Instruments used to detect and record seismic disturbances are known as seismographs. Those in use today vary somewhat in design and function, but generally a heavy mass, either a pendulum or a large permanent magnet, is connected to a mechanical or optical recording device. When earthquake tremors occur, the pendulum or the magnet, because of inertia, remains still as the earth moves beneath, with the relative motion between the earth and the instrument magnified mainly by electrical amplifying apparatus. The graphic record, called the seismogram, can be used to establish information about an earthquake, e.g., its severity and distance. By using three instruments, each set to respond to motions from a different direction (north-south horizontal, east-west horizontal, and vertical), both the distance and the direction of the earth movement can be determined. Three or more widely spaced seismographic stations are required to pinpoint the location of earthquakes in remote regions.
Although seismographs have been used since their invention by John Milne in 1880, until the end of the 20th cent. their placement was limited to land areas, creating conspicuous gaps in global seismic coverage under the oceans that cover most of the earth's surface. During the late 1990s geologists began to create an underwater network of geological observatories using undersea coaxial cables no longer used for communications. This enabled the more precise detection and measurement of seismic disturbances occurring between the continental land masses.
The American scientist John Winthrop (1714-79), often called the founder of seismology, was one of the first to make scientific studies of earthquakes. By analyzing seismic data from a 1909 earthquake near Zagreb (now in Croatia), the Austro-Hungarian meteorologist Andrija Mohorovičić discovered a boundary between the crust and mantle, now called the Mohorovičić discontinuity or Moho. Seismological studies were furthered by the U.S. seismologist Charles F. Richter, who invented the Richter scale to determine an earthquake's magnitude. Each successive point on the logarithmic scale represents an increase by a factor of 10 in wave amplitude. A modified Mercalli scale, originally developed by the Italian seismologist Giuseppe Mercalli, is also based on the earthquake's effects on the surface.
One aspect of seismology is concerned with measuring the speeds at which seismic waves travel through the earth. Past earthquake studies have shown that P, or primary/compressional, waves travel fastest through the earth; S, or secondary/transverse, waves cannot pass through liquids, allowing scientists to discern the earth's many boundary layers known as the crust, mantle, and core. For example, the disappearance of S waves below 1,800 mi (2,900 km) shows that the outer core of the earth is liquid. Seismologists also prepare seismic risk maps for earthquake-prone countries; these indicate the degree of seismic danger. In addition, seismologists use earthquake data to determine plate boundaries (see plate tectonics); active earthquake areas generally coincide with plate margins, both destructive and growing, and transform faults.
An important commercial application of seismology is its use in prospecting for oil deposits. The first oil field to be discovered by this method was found in Texas in 1924. A portable seismograph is set up in the area to be investigated, and an explosive energy source is activated nearby; formerly, explosives such as dynamite were used to create the seismic waves, but they have been largely replaced by high-energy vibrators on land and air-gun arrays at sea. The waves generated are received by detectors known as geophones; on land, these are commonly placed in a fan-shaped pattern on the ground. From an interpretation of the waves created by the energy source and recorded by the seismograph, the detection of geological structures in which oil may be trapped is possible.
Seismic methods are sometimes used to locate subsurface water and to detect the underlying structure of the oceanic and continental crust. With the development of underground testing of nuclear devices, seismographic stations for their detection were set up throughout the world. Under the Comprehensive Test Ban Treaty (signed 1996 but not yet in force) an international monitoring system has been set up which includes many seismic stations; the detailed data collected is also used by contributing nations for purposes other than monitoring nuclear tests.
See B. F. Howell, An Introduction to Seismological Research: History and Development (1990); T. Lay and T. C. Wallace, eds., Modern Global Seismology (1995); H. A. Doyle, Seismology (1996). See also bibliography under earthquake.
S-waves, also called Shear waves or secondary waves, are transverse waves that travel more slowly than P-waves and thus appear later than P-waves on a seismogram. Particle motion is perpendicular to the direction of wave propagation. Shear waves do not exist in fluids such as air or water.
Surface waves travel more slowly than P-waves and S-waves, but because they are guided by the surface of the Earth (and their energy is thus trapped near the Earth's surface) 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 the seismic source is close to the surface of the Earth, such as the case of a shallow earthquake.
For large enough earthquakes, one can observe the normal modes of the Earth. These modes are excited as discrete frequencies and can be observed for days after the generating event. The first observations 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 Great Chilean earthquake and the 1964 Great Alaskan earthquake. Since then, the normal modes of the Earth have given us some of the strongest constraints on the deep structure of the Earth.
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. Pressure waves (P-waves) pass through the core. Transverse or shear waves (S-waves) that shake side-to-side require rigid material so they do not pass through the outer core. Thus, the liquid core causes a "shadow" on the side of the planet opposite of the earthquake where no direct S-waves are observed. The reduction in P-wave velocity of the outer core also causes a substantial delay for P waves penetrating the core from the (seismically faster velocity) mantle.
Seismic waves produced by explosions or vibrating controlled sources are the primary method of underground exploration. Controlled source seismology has been used to map salt domes, faults, anticlines and other geologic traps in petroleum-bearing rocks, geological faults, rock types, and long-buried giant meteor craters. For example, the Chicxulub impactor, which is believed to have killed the dinosaurs, was localized to Central America by analyzing ejecta in the cretaceous boundary, and then physically proven to exist using seismic maps from oil exploration.
Using seismic tomography with earthquake waves, the interior of the Earth has been completely mapped to a resolution of several hundred kilometers. This process has enabled scientists to identify convection cells, mantle plumes and other large-scale features of the inner Earth.
Seismographs are instruments that sense and record the motion of the Earth. Networks of seismographs today continuously monitor the seismic environment of the planet, allowing for the monitoring and analysis of global earthquakes and tsunami warnings, as well as recording a variety of seismic signals arising from non-earthquake sources ranging from explosions (nuclear and chemical), to pressure variations on the ocean floor induced by ocean waves (the global microseism), to cryospheric events associated with large icebergs and glaciers. Above-ocean meteor strikes as large as ten kilotons of TNT, (equivalent to about 4.2 × 1013 J of effective explosive force) have been recorded by seismographs. A major motivation for the global instrumentation of the Earth with seismographs has been for the monitoring of nuclear testing.
One of the first attempts at the scientific study of earthquakes followed the 1755 Lisbon earthquake. Other especially notable earthquakes that spurred major developments in the science of seismology include the 1906 San Francisco earthquake, the 1964 Alaska earthquake and the 2004 Sumatra-Andaman earthquake. An extensive list of famous earthquakes can be found on the earthquake page.
Various attempts have been made by seismologists and others to create effective systems for precise earthquake predictions, including the VAN method. Such methods have yet to be generally accepted in the seismology community.