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Jai Ganesh

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Seismology

Seismology

Gist

Seismology is the scientific study of earthquakes, including their causes, effects, and the waves they generate. It also encompasses the study of seismic waves and their propagation through the Earth and other planetary bodies.

Seismology is the study of earthquakes and seismic waves that move through and around the Earth. A seismologist is a scientist who studies earthquakes and seismic waves.

Seismologists study earthquakes by looking at the damage that was caused and by using seismometers. A seismometer is an instrument that records the shaking of the Earth's surface caused by seismic waves. The term seismograph usually refers to the combined seismometer and recording device.

Summary

Seismology is the scientific discipline that is concerned with the study of earthquakes and of the propagation of seismic waves within the Earth. A branch of geophysics, it has provided much information about the composition and state of the planet’s interior.

The goals of seismological investigations may be local or regional, as in the attempt to determine subsurface faults and other structures in petroleum or mineral exploration, or they may be of global significance, as in attempts to determine structural discontinuities in the Earth’s interior, the geophysical characteristics of island arcs, oceanic trenches, or mid-oceanic ridges, or the elastic properties of Earth materials generally.

In recent years attention has been devoted to earthquake prediction and, more successfully, to assessing seismic hazards at different geographic sites in an effort to reduce the risks of earthquakes. The physics of seismic fault sources have been better determined and modeled for computer analysis. Moreover, seismologists have studied quakes induced by human activities, such as impounding water behind high dams and detonating underground nuclear explosions. The objective of the latter research is to find ways of discriminating between explosions and natural earthquakes.

Details

Seismology (/saɪzˈmɒlədʒi, saɪs-/; from Ancient Greek (seismós) meaning "earthquake" and (-logía) meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic waves through planetary bodies. It also includes studies of the environmental effects of earthquakes such as tsunamis; other seismic sources such as volcanoes, plate tectonics, glaciers, rivers, oceanic microseisms, and the atmosphere; and artificial processes such as explosions.

Paleoseismology is a related field that uses geology to infer information regarding past earthquakes. A recording of Earth's motion as a function of time, created by a seismograph is called a seismogram. A seismologist is a scientist who 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.

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.

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".

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. 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.

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." He is widely considered to be the "Father of Seismology".

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

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 1909, Andrija Mohorovičić, one of the founders of modern seismology, discovered and defined the Mohorovičić discontinuity. 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.

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.

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.

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.

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

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

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.

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, pressure waves or primary waves (P waves) and shear or secondary waves (S waves). P waves are longitudinal waves associated with compression and expansion, and involve particle motion parallel to the direction of wave propagation. P waves are always the first waves to appear on a seismogram as they are the waves that travel fastest through solids. S waves are transverse waves associated with shear, and involve particle motion perpendicular to the direction of wave propagation. S waves travel more slowly than P waves so they appear later than P waves on a seismogram. Because of their low shear strength, fluids cannot support transverse elastic waves, so S waves travel only in solids.

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, 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.

Normal modes

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. 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, the 1906 San Francisco earthquake, the 1964 Alaska earthquake, the 2004 Sumatra-Andaman earthquake, and the 2011 Great East Japan earthquake.

Additional Information:

What is Seismology?

Seismology is the study of earthquakes and seismic waves that move through and around the Earth. A seismologist is a scientist who studies earthquakes and seismic waves.

What are Seismic Waves?

Seismic waves are caused by the sudden movement of materials  within the Earth, such as slip along a fault during an earthquake. Volcanic eruptions, explosions, landslides, avalanches, and even rushing rivers can also cause seismic waves. Seismic waves travel through and around the Earth and can be recorded with seismometers.

Types of Seismic Waves

There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the Earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes send out seismic energy as body waves (P and S). When body waves reach the surface, some of that energy is converted to surface waves.

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