s, 1963–1998]] An
earthquake (also known as a
quake,
tremor or
temblor) is the result of a sudden release of energy in the
Earth's crust that creates
seismic waves. The
seismicity or
seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured with a
seismometer; a device which also records is known as a
seismograph. The
moment magnitude (or the related and mostly obsolete
Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified
Mercalli scale. The depth of the earthquake also matters: the more shallow the earthquake, the more damage to structures (all else being equal).
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.
Naturally occurring earthquakes
Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of stick-slip behaviour. Once the boundary 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 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 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.
Earthquake fault types
There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of
dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being
extended such as a
divergent boundary. Reverse faults occur in areas where the crust is being
shortened such as at a convergent boundary. 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. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.
Earthquakes away from plate boundaries
Where plate boundaries occur within
continental lithosphere, deformation is spread out over a much larger area than the plate boundary itself. In the case of the
San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g. the “Big bend” region). The
Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the
Arabian and
Eurasian plates where it runs through the northwestern part of the
Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake
focal mechanisms.
All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.
Shallow-focus and deep-focus earthquakes
The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km 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 up to 700 kilometers). These seismically active areas of subduction are known as
Wadati-Benioff zones. Deep-focus earthquakes occur at a depth at which 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 and volcanic activity
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 the
Mount St. Helens eruption of 1980. 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 which measures the ground slope) and used as sensors to predict imminent or upcoming eruptions.
Rupture dynamics
A tectonic earthquake begins by an initial rupture at a point on the fault surface, a process known as nucleation. The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than 100 m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger. The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks. Once the rupture has initiated it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone.
Earthquake clusters
Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.
Aftershocks
An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock 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 main shock.
Earthquake storms
Sometimes a series of earthquakes occur in a sort of
earthquake storm, where 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, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the
North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.
Size and frequency of occurrence
There are around 500,000 earthquakes each year. About 100,000 of these can actually be felt. Minor earthquakes occur nearly constantly around the world in places like
California and
Alaska in the U.S., as well as in
Guatemala.
Chile,
Peru,
Indonesia,
Iran,
Pakistan, the
Azores in
Portugal,
Turkey,
New Zealand,
Greece,
Italy, and
Japan, but earthquakes can occur almost anywhere, including
New York City,
London, and Australia. Larger earthquakes occur less frequently, the relationship being
exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that 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. and tsunami took as many as 200,000 lives on December 28, 1908 in
Sicily and
Calabria.]] 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 the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The
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, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS.
Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, 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 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 quake may claim the lives of up to 3 million people.
Induced seismicity
While most earthquakes are caused by movement of the Earth's
tectonic plates, human activity can also produce earthquakes. Four main activities contribute to this phenomenon: constructing large
dams and
buildings, drilling and injecting liquid into
wells, and by
coal mining and
oil drilling. Perhaps the best known example is the
2008 Sichuan earthquake in China's
Sichuan Province in May; this tremor resulted in 69,227 fatalities and is the
19th deadliest earthquake of all time. The
Zipingpu Dam is believed to have fluctuated the pressure of the fault away; this pressure probably increased the power of the earthquake and accelerated the rate of movement for the fault. The greatest earthquake in Australia's history was also induced by humanity, through coal mining.
The city of Newcastle was built over a large sector of coal mining areas. The earthquake was spawned from a fault which reactivated due to the millions of tonnes of rock removed in the mining process.
Measuring and locating earthquakes
Earthquakes can be recorded by seismometers up to great distances, because
seismic waves travel through the whole
Earth's interior. The absolute magnitude of a quake is conventionally reported by numbers on the
Moment magnitude scale (formerly Richter scale, magnitude 7 causing serious damage over large areas), whereas the felt magnitude is reported using the modified
Mercalli intensity scale (intensity II-XII).
Every tremor produces different types of seismic waves which travel through rock with different velocities: the longitudinal P-waves (shock- or pressure waves), the transverse S-waves (both body waves) and several surface waves (Rayleigh and Love waves). The propagation velocity of the seismic waves ranges from approx. 3 km/s up to 13 km/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 epicentre to the observatory are a measure of the distance and can be used to image both sources of quakes and structures within the Earth. Also the depth of the hypocenter can be computed roughly.
In solid rock P-waves travel at about 6 to 7 km per second; the velocity increases within the deep mantle to ~13 km/s. The velocity of S-waves ranges from 2–3 km/s in light sediments and 4–5 km/s in the Earth's crust up to 7 km/s in the deep mantle. As a consequence, the first waves of a distant earth quake arrive at an observatory via the Earth's mantle.
Rule of thumb: On the 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 analyses of seismograms the Earth's core was located in 1913 by Beno Gutenberg.
Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world 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.
Effects/impacts of earthquakes
in ruins and in flames after the
1755 Lisbon earthquake, which killed an estimated 60,000 people. A
tsunami overwhelms the ships in the harbor.]]
The effects of earthquakes include, but are not limited to, the following:
Shaking and ground rupture
Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake
magnitude, the distance from the
epicenter, and the local geological and geomorphological conditions, which may amplify or reduce
wave propagation. The ground-shaking is measured by ground
acceleration.
Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.
Ground rupture is a visible breaking and displacement of the Earth's surface along the trace of the fault, which may be of the order of several metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.
Landslides and avalanches
Earthquakes, along with severe storms, volcanic activity, coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue.
Fires
Earthquakes can cause
fires by damaging
electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, more deaths in the
1906 San Francisco earthquake were caused by fire than by the earthquake itself.
Soil liquefaction
Soil liquefaction occurs when, because of the shaking, water-saturated
granular material (such as sand) temporarily loses its strength and transforms from a
solid to a
liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits. This can be a devastating effect of earthquakes. For example, in the
1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.
Tsunami
Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers (62 miles), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373–497 miles per hour), depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.
Ordinarily, subduction earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more. Floods occur usually when the volume of water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and as a result some of the water flows or sits outside of the normal perimeter of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which then collapse and cause floods.
The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslide dam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly 5 million people.
Tidal forces
Research work has shown a robust correlation between small tidally induced forces and non-volcanic tremor activity.
Human impacts
Earthquakes may lead to
disease, lack of basic necessities, loss of life, higher insurance premiums, general
property damage, road and bridge damage, and collapse or destabilization (potentially leading to future collapse) of buildings. Earthquakes can also precede volcanic eruptions, which cause further problems; for example, substantial crop damage, as in the "
Year Without a Summer" (1816).
Major earthquakes
The largest earthquake that has been measured on a seismograph reached 9.5 magnitude, occurring on 22 May 1960. The ten largest recorded earthquakes have all been megathrust earthquakes; however, of these ten, only the 2004 Indian Ocean earthquake is simultaneously one of the deadliest earthquakes in history.
The earthquakes with the greatest amount of loss of life, while powerful, were deadly because of their proximity to either heavily populated areas or the ocean, where earthquakes can potentially create tsunamis which can devastate communities thousands of miles away. Regions that are 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.
Preparation
In order to determine the likelihood of future seismic activity,
geologists and other scientists examine the rock of an area to determine if the rock appears to be "strained". Studying the faults of an area to study the buildup time it takes for the fault to build up stress sufficient for an earthquake also serves as an effective prediction technique. Measurements of the amount of accumulated
strain energy on the fault each year, time passed since the last major temblor, and the energy and power of the last earthquake are made. many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. Furthermore, state-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world - such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines.
History
Pre-Middle Ages
From the lifetime of the Greek philosopher
Anaxagoras in the 5th century BCE to the 14th century CE, earthquakes were usually attributed to "air (vapors) in the cavities of the Earth".
Thales of Miletus, who lived from 625-547 (BCE) was the only documented person who believed that earthquakes were caused by tension between the earth and water.
In Greek mythology, Poseidon was the cause and god of earthquakes. When he was in a bad mood, he would strike the ground with a trident, causing this and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge.
In Japanese mythology, Namazu (鯰) is a giant catfish who causes earthquakes. Namazu lives in the mud beneath the earth, and is guarded by the god Kashima who restrains the fish with a stone. When Kashima lets his guard fall, Namazu thrashes about, causing violent earthquakes.
Popular culture
In modern
popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as
Kobe in 1995 or
San Francisco in 1906. Fictional earthquakes tend to strike suddenly and without warning. In
Pleasure Boating in Lituya Bay, one of the stories in
Jim Shepard's
Like You'd Understand, Anyway, the "Big One" leads to an even more devastating tsunami.
In the film 2012 (2009), solar flares (geologically implausibly) affecting the Earth's core caused massive destabilization of the Earth's crust layers. This created destruction planet-wide with earthquakes and tsunamis, foreseen by the Mayan culture and myth surrounding the last year noted in the Mesoamerican calendar - 2012.
Contemporary depictions of earthquakes in film are variable in the manner in which they reflect human psychological reactions to the actual trauma that can be caused to directly afflicted families and their loved ones. Disaster mental health response research emphasizes the need to be aware of the different roles of loss of family and key community members, loss of home and familiar surroundings, loss of essential supplies and services to maintain survival. Particularly for children, the clear availability of caregiving adults who are able to protect, nourish, and clothe them in the aftermath of the earthquake, and to help them make sense of what has befallen them has been shown to be even more important to their emotional and physical health than the simple giving of provisions. As was observed after other disasters involving destruction and loss of life and their media depictions, such as those of the 2001 World Trade Center Attacks or Hurricane Katrina—and has been recently observed in the 2010 Haiti Earthquake, it is also important not to pathologize the reactions to loss and displacement or disruption of governmental administration and services, but rather to validate these reactions, to support constructive problem-solving and reflection as to how one might improve the conditions of those affected.
See also
Notes
General references
External links
Educational
How to survive an earthquake - Guide for children and youth Earthquakes">Earthquakes—an educational booklet by Kaye M. Shedlock & Louis C. Pakiser The Severity of an Earthquake USGS Earthquake FAQs IRIS Seismic Monitor - maps all earthquakes in the past five years. Latest Earthquakes in the World - maps all earthquakes in the past week. Earthquake Information from the Deep Ocean Exploration Institute, Woods Hole Oceanographic Institution Geo.Mtu.Edu—How to locate an earthquake's epicenter Photos/images of historic earthquakes earthquakecountry.info Answers to FAQs about Earthquakes and Earthquake Preparedness Interactive guide: Earthquakes - an educational presentation by Guardian Unlimited Geowall—an educational 3D presentation system for looking at and understanding earthquake data Virtual Earthquake - educational site explaining how epicenters are located and magnitude is determined CBC Digital Archives—Canada's Earthquakes and Tsunamis Earthquakes Educational Resources - dmoz USGS: Earthquakes for Kids
Seismological data centers
Europe
International Seismological Centre (ISC) European-Mediterranean Seismological Centre (EMSC) Global Seismic Monitor at GFZ Potsdam Global Earthquake Report–chart Earthquakes in Iceland during the last 48 hours Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy Portuguese Meteorological Institute (Seismic activity during the last month)
Japan
Earthquake Information of Japan, Japan Meteorological Agency International Institute of Seismology and Earthquake Engineering (IISEE) Building Research Institute Database for the damage of world earthquake, ancient period (3000 BC) to year of 2006- Building Research Institute (Japan) (建築研究所) in Japanese Seismic activity in last 7 days - Weathernews Inc., indicated with circled shindo (震度）) scale, magnitude (M) and its location. * Weathernews Inc, Global web site
New Zealand
GeoNet - New Zealand Earthquake Report (latest and recent quakes)
United States
The U.S. National Earthquake Information Center Southern California Earthquake Data Center The Southern California Earthquake Center (SCEC) Broadband Seismic Data Collection Center, San Diego, California (ANZA network) Putting Down Roots in Earthquake Country An Earthquake Science and Preparedness Handbook produced by SCEC Recent earthquakes in California and Nevada Seismograms for recent earthquakes via REV, the Rapid Earthquake Viewer Incorporated Research Institutions for Seismology (IRIS), earthquake database and software IRIS Seismic Monitor - world map of recent earthquakes SeismoArchives - seismogram archives of significant earthquakes of the world
Seismic scales
The European Macroseismic Scale
Scientific information
Miscellaneous
Reports on China Sichuan earthquake 12/05/2008 Kashmir Relief & Development Foundation (KRDF) PBS NewsHour - Predicting Earthquakes USGS–Largest earthquakes in the world since 1900 The Destruction of Earthquakes - a list of the worst earthquakes ever recorded Los Angeles Earthquakes plotted on a Google map the EM-DAT International Disaster Database Earthquake Newspaper Articles Archive Earth-quake.org PetQuake.org- official PETSAAF system which relies on strange or atypical animal behavior to predict earthquakes. A series of earthquakes in southern Italy - 23 November 1980, Gesualdo Recent Quakes WorldWide Real-time earthquakes on Google Map, Australia and rest of the world Earthquake Information - detailed statistics and integrated with Google Maps and Google Earth Kharita - INGV portal for Digital Cartography - Last earthquakes recorded by INGV Italian Network (with Google Maps) Kharita - INGV portal for Digital Cartography - Italian Seismicity by region 1981-2006 (with Google Maps) Earthquakes In The Last Week ">Interactive world map, showing recent earthquakes (day/week/month)–Quake-Catcher Network, BOINC Category:Seismology Category:Geological hazards Category:Earthquake engineering
