Sociobiology investigates social behaviors such as mating patterns, territorial fights, pack hunting, and the hive society of social insects. It argues that just as selection pressure led to animals evolving useful ways of interacting with the natural environment, so also it led to the genetic evolution of advantageous social behavior.
While the term "sociobiology" originated at least as early as the
1940s; the concept did not gain major recognition until the publication
of E. O. Wilson's book Sociobiology: The New Synthesis in 1975. The new field quickly became the subject of controversy. Critics, led by Richard Lewontin and Stephen Jay Gould, argued that genes played a role in human behavior, but that traits such as aggressiveness
could be explained by social environment rather than by biology.
Sociobiologists responded by pointing to the complex relationship
between nature and nurture.
Among sociobiologists, the controversy between laying weight to
different levels of selection was settled between D.S. Wilson and E.O.
Wilson in 2007.
Definition
E. O. Wilson defined sociobiology as "the extension of population biology and evolutionary theory to social organization".
Sociobiology is based on the premise that some behaviors (social
and individual) are at least partly inherited and can be affected by natural selection.
It begins with the idea that behaviors have evolved over time, similar
to the way that physical traits are thought to have evolved. It predicts
that animals will act in ways that have proven to be evolutionarily
successful over time. This can, among other things, result in the
formation of complex social processes conducive to evolutionary fitness.
The discipline seeks to explain behavior as a product of natural
selection. Behavior is therefore seen as an effort to preserve one's
genes in the population. Inherent in sociobiological reasoning is the
idea that certain genes or gene combinations that influence particular
behavioral traits can be inherited from generation to generation.
For example, newly dominant male lions often kill cubs in the pride that they did not sire. This behavior is adaptive because killing the cubs eliminates competition
for their own offspring and causes the nursing females to come into
heat faster, thus allowing more of his genes to enter into the
population. Sociobiologists would view this instinctual cub-killing
behavior as being inherited through the genes of successfully
reproducing male lions, whereas non-killing behavior may have died out
as those lions were less successful in reproducing.
The philosopher of biology Daniel Dennett suggested that the political philosopher Thomas Hobbes was the first proto-sociobiologist, arguing that in his 1651 book Leviathan Hobbes had explained the origins of morals in human society from an amoral sociobiological perspective.
The geneticist of animal behavior John Paul Scott coined the word sociobiology
at a 1948 conference on genetics and social behavior, which called for a
conjoint development of field and laboratory studies in animal behavior
research.
With John Paul Scott's organizational efforts, a "Section of Animal
Behavior and Sociobiology" of the Ecological Society of America was
created in 1956, which became a Division of Animal Behavior of the
American Society of Zoology in 1958. In 1956, E. O. Wilson
came in contact with this emerging sociobiology through his PhD student
Stuart A. Altmann, who had been in close relation with the participants
to the 1948 conference. Altmann developed his own brand of sociobiology
to study the social behavior of rhesus macaques, using statistics, and
was hired as a "sociobiologist" at the Yerkes Regional Primate Research
Center in 1965.
Wilson's sociobiology is different from John Paul Scott's
or Altmann's, insofar as he drew on mathematical models of social
behavior centered on the maximization of the genetic fitness by W. D. Hamilton, Robert Trivers, John Maynard Smith, and George R. Price.
The three sociobiologies by Scott, Altmann and Wilson have in common to
place naturalist studies at the core of the research on animal social
behavior and by drawing alliances with emerging research methodologies,
at a time when "biology in the field" was threatened to be made
old-fashioned by "modern" practices of science (laboratory studies,
mathematical biology, molecular biology).
Once a specialist term, "sociobiology" became widely known in 1975 when Wilson published his book Sociobiology: The New Synthesis,
which sparked an intense controversy. Since then "sociobiology" has
largely been equated with Wilson's vision. The book pioneered and
popularized the attempt to explain the evolutionary mechanics behind
social behaviors such as altruism, aggression, and nurturance, primarily in ants (Wilson's own research specialty) and other Hymenoptera,
but also in other animals. However, the influence of evolution on
behavior has been of interest to biologists and philosophers since soon
after the discovery of evolution itself. Peter Kropotkin's Mutual Aid: A Factor of Evolution,
written in the early 1890s, is a popular example. The final chapter of
the book is devoted to sociobiological explanations of human behavior,
and Wilson later wrote a Pulitzer Prize winning book, On Human Nature, that addressed human behavior specifically.
Edward H. Hagen writes in The Handbook of Evolutionary Psychology
that sociobiology is, despite the public controversy regarding the
applications to humans, "one of the scientific triumphs of the twentieth
century." "Sociobiology is now part of the core research and curriculum
of virtually all biology departments, and it is a foundation of the
work of almost all field biologists.
" Sociobiological research on nonhuman organisms has increased
dramatically and continuously in the world's top scientific journals
such as Nature and Science. The more general term behavioral ecology is commonly substituted for the term sociobiology in order to avoid the public controversy.
Theory
Sociobiologists maintain that human behavior,
as well as nonhuman animal behavior, can be partly explained as the
outcome of natural selection. They contend that in order to fully
understand behavior, it must be analyzed in terms of evolutionary
considerations.
Natural selection
is fundamental to evolutionary theory. Variants of hereditary traits
which increase an organism's ability to survive and reproduce will be
more greatly represented in subsequent generations, i.e., they will be
"selected for". Thus, inherited behavioral mechanisms that allowed an organism
a greater chance of surviving and/or reproducing in the past are more
likely to survive in present organisms. That inherited adaptive
behaviors are present in nonhuman animal species has been multiply demonstrated by biologists, and it has become a foundation of evolutionary biology.
However, there is continued resistance by some researchers over the
application of evolutionary models to humans, particularly from within
the social sciences, where culture has long been assumed to be the
predominant driver of behavior.
Sociobiology is based upon two fundamental premises:
Certain behavioral traits are inherited,
Inherited behavioral traits have been honed by natural selection.
Therefore, these traits were probably "adaptive" in the environment in
which the species evolved.
Sociobiology uses Nikolaas Tinbergen's four categories of questions
and explanations of animal behavior. Two categories are at the species
level; two, at the individual level. The species-level categories (often
called "ultimate explanations") are
the function (i.e., adaptation) that a behavior serves and
the evolutionary process (i.e., phylogeny) that resulted in this functionality.
The individual-level categories (often called "proximate explanations") are
the development of the individual (i.e., ontogeny) and
Sociobiologists are interested in how behavior can be explained
logically as a result of selective pressures in the history of a
species. Thus, they are often interested in instinctive, or intuitive
behavior, and in explaining the similarities, rather than the
differences, between cultures. For example, mothers within many species
of mammals – including humans – are very protective of their offspring.
Sociobiologists reason that this protective behavior likely evolved
over time because it helped the offspring of the individuals which had
the characteristic to survive. This parental protection would increase
in frequency in the population. The social behavior is believed to have
evolved in a fashion similar to other types of nonbehavioral adaptations, such as a coat of fur, or the sense of smell.
Individual genetic advantage fails to explain certain social
behaviors as a result of gene-centred selection. E.O. Wilson argued that
evolution may also act upon groups. The mechanisms responsible for group selection employ paradigms and population statistics borrowed from evolutionary game theory.
Altruism is defined as "a concern for the welfare of others". If
altruism is genetically determined, then altruistic individuals must
reproduce their own altruistic genetic traits for altruism to survive,
but when altruists lavish their resources on non-altruists at the
expense of their own kind, the altruists tend to die out and the others
tend to increase. An extreme example is a soldier losing his life trying
to help a fellow soldier. This example raises the question of how
altruistic genes can be passed on if this soldier dies without having
any children.
Within sociobiology, a social behavior is first explained as a sociobiological hypothesis by finding an evolutionarily stable strategy
that matches the observed behavior. Stability of a strategy can be
difficult to prove, but usually, it will predict gene frequencies. The
hypothesis can be supported by establishing a correlation between the
gene frequencies predicted by the strategy, and those expressed in a
population.
Altruism between social insects
and littermates has been explained in such a way. Altruistic behavior,
behavior that increases the reproductive fitness of others at the
apparent expense of the altruist, in some animals has been correlated to
the degree of genome shared between altruistic individuals. A quantitative description of infanticide by male harem-mating animals when the alpha male is displaced as well as rodent female infanticide and fetal resorption
are active areas of study. In general, females with more bearing
opportunities may value offspring less, and may also arrange bearing
opportunities to maximize the food and protection from mates.
An important concept in sociobiology is that temperament traits exist in an ecological balance. Just as an expansion of a sheep population might encourage the expansion of a wolf
population, an expansion of altruistic traits within a gene pool may
also encourage increasing numbers of individuals with dependent traits.
Studies of human behavior genetics have generally found behavioral traits such as creativity, extroversion, aggressiveness, and IQ have high heritability.
The researchers who carry out those studies are careful to point out
that heritability does not constrain the influence that environmental or
cultural factors may have on those traits.
Various theorists have argued that in some environments criminal behavior might be adaptive. The evolutionary neuroandrogenic (ENA) theory, by sociologist/criminologist Lee Ellis,
posits that female sexual selection has led to increased competitive
behavior among men, sometimes resulting in criminality. In another
theory, Mark van Vugt
argues that a history of intergroup conflict for resources between men
have led to differences in violence and aggression between men and
women. The novelist Elias Canetti also has noted applications of sociobiological theory to cultural practices such as slavery and autocracy.
Support for premise
Genetic mouse mutants illustrate the power that genes exert on behavior. For example, the transcription factor FEV (aka Pet1), through its role in maintaining the serotonergic system in the brain, is required for normal aggressive and anxiety-like behavior.
Thus, when FEV is genetically deleted from the mouse genome, male mice
will instantly attack other males, whereas their wild-type counterparts
take significantly longer to initiate violent behavior. In addition, FEV
has been shown to be required for correct maternal behavior in mice,
such that offspring of mothers without the FEV factor do not survive
unless cross-fostered to other wild-type female mice.
A genetic basis for instinctive behavioral traits among non-human
species, such as in the above example, is commonly accepted among many
biologists; however, attempting to use a genetic basis to explain
complex behaviors in human societies has remained extremely
controversial.
Reception
Steven Pinker argues that critics have been overly swayed by politics and a fear of biological determinism, accusing among others Stephen Jay Gould and Richard Lewontin of being "radical scientists", whose stance on human nature is influenced by politics rather than science, while Lewontin, Steven Rose and Leon Kamin, who drew a distinction between the politics and history of an idea and its scientific validity, argue that sociobiology fails on scientific grounds. Gould grouped sociobiology with eugenics, criticizing both in his book The Mismeasure of Man. When Napoleon Chagnon scheduled sessions on sociobiology at the 1976 American Anthropological Association
convention, other scholars attempted to cancel them with what Chagnon
later described as "Impassioned accusations of racism, fascism and
Nazism"; Margaret Mead's support caused the sessions to occur as scheduled.
Noam Chomsky has expressed views on sociobiology on several occasions. During a 1976 meeting of the Sociobiology Study Group, as reported by Ullica Segerstråle, Chomsky argued for the importance of a sociobiologically informed notion of human nature. Chomsky argued that human beings are biological organisms and ought to be studied as such, with his criticism of the "blank slate"
doctrine in the social sciences (which would inspire a great deal of
Steven Pinker's and others' work in evolutionary psychology), in his
1975 Reflections on Language. Chomsky further hinted at the possible reconciliation of his anarchist political views and sociobiology in a discussion of Peter Kropotkin's Mutual Aid: A Factor of Evolution,
which focused more on altruism than aggression, suggesting that
anarchist societies were feasible because of an innate human tendency to
cooperate.
Wilson has claimed that he had never meant to imply what ought to be, only what is the case. However, some critics have argued that the language of sociobiology readily slips from "is" to "ought", an instance of the naturalistic fallacy. Pinker has argued that opposition to stances considered anti-social, such as ethnic nepotism, is based on moral assumptions, meaning that such opposition is not falsifiable by scientific advances. The history of this debate, and others related to it, are covered in detail by Cronin (1993), Segerstråle (2000), and Alcock (2001).
Earthquake epicenters occur mostly along tectonic plate boundaries, especially on the Pacific Ring of Fire.Global plate tectonic movement
An earthquake – also called 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 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 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 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 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 the universality of such events beyond Earth.
Terminology
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 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 a particular location in the Earth is the average rate of seismic energy release per unit volume.
Earthquakes (M6.0+) since 1900 through 2017Earthquakes
of magnitude 8.0 and greater from 1900 to 2018. The apparent 3D volumes
of the bubbles are linearly proportional to their respective
fatalities.
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, 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 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 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 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 strainseismic 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.
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 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 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 the lengths along subducting
plate margins, and those along normal faults are even shorter.
Normal faults
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 the thickness of the brittle layer is only about six
kilometres (3.7 mi).
Reverse faults
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 the total seismic moment
released worldwide.
Strike-slip faults
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 the brittle crust. Thus, earthquakes with magnitudes much larger than 8 are not possible.
Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles
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 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 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.
Energy released
For every unit increase in magnitude, there is a roughly thirty-fold
increase in 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, 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, 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 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 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 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.
Rupture dynamics
A tectonic earthquake begins as an area of initial slip on the fault
surface that forms 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 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 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 earthquakes
Slow earthquake ruptures travel at unusually low velocities. A particularly dangerous form of slow earthquake 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.
Co-seismic overpressuring and effect of pore pressure
During an earthquake, high temperatures can develop at the fault
plane, increasing pore pressure and consequently vaporization of the
groundwater already contained within 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 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 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.
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 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 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 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 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 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 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, 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 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 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 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 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 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 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 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 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, 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 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 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.
Seismic waves
Every earthquake produces different types of seismic waves, which travel through rock with different velocities:
Propagation velocity of 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 the Earth. Also,
the depth of the hypocenter can be computed roughly.
P-wave speed
Upper crust soils and unconsolidated sediments: 2–3 km (1.2–1.9 mi) per second
Upper crust solid rock: 3–6 km (1.9–3.7 mi) per second
Lower crust: 6–7 km (3.7–4.3 mi) per second
Deep mantle: 13 km (8.1 mi) per second.
S-waves speed
Light sediments: 2–3 km (1.2–1.9 mi) per second
Earths crust: 4–5 km (2.5–3.1 mi) per second
Deep mantle: 7 km (4.3 mi) per second
Seismic wave arrival
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, 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 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 the epicenter, geographical region, distances to population
centers, location uncertainty, several parameters that are included in
USGS earthquake reports (number of stations reporting, number of
observations, etc.), and a unique event ID.
Although relatively slow seismic waves have traditionally been
used to detect earthquakes, scientists realized in 2016 that
gravitational measurement could provide instantaneous detection of
earthquakes, and confirmed this by analyzing gravitational records
associated with the 2011 Tohoku-Oki ("Fukushima") earthquake.
Effects
1755 copper engraving depicting Lisbon 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
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 the effects
of seismic energy focalization owing to the typical geometrical setting
of such 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 meters 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 that
are likely to break the ground surface within the life of the structure.
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.
For example, in the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.
Physical damage from an earthquake will vary depending on the
intensity of shaking in a given area and the type of population.
Underserved and developing communities frequently experience more severe
impacts (and longer lasting) from a seismic event compared to
well-developed communities. Impacts may include:
Injuries and loss of life
Damage to critical infrastructure (short and long-term)
Roads, bridges, and public transportation networks
Water, power, sewer and gas interruption
Communication systems
Loss of critical community services including hospitals, police, and fire
Collapse or destabilization (potentially leading to future collapse) of buildings
With these impacts and others, the aftermath may bring disease, a
lack of basic necessities, mental consequences such as panic attacks and
depression to survivors,
and higher insurance premiums. Recovery times will vary based on the
level of damage and the socioeconomic status of the impacted community.
Earthquakes can produce slope instability leading to landslides, a
major geological hazard. Landslide danger may persist while emergency
personnel is attempting rescue work.
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.
Tsunamis are long-wavelength, long-period sea waves produced by the
sudden or abrupt movement of large volumes of water—including when an
earthquake occurs at sea.
In the open ocean, the distance between wave crests can surpass 100
kilometres (62 mi), 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 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 may be secondary effects of earthquakes if dams are damaged.
Earthquakes may cause landslips to dam rivers, which collapse and cause
floods.
The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flooding 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 five million people.
Earthquake prediction is a branch of the science of seismology concerned with the specification of the time, location, and magnitude of future earthquakes within stated limits.
Many methods have been developed for predicting the time and place in
which earthquakes will occur. Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month. Popular belief holds earthquakes are preceded by earthquake weather, in the early morning.
While forecasting is usually considered to be a type of prediction, earthquake forecasting is often differentiated from earthquake prediction.
Earthquake forecasting is concerned with the probabilistic assessment
of general earthquake hazards, including the frequency and magnitude of
damaging earthquakes in a given area over years or decades. For well-understood faults the probability that a segment may rupture during the next few decades can be estimated.
Earthquake warning systems
have been developed that can provide regional notification of an
earthquake in progress, but before the ground surface has begun to move,
potentially allowing people within the system's range to seek shelter
before the earthquake's impact is felt.
The objective of earthquake engineering
is to foresee the impact of earthquakes on buildings, bridges, tunnels,
roadways, and other structures, and to design such structures to
minimize the risk of damage. Existing structures can be modified by seismic retrofitting to improve their resistance to earthquakes. Earthquake insurance can provide building owners with financial protection against losses resulting from earthquakes. Emergency management strategies can be employed by a government or organization to mitigate risks and prepare for consequences.
Artificial intelligence may help to assess buildings and plan precautionary operations. The Igor expert system
is part of a mobile laboratory that supports the procedures leading to
the seismic assessment of masonry buildings and the planning of
retrofitting operations on them. It has been applied to assess buildings
in Lisbon, Rhodes, and Naples.
Individuals can also take preparedness steps like securing water heaters
and heavy items that could injure someone, locating shutoffs for
utilities, and being educated about what to do when the shaking starts.
For areas near large bodies of water, earthquake preparedness
encompasses the possibility of a tsunami caused by a large earthquake.
In culture
Historical views
An image from a 1557 book depicting an earthquake in Italy in the 4th century BCE
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 (625–547 BCE) was the only documented person who believed
that earthquakes were caused by tension between the earth and water.
Other theories existed, including the Greek philosopher Anaxamines'
(585–526 BCE) beliefs that short incline episodes of dryness and wetness
caused seismic activity. The Greek philosopher Democritus (460–371 BCE)
blamed water in general for earthquakes. Pliny the Elder called earthquakes "underground thunderstorms".
Mythology and religion
In Norse mythology, earthquakes were explained as the violent struggle of the god Loki. When Loki, god of mischief and strife, murdered Baldr,
god of beauty and light, he was punished by being bound in a cave with a
poisonous serpent placed above his head dripping venom. Loki's wife Sigyn
stood by him with a bowl to catch the poison, but whenever she had to
empty the bowl the poison dripped on Loki's face, forcing him to jerk
his head away and thrash against his bonds, which caused the earth to
tremble.
In Greek mythology, Poseidon was the cause and god of earthquakes. When he was in a bad mood, he struck the ground with a trident, causing earthquakes 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.
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. For this reason, stories about earthquakes generally begin with the disaster and focus on its immediate aftermath, as in Short Walk to Daylight (1972), The Ragged Edge (1968) or Aftershock: Earthquake in New York (1999). A notable example is Heinrich von Kleist's classic novella, The Earthquake in Chile, which describes the destruction of Santiago in 1647. Haruki Murakami's short fiction collection After the Quake depicts the consequences of the Kobe earthquake of 1995.
The most popular single earthquake in fiction is the hypothetical "Big One" expected of California's San Andreas Fault someday, as depicted in the novels Richter 10 (1996), Goodbye California (1977), 2012 (2009), and San Andreas (2015), among other works. Jacob M. Appel's widely anthologized short story, A Comparative Seismology, features a con artist who convinces an elderly woman that an apocalyptic earthquake is imminent.
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, and loss of essential
supplies and services to maintain survival.
Particularly for children, the clear availability of caregiving adults
who can protect, nourish, and clothe them in the aftermath of the
earthquake and help them make sense of what has befallen them has been
shown to be 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, recently observed in the 2010 Haiti earthquake,
it is also believed to be important not to pathologize the reactions to
loss and displacement or disruption of governmental administration and
services, but rather to validate the reactions to support constructive
problem-solving and reflection.
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