude, there is a roughly thirty-fold increase in
6375-483: The neritic zone , and the benthic (sea floor) province of the shelf is the sublittoral zone . The shelves make up less than 10% of the ocean, and a rough estimate suggests that only about 30% of the continental shelf sea floor receives enough sunlight to allow benthic photosynthesis. Though the shelves are usually fertile, if anoxic conditions prevail during sedimentation, the deposits may over geologic time become sources for fossil fuels . The continental shelf
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#17327651778596500-462: The ocean , it is not part of the deep ocean basin proper, but the flooded margins of the continent. Passive continental margins such as most of the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent. Active continental margins have narrow, relatively steep shelves, due to frequent earthquakes that move sediment to
6625-422: The tectonic plates , but exclude small continental fragments such as Madagascar that are generally referred to as microcontinents . Continental crust is only known to exist on Earth. The idea of continental drift gained recognition in the 20th century. It postulates that the current continents formed from the breaking up of a supercontinent ( Pangaea ) that formed hundreds of millions of years ago. From
6750-492: The "subcontinents" concerned are on different tectonic plates from the rest of the continent, providing a geological justification for the terminology. Greenland , generally considered the world's largest island on the northeastern periphery of the North American Plate , is sometimes referred to as a subcontinent. This is a significant departure from the more conventional view of a subcontinent as comprising
6875-638: The 16th century the English noun continent was derived from the term continent land , meaning continuous or connected land and translated from the Latin terra continens . The noun was used to mean "a connected or continuous tract of land" or mainland . It was not applied only to very large areas of land—in the 17th century, references were made to the continents (or mainlands) of the Isle of Man , Ireland and Wales and in 1745 to Sumatra . The word continent
7000-410: The 1960s. Continental shelves cover an area of about 27 million km (10 million sq mi), equal to about 7% of the surface area of the oceans. The width of the continental shelf varies considerably—it is not uncommon for an area to have virtually no shelf at all, particularly where the forward edge of an advancing oceanic plate dives beneath continental crust in an offshore subduction zone such as off
7125-553: The 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East. It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt. Minor earthquakes occur very frequently around the world in places like California and Alaska in the U.S., as well as in El Salvador, Mexico, Guatemala, Chile, Peru, Indonesia,
7250-433: The 21st century. Seismic waves travel through the Earth's interior and can be recorded by seismometers at great distances. The surface-wave magnitude was developed in the 1950s as a means to measure remote earthquakes and to improve the accuracy for larger events. The moment magnitude scale not only measures the amplitude of the shock but also takes into account the seismic moment (total rupture area, average slip of
7375-533: The Americas, Columbus never believed he had reached a new continent—he always thought it was part of Asia. In 1501, Amerigo Vespucci and Gonçalo Coelho attempted to sail around what they considered the southern end of the Asian mainland into the Indian Ocean , passing through Fernando de Noronha . After reaching the coast of Brazil , they sailed along the coast of South America much farther south than Asia
7500-697: The Black Sea and the Caspian Sea , and the border between Asia and Africa at the isthmus between the Red Sea and the mouth of Lake Bardawil on the Mediterranean Sea . The Roman author Pliny the Elder , writing in the 1st century CE, stated that "The whole globe is divided into three parts, Europe, Asia, and Africa", adding: I shall first then speak of Europe, the foster-mother of that people which has conquered all other nations, and itself by far
7625-691: The Catholic church, began to associate itself with the concept of Europe. Through the Roman period and the Middle Ages , a few writers took the Isthmus of Suez as the boundary between Asia and Africa, but most writers continued to consider it the Nile or the western border of Egypt (Gibbon). In the Middle Ages, the world was usually portrayed on T and O maps , with the T representing the waters dividing
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#17327651778597750-399: The Earth's core was located in 1913 by Beno Gutenberg . S waves and later arriving surface waves do most of the damage compared to P waves. P waves squeeze and expand the material in the same direction they are traveling, whereas S waves shake the ground up and down and back and forth. Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world
7875-446: The Earth, the text claims that the Earth has seven continents and Lord Vishnu Measured the entire universe from his first foot from the land of Earth which has 7 continents. ato devā avantu no yato viṣṇurvicakrame | pṛthivyāḥ saptadhāmabhiḥ || idaṃ viṣṇurvi cakrame tredhā ni dadhe padam | samūḷhamasya pāṃsure || trīṇi padā vi cakrame viṣṇurghopā adābhyaḥ | ato dharmāṇi dhārayan || The Gods be gracious unto us even from
8000-484: The Earth. Also, the depth of the hypocenter can be computed roughly. P wave speed S waves speed As a consequence, the first waves of a distant earthquake arrive at an observatory via the Earth's mantle. On average, the kilometer distance to the earthquake is the number of seconds between the P- and S wave times 8. Slight deviations are caused by inhomogeneities of subsurface structure. By such analysis of seismograms,
8125-619: The Greek word ἤπειρος , meaning "landmass, terra firma", the proper name of Epirus and later especially used for Asia (i.e. Asia Minor ). The first distinction between continents was made by ancient Greek mariners who gave the names Europe and Asia to the lands on either side of the waterways of the Aegean Sea , the Dardanelles strait, the Sea of Marmara , the Bosporus strait and
8250-573: The Philippines, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, India, Nepal, and Japan. Larger earthquakes occur less frequently, the relationship being exponential ; for example, roughly ten times as many earthquakes larger than magnitude 4 occur than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that
8375-463: The average recurrences are: an earthquake of 3.7–4.6 every year, an earthquake of 4.7–5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years. This is an example of the Gutenberg–Richter law . The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of
8500-404: The brittle crust. Thus, earthquakes with magnitudes much larger than 8 are not possible. In addition, there exists a hierarchy of stress levels in the three fault types. Thrust faults are generated by the highest, strike-slip by intermediate, and normal faults by the lowest stress levels. This can easily be understood by considering the direction of the greatest principal stress, the direction of
8625-566: The cause of other earthquakes in the past century. A Columbia University paper suggested that the 8.0 magnitude 2008 Sichuan earthquake was induced by loading from the Zipingpu Dam , though the link has not been conclusively proved. The instrumental scales used to describe the size of an earthquake began with the Richter scale in the 1930s. It is a relatively simple measurement of an event's amplitude, and its use has become minimal in
8750-594: The coast of Chile or the west coast of Sumatra . The largest shelf—the Siberian Shelf in the Arctic Ocean —stretches to 1,500 kilometers (930 mi) in width. The South China Sea lies over another extensive area of continental shelf, the Sunda Shelf , which joins Borneo , Sumatra, and Java to the Asian mainland. Other familiar bodies of water that overlie continental shelves are the North Sea and
8875-565: The coast; sand is limited to shallow, wave-agitated waters, while silt and clays are deposited in quieter, deep water far offshore. These accumulate 15–40 centimetres (5.9–15.7 in) every millennium, much faster than deep-sea pelagic sediments . "Shelf seas" are the ocean waters on the continental shelf. Their motion is controlled by the combined influences of the tides , wind-forcing and brackish water formed from river inflows ( Regions of Freshwater Influence ). These regions can often be biologically highly productive due to mixing caused by
9000-477: The concept of a continent may go beyond the continental shelf to include oceanic islands and continental fragments. In this way, Iceland is considered a part of Europe, and Madagascar a part of Africa. Extrapolating the concept to its extreme, some geographers group the Australian continental landmass with other islands in the Pacific Ocean into Oceania , which is usually considered a region rather than
9125-422: The deep ocean floor, the abyssal plain . The continental shelf and the slope are part of the continental margin . The shelf area is commonly subdivided into the inner continental shelf , mid continental shelf , and outer continental shelf , each with their specific geomorphology and marine biology . The character of the shelf changes dramatically at the shelf break, where the continental slope begins. With
9250-415: The deep sea. The continental shelves are covered by terrigenous sediments ; that is, those derived from erosion of the continents. However, little of the sediment is from current rivers ; some 60–70% of the sediment on the world's shelves is relict sediment , deposited during the last ice age, when sea level was 100–120 m lower than it is now. Sediments usually become increasingly fine with distance from
9375-529: The dip angle of the rupture plane is very shallow, typically about 10 degrees. Thus, the width of the plane within the top brittle crust of the Earth can reach 50–100 km (31–62 mi) (such as in Japan, 2011 , or in Alaska, 1964 ), making the most powerful earthquakes possible. The majority of tectonic earthquakes originate in the Ring of Fire at depths not exceeding tens of kilometers. Earthquakes occurring at
9500-678: The displacement along the fault is in the direction of dip and where movement on them involves a vertical component. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip. The topmost, brittle part of the Earth's crust, and the cool slabs of the tectonic plates that are descending into the hot mantle, are the only parts of our planet that can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 °C (572 °F) flow in response to stress; they do not rupture in earthquakes. The maximum observed lengths of ruptures and mapped faults (which may break in
9625-439: The distance from the earthquake and the underlying rock or soil makeup. The first scale for measuring earthquake magnitudes was developed by Charles Francis Richter in 1935. Subsequent scales ( seismic magnitude scales ) have retained a key feature, where each unit represents a ten-fold difference in the amplitude of the ground shaking and a 32-fold difference in energy. Subsequent scales are also adjusted to have approximately
9750-405: The division into three of what is really a single landmass, a debate that continues nearly two and a half millennia later. Herodotus believed Europe to be larger (at least in width) than the other two continents: I wonder, then, at those who have mapped out and divided the world into Libya, Asia, and Europe; for the difference between them is great, seeing that in length Europe stretches along both
9875-554: The early 1900s, so it is too early to categorically state that this is the case. Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-kilometre-long (25,000 mi), horseshoe-shaped zone called the circum-Pacific seismic belt, known as the Pacific Ring of Fire , which for the most part bounds the Pacific plate . Massive earthquakes tend to occur along other plate boundaries too, such as along
10000-403: The earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane . The sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the fault surface that increases the frictional resistance. Most fault surfaces do have such asperities, which leads to a form of stick-slip behavior . Once
10125-612: The earth'. A. Glucklich adds that 'In the Matsya Purana , for instance, there is a seven-part map of the world ... [it has] one centre, where an immense mountain – Mount Meru (or Maha Meru, Great Meru) – stands ... The continents encircle the mountain in seven concentric circles ... It seems clear that the Himalayas were the approximate location of Mt. Meru and the text is clear that the earth has seven continents. Earthquake An earthquake – also called
10250-426: The earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, 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 North Anatolian Fault in Turkey in
10375-504: The energy released. For instance, an earthquake of magnitude 6.0 releases approximately 32 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 1,000 times more energy than a 5.0 magnitude earthquake. An 8.6-magnitude earthquake releases the same amount of energy as 10,000 atomic bombs of the size used in World War II . This is so because the energy released in an earthquake, and thus its magnitude,
10500-458: The fact that no single earthquake in the sequence is the main shock, so none has a notably higher magnitude than another. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park . In August 2012, a swarm of earthquakes shook Southern California 's Imperial Valley , showing the most recorded activity in the area since the 1970s. Sometimes a series of earthquakes occur in what has been called an earthquake storm , where
10625-462: The fault has locked, continued relative motion between the plates leads to increasing stress and, therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy . This energy is released as a combination of radiated elastic strain seismic waves , frictional heating of
10750-411: The fault plane that holds it in place, and fluids can exert a lubricating effect. As thermal overpressurization may provide positive feedback between slip and strength fall at the fault plane, a common opinion is that it may enhance the faulting process instability. After the mainshock, the pressure gradient between the fault plane and the neighboring rock causes a fluid flow that increases pore pressure in
10875-418: The fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the elastic-rebound theory . It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or
11000-541: The fault, and rigidity of the rock). The Japan Meteorological Agency seismic intensity scale , the Medvedev–Sponheuer–Karnik scale , and the Mercalli intensity scale are based on the observed effects and are related to the intensity of shaking. The shaking of the earth is a common phenomenon that has been experienced by humans from the earliest of times. Before the development of strong-motion accelerometers,
11125-408: The flatter continental rise , in which sediment from the continent above cascades down the slope and accumulates as a pile of sediment at the base of the slope. Extending as far as 500 km (310 mi) from the slope, it consists of thick sediments deposited by turbidity currents from the shelf and slope. The continental rise 's gradient is intermediate between the gradients of the slope and
11250-417: The focus. Once the rupture has been initiated, it begins to propagate away from the focus, spreading out along the fault surface. Lateral propagation will continue until either the rupture reaches a barrier, such as the end of a fault segment, or a region on the fault where there is insufficient stress to allow continued rupture. For larger earthquakes, the depth extent of rupture will be constrained downwards by
11375-406: The force that "pushes" the rock mass during the faulting. In the case of normal faults, the rock mass is pushed down in a vertical direction, thus the pushing force ( greatest principal stress) equals the weight of the rock mass itself. In the case of thrusting, the rock mass "escapes" in the direction of the least principal stress, namely upward, lifting the rock mass, and thus, the overburden equals
11500-410: The intensity of a seismic event was estimated based on the observed effects. Magnitude and intensity are not directly related and calculated using different methods. The magnitude of an earthquake is a single value that describes the size of the earthquake at its source. Intensity is the measure of shaking at different locations around the earthquake. Intensity values vary from place to place, depending on
11625-476: The lengths along subducting plate margins, and those along normal faults are even shorter. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary . Earthquakes associated with normal faults are generally less than magnitude 7. Maximum magnitudes along many normal faults are even more limited because many of them are located along spreading centers, as in Iceland, where
11750-402: The main causes of these aftershocks, along with the crust around the ruptured fault plane as it adjusts to the effects of the mainshock. An aftershock is in the same region as the main shock but always of a smaller magnitude, however, they can still be powerful enough to cause even more damage to buildings that were already previously damaged from the mainshock. If an aftershock is larger than
11875-490: The mainlands of their continents. At that time, there were just three discrete landmasses in the world: Africa - Eurasia - America , Antarctica , and Australia - New Guinea ( Sahul ). There are several ways of distinguishing the continents: In the English-speaking countries, geographers often use the term Oceania to denote a geographical region which includes most of the island countries and territories in
12000-414: The mainshock, the aftershock is redesignated as the mainshock and the original main shock is redesignated as a foreshock . Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the mainshock. Earthquake swarms are sequences of earthquakes striking in a specific area within a short period. They are different from earthquakes followed by a series of aftershocks by
12125-429: The map and Asia on the right end. In the accompanying book Cosmographiae Introductio , Waldseemüller noted that the earth is divided into four parts, Europe, Asia, Africa, and the fourth part, which he named "America" after Amerigo Vespucci's first name. On the map, the word "America" was placed on part of South America. The Sanskrit text Rig Veda often dated 1500 BCE has the earliest mention of seven continents in
12250-742: The marine nations that signed the Convention on the Continental Shelf drawn up by the UN's International Law Commission in 1958. This was partly superseded by the 1982 United Nations Convention on the Law of the Sea (UNCLOS). The 1982 convention created the 200 nautical miles (370 km; 230 mi) exclusive economic zone, plus continental shelf rights for states with physical continental shelves that extend beyond that distance. The legal definition of
12375-549: The mid-17th century, Peter Heylin wrote in his Cosmographie that "A Continent is a great quantity of Land, not separated by any Sea from the rest of the World, as the whole Continent of Europe, Asia, Africa." In 1727, Ephraim Chambers wrote in his Cyclopædia, "The world is ordinarily divided into two grand continents: the Old and the New ." And in his 1752 atlas, Emanuel Bowen defined
12500-543: The most beauteous portion of the earth. Indeed, many persons have, not without reason, considered it, not as a third part only of the earth, but as equal to all the rest, looking upon the whole of our globe as divided into two parts only, by a line drawn from the river Tanais to the Straits of Gades. Following the fall of the Western Roman Empire , the culture that developed in its place , linked to Latin and
12625-475: The ocean primarily in New Zealand and New Caledonia , and the almost completely submerged Kerguelen Plateau in the southern Indian Ocean . Some islands lie on sections of continental crust that have rifted and drifted apart from a main continental landmass. While not considered continents because of their relatively small size, they may be considered microcontinents . Madagascar, the largest example,
12750-455: The others together, and it appears to me to be wider beyond all comparison. Eratosthenes , in the 3rd century BCE, noted that some geographers divided the continents by rivers (the Nile and the Don), thus considering them "islands". Others divided the continents by isthmuses , calling the continents "peninsulas". These latter geographers set the border between Europe and Asia at the isthmus between
12875-479: The place whence Vishnu strode Through the seven regions of the earth! Through all this world strode Vishnu; thrice his foot he planted, and the whole Was gathered in his footstep's dust. Vishnu, the Guardian, he whom none deceiveth , made three steps; thenceforth Establishing his high decrees. In regard to the above-quoted verses, it is commonly accepted that there are Seven Continents or 'regions of
13000-558: The present continents. Certain parts of continents are recognized as subcontinents, especially the large peninsulas separated from the main continental landmass by geographical features. The most widely recognized example is the Indian subcontinent . The Arabian Peninsula , Southern Africa , the Southern Cone of South America, and Alaska in North America might be considered further examples. In many of these cases,
13125-530: The rock. In the coseismic phase, such an increase can significantly affect slip evolution and speed, in the post-seismic phase it can control the Aftershock sequence because, after the main event, pore pressure increase slowly propagates into the surrounding fracture network. From the point of view of the Mohr-Coulomb strength theory , an increase in fluid pressure reduces the normal stress acting on
13250-437: The rupture of geological faults but also by other events such as volcanic activity, landslides, mine blasts, fracking and nuclear tests . An earthquake's point of initial rupture is called its hypocenter or focus. The epicenter is the point at ground level directly above the hypocenter. The seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at
13375-522: The same numeric value within the limits of the scale. Although the mass media commonly reports earthquake magnitudes as "Richter magnitude" or "Richter scale", standard practice by most seismological authorities is to express an earthquake's strength on the moment magnitude scale, which is based on the actual energy released by an earthquake, the static seismic moment. Every earthquake produces different types of seismic waves, which travel through rock with different velocities: Propagation velocity of
13500-464: The seismic waves through solid rock ranges from approx. 3 km/s (1.9 mi/s) up to 13 km/s (8.1 mi/s), depending on the density and elasticity of the medium. In the Earth's interior, the shock- or P waves travel much faster than the S waves (approx. relation 1.7:1). The differences in travel time from the epicenter to the observatory are a measure of the distance and can be used to image both sources of earthquakes and structures within
13625-552: The seven-continent model. However, they may also be viewed as a single continent known as America . This viewpoint was common in the United States until World War II , and remains prevalent in some Asian six-continent models. The single American continent model remains a common view in European countries like France, Greece, Hungary, Italy, Malta, Portugal, Spain, Latin American countries and some Asian countries. The criterion of
13750-415: The shallower waters and the enhanced current speeds. Despite covering only about 8% of Earth's ocean surface area, shelf seas support 15–20% of global primary productivity . In temperate continental shelf seas, three distinctive oceanographic regimes are found, as a consequence of the interplay between surface heating, lateral buoyancy gradients (due to river inflow), and turbulent mixing by the tides and to
13875-415: The shelf ( continental islands ), as they are structurally part of the continent. From this perspective, the edge of the continental shelf is the true edge of the continent, as shorelines vary with changes in sea level. In this sense the islands of Great Britain and Ireland are part of Europe, while Australia and the island of New Guinea together form a continent. Taken to its limit, this view could support
14000-517: The shelf. Under the United Nations Convention on the Law of the Sea , the name continental shelf was given a legal definition as the stretch of the seabed adjacent to the shores of a particular country to which it belongs. The shelf usually ends at a point of increasing slope (called the shelf break ). The sea floor below the break is the continental slope . Below the slope is the continental rise , which finally merges into
14125-422: The stresses and strains on the crust, including building reservoirs, extracting resources such as coal or oil, and injecting fluids underground for waste disposal or fracking . Most of these earthquakes have small magnitudes. The 5.7 magnitude 2011 Oklahoma earthquake is thought to have been caused by disposing wastewater from oil production into injection wells , and studies point to the state's oil industry as
14250-490: The surrounding fracture networks; such an increase may trigger new faulting processes by reactivating adjacent faults, giving rise to aftershocks. Analogously, artificial pore pressure increase, by fluid injection in Earth's crust, may induce seismicity . Tides may trigger some seismicity . Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors that cause little to no damage, but there
14375-443: The thickness of the brittle layer is only about six kilometres (3.7 mi). Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary . Reverse faults, particularly those along convergent boundaries, are associated with the most powerful earthquakes (called megathrust earthquakes ) including almost all of those of magnitude 8 or more. Megathrust earthquakes are responsible for about 90% of
14500-651: The three continents. By the middle of the 18th century, "the fashion of dividing Asia and Africa at the Nile, or at the Great Catabathmus [the boundary between Egypt and Libya ] farther west, had even then scarcely passed away". Christopher Columbus sailed across the Atlantic Ocean to the Caribbean in 1492, sparking a period of European exploration of the Americas . But despite four voyages to
14625-461: The total seismic moment released worldwide. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other; transform boundaries are a particular type of strike-slip fault. Strike-slip faults, particularly continental transforms , can produce major earthquakes up to about magnitude 8. Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km (6.2 mi) within
14750-505: The universality of such events beyond Earth. An earthquake is the shaking of the surface of Earth resulting from a sudden release of energy in the lithosphere that creates seismic waves . Earthquakes may also be referred to as quakes , tremors , or temblors . The word tremor is also used for non-earthquake seismic rumbling . In its most general sense, an earthquake is any seismic event—whether natural or caused by humans—that generates seismic waves. Earthquakes are caused mostly by
14875-436: The vast improvement in instrumentation, rather than an increase in the number of earthquakes. The United States Geological Survey (USGS) estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0–7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable. In recent years, the number of major earthquakes per year has decreased, though this
15000-479: The view of Europe as a separate continent continues in almost all categorizations. If continents are defined strictly as discrete landmasses, embracing all the contiguous land of a body, then Africa, Asia, and Europe form a single continent which may be referred to as Afro-Eurasia . Combined with the consolidation of the Americas, this would produce a four-continent model consisting of Afro-Eurasia, America, Antarctica, and Australia. When sea levels were lower during
15125-471: The view that there are only three continents: Antarctica, Australia-New Guinea, and a single mega-continent which joins Afro-Eurasia and America via the contiguous continental shelf in and around the Bering Sea. The vast size of the latter compared to the first two might even lead some to say it is the only continent, the others being more comparable to Greenland or New Zealand. As a cultural construct ,
15250-438: Was known to extend, confirming that this was a land of continental proportions. On return to Europe, an account of the voyage, called Mundus Novus ("New World"), was published under Vespucci's name in 1502 or 1503, although it seems that it had additions or alterations by another writer. Regardless of who penned the words, Mundus Novus credited Vespucci with saying, "I have discovered a continent in those southern regions that
15375-641: Was the deadliest of the 20th century. The 1960 Chilean earthquake is the largest earthquake that has been measured on a seismograph, reaching 9.5 magnitude on 22 May 1960. Its epicenter was near Cañete, Chile. The energy released was approximately twice that of the next most powerful earthquake, the Good Friday earthquake (27 March 1964), which was centered in Prince William Sound , Alaska. The ten largest recorded earthquakes have all been megathrust earthquakes ; however, of these ten, only
15500-417: Was used in translating Greek and Latin writings about the three "parts" of the world, although in the original languages no word of exactly the same meaning as continent was used. While continent was used on the one hand for relatively small areas of continuous land, on the other hand geographers again raised Herodotus's query about why a single large landmass should be divided into separate continents. In
15625-685: Was viewed as running from the Black Sea through Kerch Strait , the Sea of Azov and along the Don River (known then as the Tanais ) in Russia . The boundary between Asia and Africa was generally taken to be the Nile River. Herodotus in the 5th century BCE objected to the whole of Egypt being split between Asia and Africa ("Libya") and took the boundary to lie along the western border of Egypt, regarding Egypt as part of Asia. He also questioned
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