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 to 1712) and Nicolas Lemery (1645 to 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]
From 1857, Robert Mallet laid the foundation of instrumental seismology and carried out seismological
experiments using explosives. He is also responsible for coining the word "seismology".[6]
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.[7]
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.[8]
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 behaviour of elastic
materials and in mathematics.[9]
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.[10]
In 1937, Inge Lehmann determined that within the earth's liquid outer core there is a solid inner core.[11]
By the 1960s, earth science had developed to the point where a comprehensive theory of the causation of
seismic events had come together in the now well-established theory of plate tectonics.

Types of seismic wave[edit]

Main article: Seismic wave

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[edit]
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 perpendicular motion, so S-waves only travel in solids.[12]

Surface waves[edit]
The two main surface wave types are Rayleigh waves, which have some compressional motion, and Love
waves, which do not. Rayleigh waves result from the interaction of vertically polarized P- and S-waves that
satisfy the boundary conditions on the surface. Love waves can exist in the presence of a subsurface layer,
and are only formed by horizontally polarized S-waves. Surface waves travel more slowly than P-waves and S-
waves; however, because they are guided by the Earth's surface and their energy is thus trapped near the
surface, they can be much stronger than body waves, and can be 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.[12]

Normal modes[edit]
See also: Free oscillations of the Earth
Both body and surface waves are traveling waves; however, large earthquakes can also make the Earth "ring"
like a bell. This ringing is a mixture of normal modes with discrete frequencies and periods of an hour or
shorter. Motion caused by a large earthquake can be observed for up to a month after the event.[12] 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[edit]
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, 1906 San Francisco earthquake, the 1964 Alaska earthquake, the 2004 Sumatra-Andaman
earthquake, and the 2011 Great East Japan earthquake.

Controlled seismic sources[edit]

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.[13]

Detection of seismic waves[edit]

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.
underwater A
complete instrument package that records seismic signals is called a seismograph.. Networks of seismographs
continuously record ground motions arou aroundnd 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
smic waves travel considerably faster than tsunami waves. Seismometers also record
signals from non-earthquake
earthquake sources ranging from explosions (nuclear and chemical), to local noise from
wind[14] 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.
ocean meteor strikes with energies as high as 4.2 × 1013 J (equivalent to that released by an explosion
Above-ocean
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
long motivation
for the global seismographic monitoring has been for the detection and study of nuclear testing.testing

Mapping the earth's interior[edit]

Main article: Earth's interior

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
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
waves do not pass through liquids, the
liquid core causes a "shadow" on the side of the planet opposite the earthquake where no direct
di S-waves are
observed. In addition, P-waves
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.[15]

Seismology and society[edit]

Earthquake prediction[edit]
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. The indictment has been widely perceived[by whom?] as an indictment for failing to predict the
earthquake and has drawn condemnation from the American Association for the Advancement of Science and
the American Geophysical Union. 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 preparedness.[16]

Engineering seismology[edit]
Engineering seismology is the study and application of seismology for engineering purposes.[17] It generally
applied to the branch of seismology that deals with the assessment of the seismic hazard of a site or region for
the purposes of earthquake engineering. It is, therefore, a link between earth science and civil
engineering.[18] There are two principal components of engineering seismology. Firstly, studying earthquake
history (e.g. historical[18] and instrumental catalogs[19] of seismicity) and tectonics[20] to assess
the earthquakesthat could occur in a region and their characteristics and frequency of occurrence. Secondly,
studying strong ground motions generated by earthquakes to assess the expected shaking from future
earthquakes with similar characteristics. These strong ground motions could either be observations
from accelerometers or seismometers or those simulated by computers using various techniques[21], which are
then often used to develop ground motion prediction equations[22] (or ground-motion models)[1].

Tools