Earthquake
Earthquake is the sudden vibrations or oscillations of the earth’s crust Disambiguation – If you are looking for other meanings, see Earthquake (disambiguation) .
In geophysics , earthquakes (from the Latin : terrae motus , which means ” movement of the earth “), also called earthquakes or telluric tremors (from the Latin Tellus , the Roman goddess of the Earth ), are vibrations or settlements of the earth’s crust , caused by the sudden displacement impact of a rock mass underground.
Scheme of generation of an earthquake: the sudden displacement of a rock mass, usually not superficial, generate sseismic waves that reach the earth’s surface in a short time, making the rock layers and the soil above vibrate
This displacement is generated by tectonic forces which act constantly inside the earth’s crust causing a slow deformation until the breaking load is reached with the consequent release of elastic energy in an internal area of the earth called the hypocenter , typically located in correspondence with fractures pre-existing parts of the crust called faults . Starting from the fracture created, a series of elastic waves, called seismic waves , propagated in all directions from the hypocenter, giving rise to the phenomenon observed on the surface with the place on the earth’s surface placed vertically above the hypocenter, called the epicenter , which is generally the one most affected by the phenomenon. The branch of geophysics that studies these phenomena is seismology .
Almost all the earthquakes that occur on the earth’s surface are concentrated near the boundaries between two tectonic plates , where the contact is made up of faults: these are in fact the tectonically active areas, ie where the plates move more or less by “rubbing” or ” colliding” with respect to each other, thus generating interplate earthquakes . More rarely, earthquakes occur far from the border areas between plates, due to tectonic rearrangements. Localized and less intense earthquakes can be recorded in volcanic areas due to the movement of magmatic masses in depth.
According to the plate tectonics model, the movement of plates is slow, constant and imperceptible (unless with special tools), and shapes and distorts rocks both above and below ground. However, in some moments and in some areas, due to the internal forces (pressures, tensions and frictions ) between the rock masses, these modeling stops and the surface involved accumulates tension and energy for tens or hundreds of years until, upon reaching of the breaking load , the accumulated energy is sufficient to overcome the resistant forces causing the sudden and sudden displacement of the rock mass involved.This sudden movement, which in a few seconds releases energy accumulated for tens or hundreds of years, thus generates seismic waves and the associated earthquake.
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- Description
An earthquake (or earthquake) originates when the collision between two crustal plates causes a rapid vibration of the earth’s crust capable of releasing very high quantities of energy, regardless of the effects it causes. Thousands of earthquakes occur on Earth every day : experimentally it is observed that the majority of earthquakes in the world, as well as volcanic eruptions, occur along the so-called Pacificring of fire , the oceanic ridges and the subduction zones or boundaries between tectonic plates and therefore often affects the oceanic crust as a trigger or fracturing zone. Only a few dozen are perceived by the population and most of the latter cause little or no harm. The average duration of an earthquake is well below 30seconds; for the strongest earthquakes, however, it can last up to a few minutes.
Earth seismic zone map
The source of the earthquake is generally distributed in an internal area of the earth’s crust. In the case of the most devastating earthquakes this can even have an extension of the order of a thousand kilometres, but it is ideally possible to identify a precise point from which the seismic waves originated: this is called the “hypocentre” and this is where the movement from the pre-existing fracture (fault ) or its sudden generation. The vertical projection of the hypocenter onto the earth’s surface is called the ” epicenter “, and is where the greatest damage usually occurs. The elastic waves that propagate during an earthquake are of different types and in some cases they can result in a predominantly horizontal movement (wave shock) or vertical movement of the ground (heavy shock).
Some earthquakes occur or are preceded by more or less long and intense seismic swarms ( foreshocks ), characterized by several earthquakes repeated over time and particularly limited in a specific area, while others occur immediately and suddenly with one or more main shocks ( main shock ); another form are seismic sequences , each characterized by several earthquakes released in close succession and not limited to a given area. Earthquakes of greater magnitude are usually accompanied by secondary events (not necessarily less destructive) that follow the main shock and are defined as aftershocks (aftershocks , often incorrectly referred to as aftershocks ). When several events occur simultaneously or nearly so, they may be induced earthquakes (the earthquake triggers the fracturing of other rock that was already close to the critical breaking point).
Furthermore, an earthquake can be accompanied by loud noises that can recall roars, rumbles, thunder, sequences of shots, etc.: these sounds are due to the passage of seismic waves to the atmosphere and are more intense near the epicenter. The most seismic countries and areas in the world are Afghanistan , Albania , Armenia , Azerbaijan , Chile , Colombia , Costa Rica , Croatia , Ecuador , Philippines , Georgia , Japan , Greece , Haiti , Indonesia, Iran , Iceland , Italy , Mexico , Montenegro , Nepal , New Zealand , Panama , Papua New Guinea , Peru , Polynesia , the Dominican Republic , United States of America , Taiwan , Tajikistan and Turkey .
Diagram illustrating the generation of an earthquake according to the theory of elastic rebound, due to the release of energy due to the relative movement of rock masses along a fault.
In general, earthquakes are caused by sudden movements of rock masses (more or less large) within the earth’s crust. The earth’s surface is in fact in slow but constant motion (see plate tectonics) and earthquakes occur when the resulting tension accumulated by mechanical stresses exceeds the ability or resistance of the rock material to bear it, ie exceeds the so-called tensile strength . This condition most often occurs at tectonic plate boundaries. Seismic events that occur at plate boundaries are called tectonic earthquakes, the less frequent ones that occur inside the plates of the lithosphere are instead called intraplate earthquakes.
Almost all the earthquakes that occur on the earth’s surface are therefore concentrated in very specific areas, ie near the boundaries between one tectonic plate and another: these are in fact the tectonically active areas, ie where the plates move more or less slowly and suddenly compared to each other. According to plate tectonics, the surface of the Earth is in fact modeled as if it were composed of about a dozen large tectonic plates that move very slowly, due to the convection currents of the earth’s mantle under the earth’s crust.. Because they do not all move in the same direction, plates often collide directly by sliding sideways along the edge of one another (transform fault). In general, the movement of the plates is slow, imperceptible (if not with special tools) and constant; however in some moments and in some areas, the movement stops and the area involved accumulates energy for decades or centuries until the so-called breaking load is reached , when due to internal forces, ie the balance between pressures, tensions and frictions between the rock masses, these movements occur suddenly and suddenly releasing the accumulated energy and thus developing an earthquake.
The disposition of the seismic zones is mostly located along the margins between the tectonic plates (e.g belt of fire) and in particular along the abyssal trenches ( subduction zones ), where the sinking of the oceanic crust below other portions of crust leads to melting by friction of part of the rocky contact zone, or along the oceanic ridges where the magma of the earth’s mantle it rises to the surface through the fractures of the oceanic crust and once solidified it “welds” the plates themselves; the earthquakes along the ridges are therefore the effect of the sudden breakage of these welds when a certain level of mechanical stress is reached. In these areas seismic phenomena are often also associated with volcanism due to the concomitance of the tectonic forces involved and for this reason volcanic eruptions are often preceded by earthquakes.
It is therefore assumed that the displacement of the plates is the triggering mechanism of earthquakes. The secondary cause is the magmatic movement inside a volcano, which can indicate an imminent eruption together with the characteristic tremor . In very rare cases, earthquakes have been associated with the accumulation of large bodies of water in dam reservoirs, such as at the Kariba Damin Zambia,Africa, and with the injection. Such earthquakes occur because the resistance of the earth’s crust can be changed by fluid pressure.
Earthquakes occur on fractures in the earth’s crust known as seismic faults, i.e where the mechanical stress induced by tectonic movements accumulates. The boundaries between tectonic plates are not in fact defined by a simple break or discontinuity, but this often manifests itself through a system of multiple fractures, which can be independent of each other and even parallel for some stretches, which represent the faults. There are different types of faults divided according to the relative movement of the tectonic portions adjacent to the fracture itself and the angle of the fault plane. The process of formation and development of the fault, as well as of the earthquakes themselves, is known as faulting and can be studied through analysis techniques typical off racture mechanics.
The intensity of an earthquake depends on the amount of energy accumulated in the breaking point which in turn depends in general on the type of rocks involved in the accumulation process, i.e on their breaking load, on the type of stress or internal stress and on the type offault.
seismic waves
There are three types of seismic waves:
Compression or longitudinal waves (P)
Longitudinal waves cause rock particles to oscillate in the same direction as the wave propagation. They therefore generate successive “compressions” and “rarefactions” in which they propagate. The speed of propagation depends on the elastic characteristics of the material and its density; however, they generally travel at a speed of between 4 and 8 km/s. Because P waves propagate fastest, they are also the first (P = Primary) to reach seismometers, and thus to be recorded by seismographs. These seismic waves pass through all types of matter longitudinally: solids, liquids and gases.
Shear or transverse waves (S)
Seismic wave pattern (body waves and surface waves)
S waves, i.e. “secondary” waves, propagate only in solids perpendicular to their direction of propagation ( shear waves ). They are slower than P waves, traveling in the earth’s crust with a speed between 2 and 4 km/s. S-waves cannot travel through fluids and gases because they do not resist shear. Unlike P waves, S waves do not cause changes in loudness.
Surface waves (R and L)
Surface waves, contrary to what one might think, do not occur in the epicenter , but only at a certain distance from it. These waves are the result of the combination of P waves and S waves, and are therefore very complex.
Rayleigh waves, also called R waves, move particles in elliptical orbits in a vertical plane along the direction of propagation, as happens for waves in water.
Love waves, also known as L waves, instead move particles transversely to the direction of propagation (like S waves), but only on the horizontal plane.
All seismic waves are subject to attenuation with distance as a function of the characteristics of the propagation medium.
Detection and measurement
Seismograph
Seismic waves are detectable and measurable through particular instruments called seismographs , commonly used by seismologists , and viewable on seismograms ; the cross-processing of data from several seismographs scattered over an area at a certain distance from the earthquake makes it possible to fairly accurately estimate the epicenter, the hypocenter and the intensity of the earthquake; the latter can be evaluated through the so-called seismic scales , mainly the Richter scale , the Mercalli scale and the seismic moment magnitude scale .
The recognition of the arrival orientation of the tremors along the three reference planes, and the understanding of whether the first arrival of the tremor was of a compressive or expansive type, allows us to determine the focal mechanism of the tremor and therefore to understand what type of fault originated the earthquake.
The tectonic displacement of the earth’s crust in the three spatial coordinates following a strong earthquake can be accurately measured through remote sensing techniques such as geodetic surveys and radar-satellite interferometry via SAR in the entire affected area starting from the epicenter.
Effects and damage
Damage caused by an earthquake
The misalignment of the columns of the Temple of Hephaestusis attributed to the effect on the building of earthquakes that occurred in the past.
Earthquakes are by far the most powerful natural events on Earth; earthquakes can release more energy than thousands of atomic bombs in seconds , usually measured in terms of seismic moment. In this regard, it is enough to think that an earthquake is able to move rock volumes of hundreds of cubic kilometers in a few seconds.
As a result of this, earthquakes can cause serious destruction and high loss of human life through a series of destructive agents, the main one being the violent movement of the ground – which can occur with accelerations that can be simplified into horizontal and vertical [3 ] – with consequent stress on the building structures in place (buildings, bridges, etc.), possibly also accompanied by other secondary effects such as floods(for example the failure of dams), subsidence of the land (landslides, landslides or liquefaction),fires or leakages of hazardous materials; if the earthquake occurs under the ocean or sea surface or near the coast line can generate tsunamis [4] . In each earthquake, one or more of these agents can therefore contribute to causing further serious damage and victims. The effects of an earthquake can be exalted and appear in a variable manner even within small distances due to phenomena of amplification of the seismic motion, due to local geological conditions, which go by the name of local seismic response or site effects.
Stronger earthquakes, such as the one in Japan of March 11,2011(2011Tōhoku earthquake ), can also move the geographic north pole by a few centimeters (this, for example, moved it by about 10 cm) due to the elasticity of the Earth’s crust. At the local level, the effects of an earthquake can also vary significantly as a result of the so-called site effects.
The single event with the most fatalities in the last thousand years is the 1556Shaanxi earthquake(China) , with a magnitude of 8.3, which killed 830,000 people . The one with the highest magnitude, however, is the 1960 Valdivia ( Chile ) earthquake , which reached a magnitude of 9.5.
The strongest earthquakes of the last two centuries
The strongest earthquakes of the 20th and 21st centuries
Ranking by magnitude. According to what is reported on the USGS website they are the following.
Valdivia,Chile- magnitude 9.5 – May 22,1960
Sumatra,Indonesia- magnitude 9.1 – December 26,2004
Tōhoku,Japan- magnitude 9.0 – March 11,2011
Kamchatka,Russia- magnitude 9.0 – November 4,1952
Concepción,Chile- magnitude 8.8 – February 27,2010
Rat Islands,Alaska- magnitude 8.7 – February 4,1965
Sumatra,Indonesia- magnitude 8.7 – March 28,2005
Sumatra,Indonesia- magnitude 8.6 – April 11,2012
Haiyuan,China- magnitude 8.6 – December 16,1920
Assam,Tibet- magnitude 8.6 – August 15,1950
Andreanof Islands,Alaska- magnitude 8.6 – March 9,1957
Atacama Region,Chile- magnitude 8.5 – November 11,1922
Kamchatka Peninsula,Russia- magnitude 8.5 – February 3,1923
Banda Sea,Indonesia- magnitude 8.5 – February 1,1938
Kuril Islands,Russia- magnitude 8.5 – October 13,1963
Sumatra,Indonesia- magnitude 8.5 – September 12,2007
Arequipa,Camaná,Peru- magnitude 8.4 – June 23,2001
Coquimbo Region,Chile- magnitude 8.3 – September 17,2015
Mexico City,Mexico- magnitude 8.3 – September 19,1985
Mexico City,Mexico- magnitude 8.2 – September 8,2017
Iquique,Chile- magnitude 8.2 – April 1,2014
Ica,Peru- magnitude 8.0 – August 15,2007
Loreto Region,Peru- magnitude 8.0 – May 26,2019
The distribution of the seismic moment in the earthquakes of the 20th and 21st centuries. Note the percentage of overall moment, expressed by the first three earthquakes with respect to the total.
The most disastrous earthquakes of the 20th and 21st centuries
Ranking based on the number of deaths declared [8] (the numbers are always to be considered approximate and almost always underestimated) .
Port-au-Prince,Haiti(2010) – 316 000 dead
Tangshan,China(1976) – 255,000 dead
North Sumatra,Indonesia(2004) – 230,000 dead
Haiyuan,China(1920) – 200,000 dead (effects-wise, this earthquake was rated the highest on the Mercalli scale, twelfth)
Qinghai,China(1927) – 200,000 dead
Kanto,Japan(1923) – 143,000 dead
Messina and Reggio Calabria,Italy(1908) – 120 000 dead
Ashgabat,Turkmenistan(1948) – 110 000 dead
Eastern Sichuan ,China(2008) – 88,000 dead
Muzzarrafad,PakistanandIndia(2005) – 86 000 dead
Gansu,China(1932) – 70,000 dead
Chimbote,Peru(1970) – 70,000 dead
Western Iran (1990) – 45 000 dead
Gulang,China(1927) – 41,000 killed
Avezzano,Italy(1915) – 33 000 dead
Erzincan,Turkey(1939) – 33,000 killed
Bam,Iran(2003) – 31 000 dead
Quetta,Pakistan(1935) – 30 000 dead
Chillán,Chile(1939) – 28,000 killed
Sendai,Japan(2011) – 27 000 dead (unconfirmed)
Spitak,Armenia(1988) – 25 000 dead
Guatemala(1976) – 23 000 dead
China(1974) – 20,000 dead
Gujarat,India(2001) – 20,000 dead
Kangra,India(1905) – 19 000 dead
Karamursel/Golyaka,Turkey(1999) – 17 000 dead
India, (1993) – 16 000 dead
Agadir,Morocco(1960) – 15,000 dead
Tabas,Iran(1978) – 15 000 dead
Qazvin,Iran(1962) – 12 500 dead
Qaratog,Tajikistan(1907) – 12,000 killed
Khait , Tajikistan ( 1949 ) – 12,000 killed
Bihar , India- Nepal ( 1934 ) – 11 000 dead
Fuyun , Xinjiang ( Sinkiang ), China ( 1931 ) – 10 000 dead
Dasht-e Bayaz , Iran ( 1968 ) – 10 000 dead
Tonghai , Yunnan , China ( 1970 ) – 10 000 dead
Strongest earthquakes by country
Some earthquakes, especially the strongest ones, are also accompanied, preceded or followed by unusual natural phenomena called seismic precursors such as: flashes or flashes ( telluric lights ); sudden changes in the magnetic , electric or local radioactivity field ( radon emission ); interference in radio communications; nervousness of animals; change in the level of groundwater or coastal waters; volcanic activity. All these manifestations have been confirmed by observations and testimonies and have been studied and partially confirmed by scientific research which has come to the explanation of each of them, even if, in the absence of unanimous consensus, they do not in fact constitute measures actually recognized and adopted in terms of forecasting.
The Haicheng earthquake of February 4, 1975 was historically the first and only earthquake predicted with such techniques [9] , but in that case the seismic precursors of a geological nature were so intense and regularly progressive as to leave no doubt about the proximity and imminence of the ‘event.
The correlations between the variations in the height of the water table and local gravity , as well as radon emissions , have also been studied since the 19th century , but unfortunately at the current state of knowledge models have not yet been elaborated which allow us to highlight signals useful for the effective prediction of an earthquake or its possible characteristics, intensity and spatiotemporal location.
Map of seismic risk in Northern Europe
In particular radon is formed from the radioactive decay of radium and being a noble gas it does not combine with other elements and chemical compounds ; therefore a large part of the radon that forms inside the rocks remains trapped in them. If movements, cracks, compressions and extensions of rocks suddenly occur, as occurs during or immediately before an earthquake, the radon contained in depth emerges on the earth’s surface, where it is already present in a certain concentration , increasing the local concentration with peaks sudden or so-called “draughts” [10]. In the hope of being able to create a short-term and reliable earthquake forecasting system, various studies are underway; for this research a network of radon detectors is used, suitably distributed on the surface of the interested areas.
The predictability of seismic phenomena has been the subject of discussions and controversies in Italy outside the scientific sphere, following the earthquake in L’Aquila of 6 April 2009 ; on the occasion of the tragic event, the press reported with emphasis the news according to which Giampaolo Giuliani (a non-graduated INAF laboratory technician who, during his spare time, carries out studies on earthquakes on his own), in the weeks preceding the earthquake, would have supported various hypotheses on the imminence of a disastrous shock, also causing some false alarms the occurrence of a seismic event would have been predicted, according to him, in March, broadly in that same region; he claimed to base his analysis on the sudden increase in radon emissions , however using forecasting instruments and methods that were not considered strictly valid by the scientific community.
The same topic in detail: Earthquake Engineering and Seismic Retrofitting .
Seismic isolator
If in practice the exact prediction of an earthquake is, at the current stage of scientific research, still far away, the most practicable and wise remedy against material and human damage caused by earthquakes is represented by active protection , i.e. by the use of effective techniques anti- seismic techniques of building construction typical of seismic engineering such as seismic isolation : these techniques at the current stage are able to minimize the damage of even extremely powerful earthquakes and are widely used in some of the most seismic areas in the world such as Japan .
To identify areas of significant seismic danger and consequent seismic risk , historical seismology studies, paleoseismology and seismic micro -zoning techniques are usually used, providing the relative risk maps, while to evaluate the effects of an earthquake, simulation techniques can be used (see earthquake simulation ).
Management with GDACS
In 2004, the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) and the European Commission established the Global Disaster Alert and Coordination System (GDACS), to improve and increase the effectiveness of the relief machine and humanitarian aid plans . [21] Born under the name of GDAS, it was initially used to replace the various existing monitoring and warning systems with a single multi-event IT platform, relating to earthquakes,
In a second step of project implementation, the monitoring system was integrated with the emergency management and intervention coordination system, known as OCHA Virtual OSOCC. This has made it possible to collect information on concrete and current risks and dangers almost in real time, promptly communicating the news to the operators involved in the interventions and to the civilian population, according to a multi-channel method (from the traditional telephone, to e-mail , to SMS , to the website). [22]
The GDACS system thus obtained has become capable of evaluating meteorological information with the economic and socio-demographic data of the predictably affected areas, in such a way as to perform an analysis not in exclusive terms of the probability of the event, but also of magnitude of the impact for the population and for other realities present in the surrounding area.
Studies and beliefs
In ancient Greece , Poseidon was considered the god of earthquakes as well as the sea. Its Roman correspondent was Neptune .
Among the Romans it was believed that earthquakes were caused by the energy of winds accumulating in caves, or by the ebb and flow of waters in the hollows of the earth. [23]
In 1626 the Italian Jesuit Niccolò Longobardi made a significant contribution to the scientific explanation of seismic phenomena with his Treatise on Earthquakes , a work written in Chinese.
The young Immanuel Kant , just thirty-one years old, when he learns of the Lisbon earthquake of 1 November 1755, publishes the first of his Writings on Earthquakes on 24 January 1756 where he will try to give a scientific guise to his reflections which he will extend in the course of his works also to moral considerations.
During the Cold War , P-waves were studied to keep countries conducting nuclear tests under control. Each of the two blocks studied the nuclear progress of the opposing block, thanks to the use of seismometers, to the point that nuclear tests (underground or in the atmosphere) were used by both the USA and the USSR as a sort of warning – or indirect communication – towards the enemy.
The Catholic Church venerates Sant’Emidio as protector from the earthquake.
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