Seismology is the scientific study of earthquakes and the spread of elastic waves through the Earth or through other planet-like bodies. The field includes studies of earthquake effects, such as tsunamis in addition to diverse seismic sources such as tectonic, volcanic, oceanic, atmospheric, and artificial processes. A related field that utilizes geology to infer information regarding past earthquakes is paleoseismology. A recording of earth motion as a function of time is a seismogram. A seismologist is a scientist who does research regarding seismology.
Early speculations of the natural causes of earthquakes began in the writings of Thales of Miletos, Anaximenes of Miletos, Aristotle and Zhang Heng. In 132 C.E., 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 in a system of channels inside the Earth. In 1703, Martin Lister and Nicolas Lemery suggested that earthquakes were caused by chemical explosions within the Earth. The Lisbon earthquake of 1755, according to the general flowering of science in Europe, set in motion intensified scientific attempts to understand the behavior 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 he carried out seismological experiments utilizing explosives. In 1897, Emil Wiechert’s theoretical calculations led him to conclude that the Earth’s interior contains 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 discovered 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 behavior of elastic materials and in mathematics. In 1926, Harold Jeffreys was the first to claim, based on his study of earthquake waves, that under the crust, the core of the Earth is liquid. In 1937, Inge Lehmann determined that in 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.
Seismic waves are elastic waves that spread throughout solid or liquid materials. They can be divided into body waves that travel through the interior of materials; surface waves that travel along surfaces or interfaces between materials; and normal modes, a form of standing wave.
There are two types of body waves, P-waves and S-waves. Pressure waves or Primary waves, or 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 known as shear or secondary waves, are transverse waves that involve motion perpendicular to the direction of propagation. S-waves 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 fluids and solids.
The two main kinds of surface waves are the Rayleigh wave, which has some compressional motion, and the Love wave, which doesn’t. Such waves can be theoretically explained in terms of cooperating P-and/or S-waves. Surface waves travel slower than P-waves and S-waves, but due to them being 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 especially strong when their source is close to the surface of the Earth, as in a shallow earthquake or explosion.
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 even shorter. Motion that is 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 arrival of higher fidelity instruments coincided with two of the largest earthquakes of the 20th century – the 1960 Great Chilean Earthquake and the 1964 Great Alaskan Earthquake. Since then, the normal modes of the Earth have provided us with some of the strongest constraints on the deep structure of the Earth.
Seismic waves that are produced by explosions or vibrating controlled sources are one of the key methods of underground exploration in geophysics. Controlled-source seismology has been utilized to map salt domes, anticlines, faults, and other geologic traps in petroleum-bearing rocks, geological faults, rock types, and long-buried giant meteor craters.
Seismometers are sensors that sense and record the motion of the Earth arising from the elastic waves. Seismometers can be deployed at the Earth’s surface, in shallow vaults, in boreholes, or under the water. A complete instrument package that records seismic signals is a seismograph. Networks of seismographs continuously record ground motions around the world to facilitate the monitoring and analysis of global earthquakes and other seismic sources. Rapid location of earthquakes makes tsunami warnings possible due to seismic waves traveling considerably faster than tsunami waves.
Seismometers also record signals from non-earthquake sources ranging from explosions, to local noise from wing or anthropogenic activities, to incessant signals that are generated at the floor of the ocean and coasts induced by ocean waves, to cryospheric events that are associated with large icebergs and glaciers. Above ocean meteor strikes have been recorded by seismographs, as well as a number of industrial accidents and terrorist bombs and events. A major long-term motivation for the global seismographic monitoring has been for the purpose of detecting and studying of nuclear testing.
Due to seismic waves commonly spreading efficiently and interacting with internal structures, they provide high-resolution noninvasive techniques for studying the Earth’s interior. One of the earliest important discoveries was that the outer core of the Earth is a liquid material. Since S-waves do not pass through liquids, the liquid core causes a “shadow” on the side of the planet that is opposite of the earthquake where no direct S-waves are observed. Additionally, P-waves travel a lot slower through the outer core than the mantle.
Processing readings from many seismometers utilizing 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, for example, the Ultra Low Velocity Zones near the core-mantle boundary.
Forecasting a probable timing, location, magnitude, and other significant features of forthcoming seismic event is called earthquake prediction. A variety of attempts have been made by seismologists and others to build effective systems for precise earthquake predictions, including the VAN method. The majority of seimologists do not believe that a system to offer timely warnings for individual earthquakes has yet been developed, and a lot of people believe that such a system would be not likely to give an important warning of impending seismic events. However, more common forecasts routinely predict seismic hazard. Such forecasts as these estimate the probability of an earthquake of a specific size affecting a particular location within a particular time-span, and they are routinely utilized 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 the 5th of 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 prior to the earthquake, scientists and officials were more interested in pacifying the population than providing adequate information about earthquake risks and preparedness.
Image Caption: 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. Credit: Crickett/Wikipedia