An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities; the seismicity, or seismic activity, of an area is the frequency and size of earthquakes experienced over a period of time. The word tremor is used for non-earthquake seismic rumbling. At the Earth's surface, earthquakes manifest themselves by shaking and displacing or disrupting the ground; when the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can trigger landslides, volcanic activity. In its most general sense, the word earthquake is used to describe any seismic event—whether natural or caused by humans—that generates seismic waves. Earthquakes are caused by rupture of geological faults, but by other events such as volcanic activity, mine blasts, nuclear tests.
An earthquake's point of initial rupture is called its hypocenter. The epicenter is the point at ground level directly above the hypocenter. Tectonic earthquakes occur anywhere in 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 increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behavior. Once 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 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, 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. There are three main types of fault, all of which may cause an interplate earthquake: normal 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 in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas.
Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip. Reverse faults those along convergent plate boundaries are associated with the most powerful earthquakes, megathrust earthquakes, including all of those of magnitude 8 or more. Strike-slip faults continental transforms, can produce major earthquakes up to about magnitude 8. Earthquakes associated with normal faults are less than magnitude 7. For every unit increase in magnitude, there is a thirtyfold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases 30 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 900 times more energy than a 5.0 magnitude of earthquake. An 8.6 magnitude earthquake releases the same amount of energy as 10,000 atomic bombs like those used in World War II. This is so because the energy released in an earthquake, thus its magnitude, 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 topmost, brittle part of the Earth's crust, the cool slabs of the tectonic plates that are descending down into the hot mantle, are the only parts of our planet which can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 °C flow in response to stress; the maximum observed lengths of ruptures and mapped faults are 1,000 km. Examples are the earthquakes in Chile, 1960; the longest earthquake ruptures on strike-slip faults, like the San Andreas Fault, the North Anatolian Fault in Turkey and the Denali Fault in Alaska, are about half to one third as long as the lengths along subducting plate margins, those along normal faults are shorter. The most important parameter controlling the maximum earthquake magnitude on a fault is however not the maximum available length, but the available width because the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is shallow about 10 de
A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. First detected using long term strain measurements, most slow earthquakes now appear to be accompanied by fluid flow and related tremor, which can be detected and located using seismometer data filtered appropriately; that is, they are quiet compared to a regular earthquake, but not "silent" as described in the past. Slow earthquakes should not be confused with tsunami earthquakes, in which slow rupture velocity produces tsunami out of proportion to the triggering earthquake. In a tsunami earthquake, the rupture propagates along the fault more than usual, but the energy release occurs on a similar timescale to other earthquakes. Earthquakes occur as a consequence of gradual stress increases in a region, once it reaches the maximum stress that the rocks can withstand a rupture generates and the resulting earthquake motion is related to a drop in the shear stress of the system.
Earthquakes generate seismic waves when the rupture in the system occurs, the seismic waves consist of different types of waves that are capable of moving through the Earth like ripples over water. The causes that lead to slow earthquakes have only been theoretically investigated, by the formation of longitudinal shear cracks that were analysed using mathematical models; the different distributions of initial stress, sliding frictional stress, specific fracture energy are all taken into account. If the initial stress minus the sliding frictional stress is low, the specific fracture energy or the strength of the crustal material is high slow earthquakes will occur regularly. In other words, slow earthquakes are caused by a variety of stick-slip and creep processes intermediated between asperity-controlled brittle and ductile fracture. Asperities are tiny protrusions along the faces of fractures, they are best documented from intermediate crustal levels of certain subduction zones, but appear to occur on other types of faults as well, notably strike-slip plate boundaries such as the San Andreas fault and "mega-landslide" normal faults on the flanks of volcanos.
Faulting takes place all over Earth. As of 2013 some of the locations that have been studied for slow earthquakes include: Cascadia, Japan, New Zealand and Alaska; the locations of slow earthquakes can provide new insights into the behavior of normal or fast earthquakes. By observing the location of tremors associated with slow-slip and slow earthquakes, seismologists can determine the extension of the system and estimate future earthquakes in the area of study. Teruyuki Kato identifies various types of slow earthquake: low frequency earthquakes low frequency earthquakes slow slip events episodic tremor and slip Low frequency earthquakes are seismic events defined by waveforms with periods far greater than those of ordinary earthquakes and abundantly occur during slow earthquakes. LFEs can be volcanic, semi-volcanic, or tectonic in origin, but only tectonic LFEs or LFEs generated during slow earthquakes are described here. Tectonic LFEs are characterized by low magnitudes and have frequencies peaked between 1 and 3 Hz.
They are the largest constituent of non-volcanic tremor at subduction zones, in some cases are the only constituent. In contrast to ordinary earthquakes, tectonic LFEs occur during long-lived slip events at subduction interfaces called slow slip events; the mechanism responsible for their generation at subduction zones is thrust-sense slip along transitional segments of the plate interface. LFEs are sensitive seismic events which can be triggered by tidal forces as well as propagating waves from distant earthquakes. LFEs have hypocenters located down-dip from the seismogenic zone, the source region of megathrust earthquakes. During SSEs, LFE foci migrate along strike at the subduction interface in concert with the primary shear slip front; the depth occurrence of low frequency earthquakes is in the range of 20–45 kilometers depending on the subduction zone, at shallower depths at strike-slip faults in California. At "warm" subduction zones like the west coast of North America, or sections in eastern Japan this depth corresponds to a transition or transient slip zone between the locked and stable slip intervals of the plate interface.
The transition zone is located at depths coincidental with the continental Mohorovicic discontinuity. At the Cascadia subduction zone, the distribution of LFEs form a surface parallel to intercrustal seismic events, but displaced 5–10 kilometers down-dip, providing evidence that LFEs are generated at the plate interface. Low frequency earthquakes are an active area of research and may be important seismic indicators for higher magnitude earthquakes. Since slow slip events and their corresponding LFE signals have been recorded, none of them have been accompanied by a megathrust earthquake, however, SSEs act to increase the stress in the seismogenic zone by forcing the locked interval between the subducting and overriding plate to accommodate for down-dip movement; some calculations find that the probability of a large earthquake occurring during a slow slip event are 30–100 times greater than background probabilities. Understanding the seismic hazard that LFEs might herald is among the primary reasons for their research.
Seismic magnitude scales
Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking caused by an earthquake at a given location. Magnitudes are determined from measurements of an earthquake's seismic waves as recorded on a seismogram. Magnitude scales vary how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, the purposes for which the magnitudes are used; the Earth's crust is stressed by tectonic forces. When this stress becomes great enough to rupture the crust, or to overcome the friction that prevents one block of crust from slipping past another, energy is released, some of it in the form of various kinds of seismic waves that cause ground-shaking, or quaking. Magnitude is an estimate of the relative "size" or strength of an earthquake, thus its potential for causing ground-shaking, it is "approximately related to the released seismic energy."
Intensity refers to the strength or force of shaking at a given location, can be related to the peak ground velocity. With an isoseismal map of the observed intensities an earthquake's magnitude can be estimated from both the maximum intensity observed, from the extent of the area where the earthquake was felt; the intensity of local ground-shaking depends on several factors besides the magnitude of the earthquake, one of the most important being soil conditions. For instance, thick layers of soft soil can amplify seismic waves at a considerable distance from the source, while sedimentary basins will resonate, increasing the duration of shaking; this is why, in the 1989 Loma Prieta earthquake, the Marina district of San Francisco was one of the most damaged areas, though it was nearly 100 km from the epicenter. Geological structures were significant, such as where seismic waves passing under the south end of San Francisco Bay reflected off the base of the Earth's crust towards San Francisco and Oakland.
A similar effect channeled seismic waves between the other major faults in the area. An earthquake radiates energy in the form of different kinds of seismic waves, whose characteristics reflect the nature of both the rupture and the earth's crust the waves travel through. Determination of an earthquake's magnitude involves identifying specific kinds of these waves on a seismogram, measuring one or more characteristics of a wave, such as its timing, amplitude, frequency, or duration. Additional adjustments are made for distance, kind of crust, the characteristics of the seismograph that recorded the seismogram; the various magnitude scales represent different ways of deriving magnitude from such information as is available. All magnitude scales retain the logarithmic scale as devised by Charles Richter, are adjusted so the mid-range correlates with the original "Richter" scale. Most magnitude scales are based on measurements of only part of an earthquake's seismic wave-train, therefore are incomplete.
This results in systematic underestimation of magnitude in certain cases, a condition called saturation. Since 2005 the International Association of Seismology and Physics of the Earth's Interior has standardized the measurement procedures and equations for the principal magnitude scales, ML , Ms , mb , mB and mbLg ; the first scale for measuring earthquake magnitudes, developed in 1935 by Charles F. Richter and popularly known as the "Richter" scale, is the Local magnitude scale, label ML or ML. Richter established two features now common to all magnitude scales. First, the scale is logarithmic, so that each unit represents a ten-fold increase in the amplitude of the seismic waves; as the energy of a wave is 101.5 times its amplitude, each unit of magnitude represents a nearly 32-fold increase in the energy of an earthquake. Second, Richter arbitrarily defined the zero point of the scale to be where an earthquake at a distance of 100 km makes a maximum horizontal displacement of 0.001 millimeters on a seismogram recorded with a Wood-Anderson torsion seismograph.
Subsequent magnitude scales are calibrated to be in accord with the original "Richter" scale around magnitude 6. All "Local" magnitudes are based on the maximum amplitude of the ground shaking, without distinguishing the different seismic waves, they underestimate the strength: of distant earthquakes because of attenuation of the S-waves, of deep earthquakes because the surface waves are smaller, of strong earthquakes because they do not take into account the duration of shaking. The original "Richter" scale, developed in the geological context of Southern California and Nevada, was found to be inaccurate for earthquakes in the central and eastern parts of the continent because of differences in the continental crust. All these problems prompted the development of other scales. Most seismological authorities, such as the United States Geological Survey, report earthquake magnitudes above 4.0 as moment magnitude, which the press describes as "Richter magnitude". Richter's original "local" scale has been adapted for other localities.
These may be with a lowercase "l", either Ml, or Ml. Whether the values are comparable depends on whether the local conditions have been adequately determined and the formula suitably adjusted. In Japan, for shallow earthquakes within 600 km, the Japanese Meteorological Agenc
Blind thrust earthquake
A blind thrust earthquake occurs along a thrust fault that does not show signs on the Earth's surface, hence the designation "blind". Such faults, being invisible at the surface, have not been mapped by standard surface geological mapping. Sometimes they are discovered as a by-product of oil exploration seismology. Although such earthquakes are not amongst the most energetic, they are sometimes the most destructive, as conditions combine to form an urban earthquake which affects urban seismic risk. A blind thrust earthquake is quite close, in meaning, to a buried rupture earthquake, if a buried rupture earthquake is not about the fault, but signs the earthquake leaves, on the Earth's surface. Blind thrust faults exist near tectonic plate margins, 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. As shown in the diagram, a weak plate under compression forms thrusting sheets, or overlapping sliding sections.
This can form a hill and valley landform, with the hills being the strong sections, the valleys being the disturbed thrust faulted and folded sections. After a long period of erosion the visible landscape may be flattened, with material eroded from the hills filling up the valleys and hiding the underlying hill-and-valley geology; the valley rock is weak and highly weathered, presenting deep, fertile soil. Reflection seismology profiles show the disturbed rock. If the region is under active compression these faults are rupturing, but any given valley might only experience a large earthquake every few hundred years. Although of magnitude 6 to 7 compared to the largest magnitude 9 earthquakes of recent times, such a temblor is destructive because the seismic waves are directed, the soft basin soil of the valley can amplify the ground motions tenfold or more, it is said that blind thrust earthquakes contribute more to urban seismic risk than the'big ones' of magnitude 8 or more. Los Angeles, California, USA, is well-studied.
In addition to surface faults, a number of blind-thrust faults have been found under the basin and metropolitan area. A NASA study which combined satellite radar images and Global Positioning System observations found that "tectonic squeezing across Los Angeles" "will produce earthquakes on either the blind Elysian Park or Puente Hills thrust fault systems". Bajo Segura Fault Zone, Spain Fukaya Fault System, Japan Uemachi Fault System, Osaka Basin, Japan 1987 Whittier Narrows earthquake 1994 Northridge earthquake 2010 Haiti earthquake 2012 Visayas earthquake Buried rupture earthquake Surface rupture earthquake
Bulgaria the Republic of Bulgaria, is a country in Southeast Europe. It is bordered by Romania to the north and North Macedonia to the west and Turkey to the south, the Black Sea to the east; the capital and largest city is Sofia. With a territory of 110,994 square kilometres, Bulgaria is Europe's 16th-largest country. One of the earliest societies in the lands of modern-day Bulgaria was the Neolithic Karanovo culture, which dates back to 6,500 BC. In the 6th to 3rd century BC the region was a battleground for Thracians, Persians and ancient Macedonians; the Eastern Roman, or Byzantine, Empire lost some of these territories to an invading Bulgar horde in the late 7th century. The Bulgars founded the First Bulgarian Empire in AD 681, which dominated most of the Balkans and influenced Slavic cultures by developing the Cyrillic script; this state lasted until the early 11th century, when Byzantine emperor Basil II conquered and dismantled it. A successful Bulgarian revolt in 1185 established a Second Bulgarian Empire, which reached its apex under Ivan Asen II.
After numerous exhausting wars and feudal strife, the Second Bulgarian Empire disintegrated in 1396 and its territories fell under Ottoman rule for nearly five centuries. The Russo-Turkish War of 1877–78 resulted in the formation of the current Third Bulgarian State. Many ethnic Bulgarian populations were left outside its borders, which led to several conflicts with its neighbours and an alliance with Germany in both world wars. In 1946 Bulgaria became part of the Soviet-led Eastern Bloc; the ruling Communist Party gave up its monopoly on power after the revolutions of 1989 and allowed multi-party elections. Bulgaria transitioned into a democracy and a market-based economy. Since adopting a democratic constitution in 1991, the sovereign state has been a unitary parliamentary republic with a high degree of political and economic centralisation; the population of seven million lives in Sofia and the capital cities of the 27 provinces, the country has suffered significant demographic decline since the late 1980s.
Bulgaria is a member of the European Union, NATO, the Council of Europe. Its market economy is part of the European Single Market and relies on services, followed by industry—especially machine building and mining—and agriculture. Widespread corruption is a major socioeconomic issue; the name Bulgaria is derived from a tribe of Turkic origin that founded the country. Their name is not understood and difficult to trace back earlier than the 4th century AD, but it is derived from the Proto-Turkic word bulģha and its derivative bulgak; the meaning may be further extended to "rebel", "incite" or "produce a state of disorder", i.e. the "disturbers". Ethnic groups in Inner Asia with phonologically similar names were described in similar terms: during the 4th century, the Buluoji, a component of the "Five Barbarian" groups in Ancient China, were portrayed as both a "mixed race" and "troublemakers". Neanderthal remains dating to around 150,000 years ago, or the Middle Paleolithic, are some of the earliest traces of human activity in the lands of modern Bulgaria.
The Karanovo culture arose circa 6,500 BC and was one of several Neolithic societies in the region that thrived on agriculture. The Copper Age Varna culture is credited with inventing gold metallurgy; the associated Varna Necropolis treasure contains the oldest golden jewellery in the world with an approximate age of over 6,000 years. The treasure has been valuable for understanding social hierarchy and stratification in the earliest European societies; the Thracians, one of the three primary ancestral groups of modern Bulgarians, appeared on the Balkan Peninsula some time before the 12th century BC. The Thracians excelled in metallurgy and gave the Greeks the Orphean and Dionysian cults, but remained tribal and stateless; the Persian Achaemenid Empire conquered most of present-day Bulgaria in the 6th century BC and retained control over the region until 479 BC. The invasion became a catalyst for Thracian unity, the bulk of their tribes united under king Teres to form the Odrysian kingdom in the 470s BC.
It was weakened and vassalized by Philip II of Macedon in 341 BC, attacked by Celts in the 3rd century, became a province of the Roman Empire in AD 45. By the end of the 1st century AD, Roman governance was established over the entire Balkan Peninsula and Christianity began spreading in the region around the 4th century; the Gothic Bible—the first Germanic language book—was created by Gothic bishop Ulfilas in what is today northern Bulgaria around 381. The region came under Byzantine control after the fall of Rome in 476; the Byzantines were engaged in prolonged warfare against Persia and could not defend their Balkan territories from barbarian incursions. This enabled the Slavs to enter the Balkan Peninsula as marauders through an area between the Danube River and the Balkan Mountains known as Moesia; the interior of the peninsula became a country of the South Slavs, who lived under a democracy. The Slavs assimilated the Hellenized and Gothicized Thracians in the rural areas. Not l
1960 Valdivia earthquake
The 1960 Valdivia earthquake or the Great Chilean earthquake of 22 May is the most powerful earthquake recorded. Various studies have placed it at 9.4–9.6 on the moment magnitude scale. It occurred in the afternoon, lasted 10 minutes; the resulting tsunami affected southern Chile, Japan, the Philippines, eastern New Zealand, southeast Australia and the Aleutian Islands. The epicenter of this megathrust earthquake was near Lumaco 570 kilometres south of Santiago, with Valdivia being the most affected city; the tremor caused localised tsunamis that battered the Chilean coast, with waves up to 25 metres. The main tsunami raced across Hawaii. Waves as high as 10.7 metres were recorded 10,000 kilometres from the epicenter, as far away as Japan and the Philippines. The death toll and monetary losses arising from this widespread disaster are not certain. Various estimates of the total number of fatalities from the earthquake and tsunamis have been published, ranging between 1,000 and 7,000 killed. Different sources have estimated the monetary cost ranged from US$400 million to 800 million.
The 1960 Chilean earthquakes were a sequence of strong earthquakes that affected Chile between 21 May and 6 June 1960, centered in the Araucanía, Aysén, Bío Bío Regions of the country. The first three quakes, all registering in the planet's top 10 by magnitude for 1960, are grouped together as the 1960 Concepción earthquakes; the first of these was the 8.1 Mw Concepción earthquake at 06:02 UTC-4 on 21 May 1960. Its epicenter was near Curanilahue. Telecommunications to southern Chile were cut off and President Jorge Alessandri cancelled the traditional ceremony of the Battle of Iquique memorial holiday to oversee the emergency assistance efforts; the second and third Concepción earthquakes occurred the next day at 06:32 UTC-4 and 14:55 UTC-4 on May 22. These earthquakes formed a southward migrating foreshock sequence to the main Valdivia shock, which occurred just 15 minutes after the third event; the earthquake interrupted and ended Lota's coal miners march on Concepción as they demanded higher salaries.
The Valdivia earthquake occurred at 15:11 UTC-4 on 22 May, affected all of Chile between Talca and Chiloé Island, more than 400,000 square kilometres. Coastal villages, such as Toltén, were struck. At Corral, the main port of Valdivia, the water level rose 4 m. At 16:20 UTC-4, a wave of 8 m struck the Chilean coast between Concepción and Chiloé. Another wave measuring 10 m was reported ten minutes later. Hundreds of people were reported dead by the time the tsunami struck. One ship, starting at the mouth of Valdivia River, sank after being moved 1.5 km backward and forward in the river. A number of Spanish-colonial fortifications were destroyed. Soil subsidence destroyed buildings, deepened local rivers, created wetlands in places like the Río Cruces and Chorocomayo, a new aquatic park north of the city. Extensive areas of the city were flooded; the electricity and water systems of Valdivia were destroyed. Witnesses reported underground water flowing up through the soil. Despite the heavy rains of 21 May, the city was without a water supply.
The river turned brown with sediment from landslides and was full of floating debris, including entire houses. The lack of potable water became a serious problem in one of Chile's rainiest regions; the earthquake did not strike all the territory with the same strength. The two most affected areas were Valdivia and Puerto Octay, near the northwest corner of Llanquihue Lake. Puerto Octay was the center of a north-south elliptical area in the Central Valley, where the intensity was at the highest outside the Valdivia Basin. East of Puerto Octay, in a hotel in Todos los Santos Lake, stacked plates were reported to have remained in place. Excepting poor building sites, the zone of Mercalli scales intensities of VII or more all lay west of the Andes in a strip running from Lota southwards; the area of intensities of VII or more did not penetrate into the Central Valley in north of Lleulleu Lake and south of Castro. Two days after the earthquake Cordón Caulle, a volcanic vent close to Puyehue volcano, erupted.
Other volcanoes may have erupted, but none were recorded due to the lack of communication in Chile at the time. The low death toll in Chile is explained in part by the low population density in the region, by building practices that took into account the area's high geological activity; the earthquake was a megathrust earthquake resulting from the release of mechanical stress between the subducting Nazca Plate and the South American Plate, on the Peru–Chile Trench. The focus was shallow at 33 km, considering that earthquakes in northern Chile and Argentina may reach depths of 70 km. Subduction zones are known to produce the strongest earthquakes on earth, as their particular structure allows more stress to build up before energy is released. Geophysicists consider it a matter of time before this earthquake will be surpassed in magnitude by another; the earthquake's rupture zone was 800 km long. Rupture velocity, the speed at which a rupture front expands across the surface of the fault, has been estimated as 3.5 km per second
Earthquake prediction is a branch of the science of seismology concerned with the specification of the time and magnitude of future earthquakes within stated limits, "the determination of parameters for the next strong earthquake to occur in a region. Earthquake prediction is sometimes distinguished from earthquake forecasting, which can be defined as the probabilistic assessment of general earthquake hazard, including the frequency and magnitude of damaging earthquakes in a given area over years or decades. Prediction can be further distinguished from earthquake warning systems, which upon detection of an earthquake, provide a real-time warning of seconds to neighboring regions that might be affected. In the 1970s, scientists were optimistic that a practical method for predicting earthquakes would soon be found, but by the 1990s continuing failure led many to question whether it was possible. Demonstrably successful predictions of large earthquakes have not occurred and the few claims of success are controversial.
For example, the most famous claim of a successful prediction is that alleged for the 1975 Haicheng earthquake. A study said that there was no valid short-term prediction. Extensive searches have reported many possible earthquake precursors, but, so far, such precursors have not been reliably identified across significant spatial and temporal scales. While part of the scientific community hold that, taking into account non-seismic precursors and given enough resources to study them extensively, prediction might be possible, most scientists are pessimistic and some maintain that earthquake prediction is inherently impossible. Predictions are deemed significant. Therefore, methods of statistical hypothesis testing are used to determine the probability that an earthquake such as is predicted would happen anyway; the predictions are evaluated by testing whether they correlate with actual earthquakes better than the null hypothesis. In many instances, the statistical nature of earthquake occurrence is not homogeneous.
Clustering occurs in both time. In southern California about 6% of M≥3.0 earthquakes are "followed by an earthquake of larger magnitude within 5 days and 10 km." In central Italy 9.5 % of M ≥ 3.0 earthquakes are followed by a larger event within 30 km. While such statistics are not satisfactory for purposes of prediction they will skew the results of any analysis that assumes that earthquakes occur randomly in time, for example, as realized from a Poisson process, it has been shown that a "naive" method based on clustering can predict about 5% of earthquakes. As the purpose of short-term prediction is to enable emergency measures to reduce death and destruction, failure to give warning of a major earthquake, that does occur, or at least an adequate evaluation of the hazard, can result in legal liability, or political purging. For example, it has been reported that members of the Chinese Academy of Sciences were purged for "having ignored scientific predictions of the disastrous Tangshan earthquake of summer 1976."
Wade 1977. Following the L'Aquila earthquake of 2009, seven scientists and technicians in Italy were convicted of manslaughter, but not so much for failing to predict the 2009 L'Aquila Earthquake as for giving undue assurance to the populace – one victim called it "anaesthetizing" – that there would not be a serious earthquake, therefore no need to take precautions, but warning of an earthquake that does not occur incurs a cost: not only the cost of the emergency measures themselves, but of civil and economic disruption. False alarms, including alarms that are canceled undermine the credibility, thereby the effectiveness, of future warnings. In 1999 it was reported that China was introducing "tough regulations intended to stamp out ‘false’ earthquake warnings, in order to prevent panic and mass evacuation of cities triggered by forecasts of major tremors." This was prompted by "more than 30 unofficial earthquake warnings... in the past three years, none of, accurate." The acceptable trade-off between missed quakes and false alarms depends on the societal valuation of these outcomes.
The rate of occurrence of both must be considered. In a 1997 study of the cost-benefit ratio of earthquake prediction research in Greece, Stathis Stiros suggested that a excellent prediction method would be of questionable social utility, because "organized evacuation of urban centers is unlikely to be accomplished", while "panic and other undesirable side-effects can be anticipated." He found that earthquakes kill less than ten people per year in Greece, that most of those fatalities occurred in large buildings with identifiable structural issues. Therefore, Stiros stated that it would be much more cost-effective to focus efforts on identifying and upgrading unsafe buildings. Since the death toll on Greek highways is more than 2300 per year on average, he argued that more lives would be saved if Greece's entire budget for earthquake prediction had been used for street and highway safety instead. Earthquake prediction is an immature science—it has not yet led to a successful prediction of an earthquake from first physical principles.
Research into methods of prediction therefore focus on empirical analysis, with two general approaches: either identifying distinctive precursors to earthquakes, or identifying some kind of geophysical trend or pattern in seismicity that might precede a large earthquake. Precursor methods are pursu