A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a recording device to form a seismograph.[1] The output of such a device—formerly recorded on paper (see picture) or film, now recorded and processed digitally—is a seismogram. Such data is used to locate and characterize earthquakes, and to study the Earth's internal structure.

Basic principles

A simple seismometer, sensitive to up-down motions of the Earth, is like a weight hanging from a spring, both suspended from a frame that moves along with any motion detected. The relative motion between the weight (called the mass) and the frame provides a measurement of the vertical ground motion. A rotating drum is attached to the frame and a pen is attached to the weight, thus recording any ground motion in a seismogram.



In other systems the weight is allowed to move, and its motion produces an electrical charge in a coil attached to the mass which voltage moves through the magnetic field of a magnet attached to the frame. This design is often used in a geophone, which is used in exploration for oil and gas.

Seismic observatories usually have instruments measuring three axes: north-south (y-axis), east-west (x-axis), and vertical (z-axis). If only one axis is measured, it is usually the vertical because it is less noisy and gives better records of some seismic waves.



The word derives from the Greek σεισμός, seismós, a shaking or quake, from the verb σείω, seíō, to shake; and μέτρον, métron, to measure, and was coined by David Milne-Home in 1841, to describe an instrument designed by Scottish physicist James David Forbes.[4]

Seismograph is another Greek term from seismós and γράφω, gráphō, to draw. It is often used to mean 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 record distinguishes them from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was.[5]

The technical discipline concerning such devices is called seismometry,[6] a branch of seismology.

The concept of measuring the "shaking" of something means that the word "seismograph" might be used in a more general sense. For example, a monitoring station that tracks changes in electromagnetic noise affecting amateur radio waves presents an rf seismograph.[7] And Helioseismology studies the "quakes" on the Sun.[8]


The first seismometer was made in China during the 2nd century.[9] The first Western description of the device comes from the French physicist and priest Jean de Hautefeuille in 1703.[10] The modern seismometer was developed in the 19th century.[3]

In December 2018, a seismometer was deployed on the planet Mars by the InSight lander, the first time a seismometer was placed onto the surface of another planet.[11]

Ancient era


Modern designs

By the 13th century, seismographic devices existed in the Maragheh observatory in Persia. French physicist and priest Jean de Hautefeuille built one in 1703.[10] After 1880, most seismometers were descended from those developed by the team of John Milne, James Alfred Ewing and Thomas Gray, who worked as foreign-government advisors in Japan from 1880 to 1895.[3] These seismometers used damped horizontal pendulums. After World War II, these were adapted into the widely used Press-Ewing seismometer.

An early special-purpose seismometer consisted of a large, stationary pendulum, with a stylus on the bottom. As the earth started to move, the heavy mass of the pendulum had the inertia to stay still within the frame. The result is that the stylus scratched 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 pressure waves and thus help find the epicenter of a local quake. Such instruments were useful in the analysis of the 1906 San Francisco earthquake. Further 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.





Modern instruments














Strong-motion seismometers

Another type of seismometer is a digital strong-motion seismometer, or accelerograph. The data from such an instrument is essential to understand how an earthquake affects man-made structures, through earthquake engineering. The recordings of such instruments are crucial for the assessment of seismic hazard, through engineering seismology.


Strong motion sensors are used for intensity meter applications.

Other forms




Interconnected seismometers






Fiber optic cables as seismometers

A new technique for detecting earthquakes has been found, using fiber optic cables.[16] In 2016 a team of metrologists running frequency metrology experiments in England observed noise with a wave-form resembling the seismic waves generated by earthquakes. This was found to match seismological observations of an Mw  6.0 earthquake in Italy, ~1400 km away. Further experiments in England, Italy, and with a submarine fiber optic cable to Malta detected additional earthquakes, including one 4,100 km away, and an ML  3.4 earthquake 89 km away from the cable.

Seismic waves are detectable because they cause micrometer-scale changes in the length of the cable. As the length changes so does the time it takes a packet of light to traverse to the far end of the cable and back (using a second fiber). Using ultra-stable metrology-grade lasers, these extremely minute shifts of timing (on the order of femtoseconds) appear as phase-changes.

The point of the cable first disturbed by an earthquake's p-wave (essentially a sound wave in rock) can be determined by sending packets in both directions in the looped pair of optical fibers; the difference in the arrival times of the first pair of perturbed packets indicates the distance along the cable. This point is also the point closest to the earthquake's epicenter, which should be on a plane perpendicular to the cable. The difference between the p-wave/s-wave arrival times provides a distance (under ideal conditions), constraining the epicenter to a circle. A second detection on a non-parallel cable is needed to resolve the ambiguity of the resulting solution. Additional observations constrain the location of the earthquake's epicenter, and may resolve the depth.

This technique is expected to be a boon in observing earthquakes, especially the smaller ones, in vast portions of the global ocean where there are no seismometers, and at a cost much cheaper than ocean bottom seismometers.




See also


  1. Agnew, Duncan Carr (2003), "Ch. 1: History of Seismology", International Handbook of Earthquake & Engineering Seismology, Part A, pp. 3–11, ISBN 978-0-12-440652-0, LCCN 2002103787, p. 269. See also the USGS Seismometers, seismographs, seismograms webpage.
  2. Erhard Wielandt's 'Seismic Sensors and their Calibration' Archived 2010-09-24 at the Wayback Machine- Current (2002) reference by a widely consulted expert.
  3. Reitherman, Robert (2012). Earthquakes and Engineers: an International History. Reston, VA: ASCE Press. pp. 122–125. ISBN 9780784410714. Archived from the original on 2012-07-26.
  5. Richter, C.F. (1958). Elementary Seismology. San Francisco: W.H. Freeman.
  7. "The RF Seismograph". Retrieved 28 March 2018.
  8. "The Singing Sun". Retrieved 28 March 2018.
  11. Cook, Jia-Rui; Good, Andrew (19 December 2018). "NASA's InSight Places First Instrument on Mars". NASA. Retrieved 20 December 2018.
  13. "Physics of the Zero-Length Spring of Geoscience". Retrieved 28 March 2018.
  14. A Biography of Lucien LaCoste, inventor of the zero-length spring Archived 2007-03-20 at the Wayback Machine
  15. "Redwood City Public Seismic Network". Retrieved 28 March 2018.
  16. Marra, Giuseppe; Clivati, Cecilia; Luckett, Richard; Tampellini, Anna; Kronjäger, Jochen; Wright, Louise; Mura, Alberto; Levi, Filippo; Robinson, Stephen; Xuereb, André; Baptie, Brian; Calonico, Davide (3 August 2016), "Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables", Science, 361 (6401): 486–490, doi:10.1126/science.aat4458, PMID 29903881.
  17. Hutton, Kate; Yu, Ellen. "NEWS FLASH!! SCSN Earthquake Catalog Completed!!" (PDF). Seismological Laboratory, Caltech. Archived from the original (PDF) on 14 July 2014. Retrieved 4 July 2014.