Seismometer is of Greek origin and comes from Seism - "the shakes" and Meteo - I measure" are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources. Body waves and surface waves Earthquake wave paths p-wave and s-wave from seismograph A seismic wave is a wave that travels through the Earth, most often as the result of a tectonic earthquake, sometimes from an explosion. ... This article is about the natural seismic phenomenon. ... A nuclear explosion (nuclear detonation) has occurred: twice using a nuclear weapon during war (during World War II, the atomic bombings of Hiroshima and Nagasaki) many times testing a nuclear weapon a series of tests of nuclear explosives for construction purposes; see Operation Plowshare Potential other applications (not yet applied... A seismic source generates controlled seismic energy that is used in both reflection and refraction seismic surveys. ... Seismology (from the Greek seismos(σεισμός) = earthquake and λόγος,logos = knowledge ) is the scientific study of earthquakes and the propagation of elastic waves through the Earth. ...

Seismograph is another Greek term from Seism - "the shakes" and Graphos - "I draw". It is often used for seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined than to modern systems, in which these functions are separated. Both types provide a continuous record of ground motion; this distinguishes them from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was.^[1]

Strong Motion seismometer that measures acceleration. This model is a K2 made by Kinemetrics and part of the Pacific Northwest Seismograph Network.

Strong Motion Seismometer K2 {pd} File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Strong Motion Seismometer K2 {pd} File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Acceleration is the time rate of change of velocity and/or direction, and at any point on a velocity-time graph, it is given by the slope of the tangent to the curve at that point. ... The Pacific Northwest Seismograph Network, or PNSN, operates seismograph stations and locates earthquakes in Washington and Oregon. ...

Basic principles

Inertial seismometers have:

• A mass, usually called the inertial mass, that can move relative to the instrument frame, but is attached to it by a system (such as a spring) that will hold it fixed relative to the frame if there is no motion, and also damp out any motions once the motion of the frame stops.
• A means of recording the motion of the mass relative to the frame, or the force needed to keep it from moving.

Any motion of the ground moves the frame. The mass tends not to move because of its inertia, and by measuring the motion between the frame and the mass the motion of the ground can be determined, even though the mass does move.

Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper.

Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop. The motion of the mass relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is recorded digitally. In other systems the mass is allowed to move, and its motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is often used in the geophones used in seismic surveys for oil and gas. A Proportional-Integral-Derivative controller is a standard feedback loop component in industrial control applications. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

Professional seismic observatories usually have instruments measuring three axes: north-south, east-west, and up-down. If only one axis can be measured, this is usually the vertical because it is less noisy and gives better records of some seismic waves.

The foundation of a seismic station is critical.^[2] A professional station is sometimes mounted on bedrock. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Amateur, or less exotic instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site should always be surveyed for ground noise with a temporary installation before pouring the pier and laying conduit.

Zhang Heng's Seismoscope

Replica of Zhang Heng's seismoscope Houfeng Didong Yi

In 132, Zhang Heng of China's Han dynasty invented the first seismoscope (by the definition above), which was called Houfeng Didong Yi (lit. instrument for measuring the seasonal winds and the movements of the Earth). The description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about 2 meters in diameter; at eight points around the top were dragon's heads holding bronze balls. When there was an earthquake, one of the mouths would open and drop its ball into a bronze toad at the base, making a sound and supposedly showing the direction of the earthquake. On at least one occasion, probably at the time of a large earthquake in Gansu in 143, the seismoscope indicated an earthquake even though one was not felt. The available text says that inside the vessel was a central column that could move along eight tracks; this is thought to refer to a pendulum, though it is not known exactly how this was linked to a mechanism that would open only one dragon's mouth.^{[3] [4]} A replica of an ancient Chinese Siesmograph from Eastern Han Dynasty (25-220 CE) This picture was taken in July 2004 from an exhibition at Chabot Space & Science Center in Oakland California. ... A replica of an ancient Chinese Siesmograph from Eastern Han Dynasty (25-220 CE) This picture was taken in July 2004 from an exhibition at Chabot Space & Science Center in Oakland California. ... For other uses, see Zhang Heng (disambiguation). ... This article is about the year 132. ... For other uses, see Zhang Heng (disambiguation). ... Han Dynasty in 87 BC Capital Changan (206 BC–9 AD) Luoyang (25 AD–220 AD) Language(s) Chinese Religion Buddhism, Taoism, Confucianism, Chinese folk religion Government Monarchy History  - Establishment 206 BC  - Battle of Gaixia; Han rule of China begins 202 BC  - Interruption of Han rule 9 - 24  - Abdication... The Book of Later Han (Chinese:后汉书) is one of the official Chinese historical works which was compiled by Fan Ye in the 5th century, using a number of earlier histories and documents as sources. ... Gansu (Simplified Chinese: 甘肃; Traditional Chinese: 甘肅; Hanyu Pinyin: ; Wade-Giles: Kan-su, Kansu, or Kan-suh) is a province located in the northwest of the Peoples Republic of China. ...

An early example

The principle can be shown by an early special purpose seismometer. This consisted of a large stationary pendulum, with a stylus on the bottom. As the earth starts to move, the heavy mass of the pendulum has the inertia to stay still in the non-earth frame of reference. The result is that the stylus scratches a pattern corresponding with the earth's movement. This type of strong motion seismometer recorded upon a smoked glass (glass with carbon soot). While not sensitive enough to detect distant earthquakes, this instrument could indicate the direction of the initial pressure waves and thus help find the epicenter of a local earthquake — such instruments were useful in the analysis of the 1906 San Francisco earthquake. Further re-analysis was performed in the 1980s using these early recordings, enabling a more precise determination of the initial fault break location in Marin county and its subsequent progression, mostly to the south. For other uses, see Pendulum (disambiguation). ... For the online music and film magazine, see Stylus Magazine. ... This article is about Earth as a planet. ... This article is about inertia as it applies to local motion. ... This article or section is in need of attention from an expert on the subject. ... Soot, also called lampblack, Pigment Black 7, carbon black or black carbon, is a dark powdery deposit of unburned fuel residues, usually composed mainly of amorphous carbon, that accumulates in chimneys, automobile mufflers and other surfaces exposed to smoke—especially from the combustion of carbon-rich organic fuels in the... Sarah San Francisco Earthquake redirects here. ... Marin County is a county located in Californias San Francisco Bay Area, across the Golden Gate Bridge from San Francisco. ...

Early designs

After 1880, most seismometers were descended from those developed by the team of John Milne, James Alfred Ewing and Thomas Gray, who worked in Japan from 1880-1895. These seismometers used damped horizontal pendulums. Later, after World War II, these were adapted into the widely used Press-Ewing seismometer. John Milne (1850 – 1913) was the English geologist and mining engineer who invented the seismograph. ... Sir James Alfred Ewing (March 27, 1855 - January 7, 1935) was a British physicist and engineer, best known for his work on the magnetic properties of metals and, in particular, for his discovery of, and coinage of the word, hysteresis. ... For Thomas Gray the classical scholar, see Thomas Gray For Thomas Gray, recipient of the Victoria Cross, see Thomas Gray (VC) Thomas Lomar Gray, of the British Imperial College of Engineering, helped John Milne and James Alfred Ewing develop the first modern seismometers in Japan from 1880-1895. ...

Later, professional suites of instruments for the world-wide standard seismographic network had one set of instruments tuned to oscillate at fifteen seconds, and the other at ninety seconds, each set measuring in three directions. Amateurs or observatories with limited means tuned their smaller, less sensitive instruments to ten seconds. The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge.

The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument. The "gate" is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted (before the damping is installed) to oscillate once per three seconds, or once per thirty seconds. The general-purpose instruments of small stations or amateurs usually oscillate once per ten seconds. A pan of oil is placed under the arm, and a small sheet of metal mounted on the underside of the arm drags in the oil to damp oscillations. The level of oil, position on the arm, and angle and size of sheet is adjusted until the damping is "critical," that is, almost having oscillation. The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents.

Zollner described torsionally-suspended horizontal pendulums as early as 1869, but developed them for gravimetry rather than seismometry.

Early seismometers had an arrangement of levers on jeweled bearings, to scratch smoked glass or paper. Later, mirrors reflected a light beam to a direct-recording plate or roll of photographic paper. Briefly, some designs returned to mechanical movements to save money. In mid-twentieth-century systems, the light was reflected to a pair of differential electronic photosensors called a photomultiplier. The voltage generated in the photomultiplier was used to drive galvanometers which had a small mirror mounted on the axis. The moving reflected light beam would strike the surface of the turning drum, which was covered with photo-sensitive paper. The expense of developing photo sensitive paper caused many seismic observatories to switch to ink or thermal-sensitive paper.

Modern instruments

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Modern instruments use electronic sensors, amplifiers, and recording instruments. Most are broadband covering a wide range of frequencies. Some seismometers can measure motions with frequencies from 30 Hz (0.03 seconds per cycle) to 1/850 Hz (850 seconds per cycle). The mechanical suspension for horizontal instruments remains the garden-gate described above. Vertical instruments use some kind of constant-force suspension such as the LaCoste suspension. The LaCoste suspension uses a zero-length spring to provide a long period (high sensitivity). ^{[5] [6]} Some modern instruments use a "triaxial" design in which three identical sensors that measure motion at the same angle to the vertical are 120 degrees apart on the horizontal. Vertical and horizontal motions can be computed from these measurements. Hookes law accurately models the physical properties of common mechanical springs for small changes in length. ...

Seismometers unavoidably introduce some distortion into the signals they measure, but professionally-designed systems have carefully-characterized frequency transforms.

Modern sensitivities come in three broad ranges: geophones, 50 to 750 V/m; local geologic seismographs, about 1,500 V/m; and teleseismographs, used for world survey, about 20,000 V/m. Instruments come in three main varieties: short period, long period and broad-band. The short and long period measure velocity and are very sensitive, however they 'clip' or go off-scale for ground motion that is strong enough to be felt by people. A 24-bit analog-to-digital conversion channel is commonplace. Practical devices are linear to roughly a part per million. This article needs to be cleaned up to conform to a higher standard of quality. ... Josephson junction array chip developed by NIST as a standard volt. ...

Delivered seismometers come with two styles of output: analog and digital. Analog seismographs require analog recording equipment, possibly including an analog-to-digital converter. Digital seismographs simply plug in to computers. They present the data in standard digital forms (often "SE2" over ethernet). Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ...

Teleseismometers

The low frequency 3-direction seismometer (cover removed). Two masses for x- and y-direction can be seen, the third one for z-direction is below.

The modern broad-band seismograph can record a very broad range of frequencies. It consists of a small 'proof mass', confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is then recorded. For other uses, see Frequency (disambiguation). ... Surface mount electronic components Electronics is the study of the flow of charge through various materials and devices such as semiconductors, resistors, inductors, capacitors, nano-structures and vacuum tubes. ... For other uses, see Feedback (disambiguation). ...

In most designs the electronics holds a mass motionless relative to the frame. This device is called a "Force Balance Accelerometer". It measures acceleration instead of velocity of ground movement. Basically, the distance between the mass and some part of the frame is measured very precisely, by a linear variable differential transformer. Some instruments use a linear variable differential capacitor). Cutaway view of an LVDT. Current is driven through the primary coil at A, causing an induction current to be generated through the secondary coils at B. The linear variable differential transformer (LVDT) is a type of electrical transformer used for measuring linear displacement. ... A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. ...

That measurement is then amplified by electronic amplifiers attached to parts of an electronic negative feedback loop. One of the amplified currents from the negative feedback loop drives a coil very like a loudspeaker, except that the coil is attached to the mass, and the magnet is mounted on the frame. The term amplifier as used in this article can mean either a circuit (or stage) using a single active device or a complete system such as a packaged audio hi-fi amplifier. ... A Proportional-Integral-Derivative controller is a standard feedback loop component in industrial control applications. ... For the Marty Friedman album, see Loudspeaker (album) An inexpensive low fidelity 3. ...

The result is that the mass stays nearly motionless.

Most instruments directly measure the ground motion using the distance sensor.

The voltage generated in a sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground.

The current to the drive coil provides a sensitive, accurate measurement of the force between the mass and frame, thus directly measuring the ground's acceleration (using F=MA of basic physics). A magnet levitating above a high-temperature superconductor demonstrates the Meissner effect. ...

One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses. The uneven changes in pressure caused by wind blowing on an open window can easily change the density of air in a room enough to cause a vertical seismograph to show spurious signals. Therefore, most professional seismographs are sealed in rigid gas-tight enclosures. For example, this is why a common Streckheisen model has a thick glass base that must be glued to its pier without bubbles in the glue.

It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors when the Earth's magnetic field moves. This is also why seismograph's moving parts are constructed from a material that minimally interacts with magnetic fields.

A seismograph is also sensitive to changes in temperature, and many instruments are constructed from low expansion materials such as nonmagnetic invar. Invar, also called FeNi36, is an alloy of iron (64%) and nickel (36%) with some carbon and chromium. ...

The hinges on a seismograph are usually patented, and by the time the patent has expired, the art has improved. The most successful public domain designs use thin foil hinges in a clamp.

Another issue is that the transfer function of a seismograph must be accurately characterized, so that its frequency response is known. This is often the crucial difference between professional and amateur instruments. Most instruments are characterized on a variable frequency shaking table. A transfer function is a mathematical representation of the relation between the input and output of a linear time-invariant system. ...

Strong-motion seismometers

Another type of seismometer is a digital strong-motion seismometer, or accelerograph. This data is essential to understand how an earthquake affects human structures. An accelerograph can be referred to as a strong motion seismograph, or simply as an earthquake accelerometer. ...

A strong-motion seismometer measures acceleration. This can be mathematically integrated later to give velocity and position. Strong-motion seismometers are not as sensitive to ground motions as teleseismic instruments but they stay on scale during the strongest seismic shaking. This article is about the concept of integrals in calculus. ...

Other forms

A Kinemetrics seismograph, formerly used by the United States Department of the Interior.

Accelerographs and geophones are often heavy cylindrical magnets with a spring-mounted coil inside. As case moves, the coil tends to stay stationary, so the magnetic field cuts the wires, inducing current in the output wires. They receive frequencies from several hundred hertz down to 4.5 Hz (cheap) to as low as 1 Hz (pretty expensive). Some have electronic damping, a low-budget way to get some of the performance of the closed-loop wide-band geologic seismographs. Image File history File linksMetadata Download high-resolution version (1024x768, 264 KB)Kinemetrics seismograph used by United States Department of the Interior. ... Image File history File linksMetadata Download high-resolution version (1024x768, 264 KB)Kinemetrics seismograph used by United States Department of the Interior. ... The United States Department of the Interior (DOI) is a Cabinet department of the United States government that manages and conserves most federally owned land. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

Strain-beam accelerometers constructed as integrated circuits are too insensitive for geologic seismographs (2002), but are widely used in geophones.

Some other sensitive designs measure the current generated by the flow of a non-corrosive ionic fluid through an electret sponge or a conductive fluid through a magnetic field. Electret (formed of elektr- from electricity and -et from magnet) is material that has been permanently electrically charged (polarised). ... For the indie-pop band, see The Magnetic Fields. ...

Modern recording

Today, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection; for amateurs, a PC with a sound card and associated software is adequate. Most systems record continuously, but some record only when a signal is detected, as shown by a short-term increase in the variation of the signal, compared to its long-term average (which can vary slowly because of changes in seismic noise).

Interconnected seismometers

Seismometers spaced in an array can also be used to precisely locate, in three dimensions, the source of an earthquake, using the time it takes for seismic waves to propagate away from the hypocenter, the initiating point of fault rupture (See also Earthquake location). Interconnected seismometers are also used to detect underground nuclear test explosions. Body waves and surface waves Earthquake wave paths p-wave and s-wave from seismograph A seismic wave is a wave that travels through the Earth, most often as the result of a tectonic earthquake, sometimes from an explosion. ... The hypocenter or hypocentre (literally: below the center from the Greek υπόκεντρον), may refer to the site of an earthquake or to that of a nuclear explosion. ... Geologic faults, fault lines or simply faults are planar rock fractures, which show evidence of relative movement. ... The primary purpose of a seismometer is to locate the initiating points of earthquake hypocenters. ... The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 kilometers (11 mi) above the hypocenter A nuclear weapon derives its destructive force from nuclear reactions of fusion or fission. ...

In reflection seismology, an array of seismometers images sub-surface features. The data are reduced to images using algorithms similar to tomography. The data reduction methods resemble those of computer-aided tomographic medical imaging X-ray machines (CAT-scans), or imaging sonars. Seismic reflection data Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earths subsurface from reflected seismic waves. ... The mathematical basis for tomographic imaging was laid down by Johann Radon. ... This article is about underwater sound propagation. ...

A world-wide array of seismometers can actually image the interior of the Earth in wave-speed and transmissivity. This type of system uses events such as earthquakes, impact events or nuclear explosions as wave sources. The first efforts at this method used manual data reduction from paper seismograph charts. Modern digital seismograph records are better adapted to direct computer use. With inexpensive seismometer designs and internet access, amateurs and small institutions have even formed a "public seismograph network."^[7] Artists impression of a major impact event. ... It has been suggested that Nuclear explosive be merged into this article or section. ...

Seismographic systems used for petroleum or other mineral exploration historically used an explosive and a wireline of geophones unrolled behind a truck. Now most short-range systems use "thumpers" that hit the ground, and some small commercial systems have such good digital signal processing that a few sledgehammer strikes provide enough signal for short-distance refractive surveys. Exotic cross or two-dimensional arrays of geophones are sometimes used to perform three-dimensional reflective imaging of subsurface features. Basic linear refractive geomapping software (once a black art) is available off-the-shelf, running on laptop computers, using strings as small as three geophones. Some systems now come in an 18" (0.5 m) plastic field case with a computer, display and printer in the cover! This article needs to be cleaned up to conform to a higher standard of quality. ...

Small seismic imaging systems are now sufficiently inexpensive to be used by civil engineers to survey foundation sites, locate bedrock, and find subsurface water.

References

1. ^ Richter, C.F. (1958). Elementary Seismology. W.H. Freeman. 
2. ^ Erhard Wielandt's 'Seismic Sensors and their Calibration'- Current (2002) reference by a widely consulted expert.
3. ^ Sleeswyk AW, Sivin N (1983). "Dragons and toads: the Chinese seismoscope of A.D. 132". Chinese Science 6: 1-19. 
4. ^ Needham, Joseph (1959). Science and Civilization in China, Volume 3: Mathematics and the Sciences of the Heavens and the Earth. Cambridge University Press, 626-635. 
5. ^ Zero Length Springs in Seismographs
6. ^ A Biography of Lucien LaCoste, inventor of the zero-length spring
7. ^ Public seismograph network- many resources for amateurs and underfunded institutions

LaCoste, Lucien, 1908-1995, scientist, inventor Many other articles will tell you the important things about Dr. LaCoste, but the purpose of this entry is to pass on two memories of a great man. ...

See also

• Seismology
• Seismogram
• Plate tectonics
• Milne, John
• Galitzine, Boris Borisovich
• Lehmann, Inge
• Oldham, Richard Dixon
• Pacific Northwest Seismograph Network
• Richter Scale

Seismology (from the Greek seismos(σεισμός) = earthquake and λόγος,logos = knowledge ) is the scientific study of earthquakes and the propagation of elastic waves through the Earth. ... A seismogram is a graph output by a seismograph. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... John Milne (1850 – 1913) was the English geologist and mining engineer who invented the seismograph. ... Prince Boris Borisovich Galitzine (b. ... Inge Lehmann (May 13, 1888 - February 21, 1993), Fellow of the Royal Society (London) 1969, was a Danish seismologist who, in 1936, argued that the Earth must not only have a molten interior, but a solid core at the center, which deflects P waves. ... Richard Dixon Oldham (July 31, 1858 – July 15, 1936) was a British geologist who, in 1906, argued that the Earth must have a molten interior as S waves were not able to travel through liquids nor through the Earths interior. ... The Pacific Northwest Seismograph Network, or PNSN, operates seismograph stations and locates earthquakes in Washington and Oregon. ... The Richter magnitude test scale (or more correctly local magnitude ML scale) assigns a single number to quantify the size of an earthquake. ...

External links

Wikimedia Commons has media related to:

Seismometers

• The history of early seismometers
• A Java code applet demonstrating the operation of a damped-mass seismometer
• http://neic.usgs.gov/neis/seismology/keeping_track.html
• Link to live Seismic Drum at Geonet's Mangatainoka River station in New Zealand
• The Lehman amateur seismograph, from Scientific American- not designed for calibrated measurement.
• USGS evaluation of Streckheisen STS-2 Seismometer models- Streckheisen is a common make of research seismometers
• Pacific Northwest Seismograph Network-PNSN is a seimograph network in the northwest USA
• SeisMac is a free tool for recent Macintosh laptop computers that implements a real-time three-axis seismograph.
• The Incorporated Research Institutions for Seismology (IRIS) is a principal U.S. seismological instrumentation and data facility, principally supported by the National Science Foundation and the U.S. Department of Energy.