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From Wikipedia, the free encyclopedia

Animation of tsunami triggered by the 2004 Indian Ocean earthquake

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Seismology (/saɪzˈmɒlədʒi, saɪs-/; from Ancient Greek σεισμός (seismós) meaning "earthquake" and -
λογία (-logía) meaning "study of") is the scientific study of earthquakes (or generally, quakes) and
the generation and propagation of elastic waves through the Earth or other planetary bodies. It also
includes studies of earthquake environmental effects such as tsunamis as well as diverse seismic
sources such as volcanic, tectonic, glacial, fluvial, oceanic microseism, atmospheric, and artificial
processes such as explosions and human activities. A related field that uses geology to infer
information regarding past earthquakes is paleoseismology. A recording of Earth motion as a
function of time, created by a seismograph is called a seismogram. A seismologist is a scientist works
in basic or applied seismology.

History

Scholarly interest in earthquakes can be traced back to antiquity. Early speculations on the natural
causes of earthquakes were included in the writings of Thales of Miletus (c. 585 BCE), Anaximenes of
Miletus (c. 550 BCE), Aristotle (c. 340 BCE), and Zhang Heng (132 CE).

In 132 CE, Zhang Heng of China's Han dynasty designed the first known seismoscope.[1][2][3]

In the 17th century, Athanasius Kircher argued that earthquakes were caused by the movement of
fire within a system of channels inside the Earth. Martin Lister (1638–1712) and Nicolas Lemery
(1645–1715) proposed that earthquakes were caused by chemical explosions within the Earth.[4]

The Lisbon earthquake of 1755, coinciding with the general flowering of science in Europe, set in
motion intensified scientific attempts to understand the behaviour and causation of earthquakes.
The earliest responses include work by John Bevis (1757) and John Michell (1761). Michell
determined that earthquakes originate within the Earth and were waves of movement caused by
"shifting masses of rock miles below the surface".[5]

In response to a series of earthquakes near Comrie in Scotland in 1839, a committee was formed in
the United Kingdom in order to produce better detection methods for earthquakes. The outcome of
this was the production of one of the first modern seismometers by James David Forbes, first
presented in a report by David Milne-Home in 1842.[6] This seismometer was an inverted pendulum,
which recorded the measurements of seismic activity through the use of a pencil placed on paper
above the pendulum. The designs provided did not prove effective, according to Milne's reports.[6]

From 1857, Robert Mallet laid the foundation of modern instrumental seismology and carried out
seismological experiments using explosives. He is also responsible for coining the word
"seismology."[7]

In 1889 Ernst von Rebeur-Paschwitz recorded the first teleseismic earthquake signal (an earthquake
in Japan recorded at Pottsdam Germany).[8]

In 1897, Emil Wiechert's theoretical calculations led him to conclude that the Earth's interior consists
of a mantle of silicates, surrounding a core of iron.[9]

In 1906 Richard Dixon Oldham identified the separate arrival of P-waves, S-waves and surface waves
on seismograms and found the first clear evidence that the Earth has a central core.[10]

In 1909, Andrija Mohorovičić, one of the founders of modern seismology,[11][12][13] discovered

and defined the Mohorovičić discontinuity.[14] Usually referred to as the "Moho discontinuity" or
the "Moho," it is the boundary between the Earth's crust and the mantle. It is defined by the distinct
change in velocity of seismological waves as they pass through changing densities of rock.[15]

In 1910, after studying the April 1906 San Francisco earthquake, Harry Fielding Reid put forward the
"elastic rebound theory" which remains the foundation for modern tectonic studies. The
development of this theory depended on the considerable progress of earlier independent streams
of work on the behavior of elastic materials and in mathematics.[16]

An early scientific study of aftershocks from a destructive earthquake came after the January 1920
Xalapa earthquake. An 80 kg (180 lb) Wiechert seismograph was brought to the Mexican city of
Xalapa by rail after the earthquake. The instrument was deployed to record its aftershocks. Data
from the seismograph would eventually determine that the mainshock was produced along a
shallow crustal fault.[17]
In 1926, Harold Jeffreys was the first to claim, based on his study of earthquake waves, that below
the mantle, the core of the Earth is liquid.[18]

In 1937, Inge Lehmann determined that within Earth's liquid outer core there is a solid inner core.
[19]

In 1950, Michael S. Longuet-Higgins elucidated the ocean processes responsible for the global
background seismic microseism.[20]

By the 1960s, Earth science had developed to the point where a comprehensive theory of the
causation of seismic events and geodetic motions had come together in the now well-established
theory of plate tectonics.[21]

Types of seismic wave

Main article: Seismic wave

Three lines with frequent vertical excursions.

Seismogram records showing the three components of ground motion. The red line marks the first
arrival of P-waves; the green line, the later arrival of S-waves.

Seismic waves are elastic waves that propagate in solid or fluid materials. They can be divided into
body waves that travel through the interior of the materials; surface waves that travel along surfaces
or interfaces between materials; and normal modes, a form of standing wave.

Body waves

There are two types of body waves, pressure waves or primary waves (P-waves) and shear or
secondary waves (S-waves). P-waves are longitudinal waves that involve compression and expansion
in the direction that the wave is moving and are always the first waves to appear on a seismogram as
they are the fastest moving waves through solids. S-waves are transverse waves that move
perpendicular to the direction of propagation. S-waves are slower than P-waves. Therefore, they
appear later than P-waves on a seismogram. Fluids cannot support transverse elastic waves because
of their low shear strength, so S-waves only travel in solids.[22]

Surface waves

Surface waves are the result of P- and S-waves interacting with the surface of the Earth. These waves
are dispersive, meaning that different frequencies have different velocities. The two main surface
wave types are Rayleigh waves, which have both compressional and shear motions, and Love waves,
which are purely shear. Rayleigh waves result from the interaction of P-waves and vertically
polarized S-waves with the surface and can exist in any solid medium. Love waves are formed by
horizontally polarized S-waves interacting with the surface, and can only exist if there is a change in
the elastic properties with depth in a solid medium, which is always the case in seismological
applications. Surface waves travel more slowly than P-waves and S-waves because they are the
result of these waves traveling along indirect paths to interact with Earth's surface. Because they
travel along the surface of the Earth, their energy decays less rapidly than body waves (1/distance2
vs. 1/distance3), and thus the shaking caused by surface waves is generally stronger than that of
body waves, and the primary surface waves are often thus the largest signals on earthquake
seismograms. Surface waves are strongly excited when their source is close to the surface, as in a
shallow earthquake or a near-surface explosion, and are much weaker for deep earthquake sources.
[22]

Normal modes

See also: Free oscillations of the Earth

Both body and surface waves are traveling waves; however, large earthquakes can also make the
entire Earth "ring" like a resonant bell. This ringing is a mixture of normal modes with discrete
frequencies and periods of approximately an hour or shorter. Normal mode motion caused by a very
large earthquake can be observed for up to a month after the event.[22] The first observations of
normal modes were made in the 1960s as the advent of higher fidelity instruments coincided with
two of the largest earthquakes of the 20th century the 1960 Valdivia earthquake and the 1964
Alaska earthquake. Since then, the normal modes of the Earth have given us some of the strongest
constraints on the deep structure of the Earth.

Earthquakes

Main articles: Earthquake and Lists of earthquakes

One of the first attempts at the scientific study of earthquakes followed the 1755 Lisbon earthquake.
Other notable earthquakes that spurred major advancements in the science of seismology include
the 1857 Basilicata earthquake, the 1906 San Francisco earthquake, the 1964 Alaska earthquake, the
2004 Sumatra-Andaman earthquake, and the 2011 Great East Japan earthquake.

Controlled seismic sources

See also: Reflection seismology

Seismic waves produced by explosions or vibrating controlled sources are one of the primary
methods of underground exploration in geophysics (in addition to many different electromagnetic
methods such as induced polarization and magnetotellurics). Controlled-source seismology has been
used to map salt domes, anticlines and other geologic traps in petroleum-bearing rocks, faults, rock
types, and long-buried giant meteor craters. For example, the Chicxulub Crater, which was caused by
an impact that has been implicated in the extinction of the dinosaurs, was localized to Central
America by analyzing ejecta in the Cretaceous–Paleogene boundary, and then physically proven to
exist using seismic maps from oil exploration.[23]
Detection of seismic waves

Installation for a temporary seismic station, north Iceland highland.

Seismometers are sensors that detect and record the motion of the Earth arising from elastic waves.
Seismometers may be deployed at the Earth's surface, in shallow vaults, in boreholes, or
underwater. A complete instrument package that records seismic signals is called a seismograph.
Networks of seismographs continuously record ground motions around the world to facilitate the
monitoring and analysis of global earthquakes and other sources of seismic activity. Rapid location of
earthquakes makes tsunami warnings possible because seismic waves travel considerably faster than
tsunami waves. Seismometers also record signals from non-earthquake sources ranging from
explosions (nuclear and chemical), to local noise from wind[24] or anthropogenic activities, to
incessant signals generated at the ocean floor and coasts induced by ocean waves (the global
microseism), to cryospheric events associated with large icebergs and glaciers. Above-ocean meteor
strikes with energies as high as 4.2 × 1013 J (equivalent to that released by an explosion of ten
kilotons of TNT) have been recorded by seismographs, as have a number of industrial accidents and
terrorist bombs and events (a field of study referred to as forensic seismology). A major long-term
motivation for the global seismographic monitoring has been for the detection and study of nuclear
testing.

Mapping Earth's interior

Main article: Earth's interior

Diagram with concentric shells and curved paths

Seismic velocities and boundaries in the interior of the Earth sampled by seismic waves

Because seismic waves commonly propagate efficiently as they interact with the internal structure of
the Earth, they provide high-resolution noninvasive methods for studying the planet's interior. One
of the earliest important discoveries (suggested by Richard Dixon Oldham in 1906 and definitively
shown by Harold Jeffreys in 1926) was that the outer core of the earth is liquid. Since S-waves do not
pass through liquids, the liquid core causes a "shadow" on the side of the planet opposite the
earthquake where no direct S-waves are observed. In addition, P-waves travel much slower through
the outer core than the mantle.

Processing readings from many seismometers using seismic tomography, seismologists have
mapped the mantle of the earth to a resolution of several hundred kilometers. This has enabled
scientists to identify convection cells and other large-scale features such as the large low-shear-
velocity provinces near the core–mantle boundary.[25]

Seismology and society

Earthquake prediction

Main article: Earthquake prediction

Forecasting a probable timing, location, magnitude and other important features of a forthcoming
seismic event is called earthquake prediction. Various attempts have been made by seismologists
and others to create effective systems for precise earthquake predictions, including the VAN
method. Most seismologists do not believe that a system to provide timely warnings for individual
earthquakes has yet been developed, and many believe that such a system would be unlikely to give
useful warning of impending seismic events. However, more general forecasts routinely predict
seismic hazard. Such forecasts estimate the probability of an earthquake of a particular size affecting
a particular location within a particular time-span, and they are routinely used in earthquake
engineering.

Public controversy over earthquake prediction erupted after Italian authorities indicted six
seismologists and one government official for manslaughter in connection with a magnitude 6.3
earthquake in L'Aquila, Italy on April 5, 2009.[26] A report in Nature stated that the indictment was
widely seen in Italy and abroad as being for failing to predict the earthquake and drew
condemnation from the American Association for the Advancement of Science and the American
Geophysical Union.[26] However, the magazine also indicated that the population of Aquila do not
consider the failure to predict the earthquake to be the reason for the indictment, but rather the
alleged failure of the scientists to evaluate and communicate risk.[26] The indictment claims that, at
a special meeting in L'Aquila the week before the earthquake occurred, scientists and officials were
more interested in pacifying the population than providing adequate information about earthquake
risk and p