Seismology is the scientific study of earthquakes and the propagation of
elastic waves through the Earth or through other planet-like bodies. The
field also includes studies of earthquake environmental effects, such
as tsunamis as well as diverse seismic sources such as volcanic, tectonic,
oceanic, atmospheric, and artificial processes. A related field that uses
geology to infer information regarding past earthquakes is paleoseismology. A
recording of earth motion as a function of time is called a seismogram. A
seismologist is a scientist who does research in 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,
Anaximenes of Miletus, Aristotle and Zhang Heng.
In 132 CE, Zhang Heng of China's Han dynasty designed the first known
seismoscope. In 1664, Athanasius Kircher argued that
earthquakes were caused by the movement of fire within a system of channels
inside the Earth. In 1703, Martin Lister and Nicolas
Lemery proposed that earthquakes were caused by chemical explosions within the
earth. 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 and John
Michell. Michell determined that earthquakes originate within the Earth
and were waves of movement caused by "shifting masses of rock miles below the
surface." From 1857, Robert Mallet laid the
foundation of instrumental seismology and carried out seismological
experiments using explosives. 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.
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. In 1910, after studying the 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. In 1926, Harold Jeffreys was the first
to claim, based on his study of earthquake waves, that below the crust,
the core of the Earth is liquid. In 1937, Inge Lehmann determined that
within the earth's liquid outer core there is a solid inner core.
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 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, P-waves and S-waves. Pressure waves or
Primary waves, are longitudinal waves that involve compression and rarefaction
in the direction that the wave is traveling. P-waves are the fastest waves
in solids and are therefore the first waves to appear on a seismogram.
S-waves, also called shear or secondary waves, are transverse waves that involve
perpendicular motion 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 this perpendicular motion, or shear, so S-waves only travel
in solids. P-waves travel in both solids and fluids.
= Surface waves = The two primary types of surface waves
are the Rayleigh waves,which has some compressional motion, and the Love wave,
which does not. Rayleigh waves can be explained theoretically in terms of
interacting P- and S-waves of vertical polarization that are required to
satisfy the boundary conditions on the free surface. Love waves can exist in
the presence of a subsurface layer, and they are formed by S-waves of horizontal
polarization only. Surface waves travel more slowly than P-waves and S-waves;
however, because they are guided by the surface of the Earth they can be much
larger in amplitude than body waves, and can be the largest signals seen in
earthquake seismograms. They are particularly strongly excited when their
source is close to the surface of the Earth, as in a shallow earthquake or
explosion. = Normal modes =
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. 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
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 Seismic waves produced by explosions or
vibrating controlled sources are one of the primary methods of underground
exploration in geophysics. 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. Detection of seismic waves
Seismometers are sensors that sense 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, to
local noise from wind or anthropogenic activities, to incessant signals
generated at the ocean floor and coasts induced by ocean waves, to cryospheric
events associated with large icebergs and glaciers. Above-ocean meteor strikes
with energies as high as 4.2 × 1013 J have been recorded by seismographs, as
have a number of industrial accidents and terrorist bombs and events. A major
long-term motivation for the global seismographic monitoring has been for
the detection and study of nuclear testing.
Mapping the earth's interior 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 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 of 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
Ultra Low Velocity Zones near the core–mantle boundary.
Seismology and society = 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 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. = Engineering seismology =
Engineering seismology is the study and application of seismology for
engineering purposes. 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. There are two principal components of engineering
seismology. Firstly, studying earthquake history and tectonics to assess the
earthquakes that 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. Tools
Seismological instruments can generate large amounts of data. Systems for
processing such data include: CUSP
RadExPro seismic software SeisComP3
Notable seismologists See also
Notes References
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External links European-Mediterranean Seismological
Center, real-time earthquake information website.
Seismological Society of America. Incorporated Research Institutions for
Seismology. USGS Earthquake Hazards Program.