A siesmometer, in general, is any instrument for recording minute motions or vibrations of the ground over time, especially during an earthquake. A modern seismometer is a transducer for converting ground motion to an electrical impulse for the purposes of recording. In plain English, a seismometer is similar to a microphone or stethoscope that is specialised for recording vibrations in the ground.

Looks like a duck, but 'tain't necessarily so

Seismometers are distinguished from geophones in that they must be calibrated. A seismometer may be presicely calibrated so that a voltage produced by the seismometer can be translated directly to ground motion. Geophones have no capacity for calibration, and are used in situations where the exact earth-motion is of little interest. Siesmometers are typically designed to record fairly low frequency movements; so low, in fact, that their response is typically reported in terms of period, rather than frequency. A geophone is meant to be portable and easily redeployable, but a siesmometer is generally installed in a monitoring station permanently, and is therefore much less rugged, and much more sensitive.

Build your own

Your average, run of the mill seismometer consists of a coil of wire and a magnet attached to a large mass, suspended on a spring (or vice versa). As the earth moves, the coil of wire is moved, while the mass remains in place. The movement of the coil relative to the magnet creates a voltage, which is then converted to a digital signal and recorded. There is also a second coil which can be used to drive the magnet for the purposes of calibration. This is very important as precise measurements of ground motion are required. Seismometers may also be used in large arrays, creating the geo-mechanical equivalent to a radio telescope array.

Most seismometers record 3 components of motion, vertical, north/south, and east-west. It is important for the seismometer to have good coupling with the earth, so most seismometers are buried or installed in boreholes.

How seismologists learned to stop worrying and love the bomb

Suprisingly, the Cold War was very good for seismometer design and deployment. Above-ground nuclear testing can be detected by satelites or acoustic sensors, but underground nuclear testing can only be detected by seismology. An underground nuclear blast creates a small earthquake. In order to detect and pinpoint a nuclear blast, it is imperitive to record the seismic waves from the blast at many locations. However, since they are small, many locations will not be able to record them. Therefore, the US Government (and other governments interested in keeping track of nuclear proliferation) funded the creation of world-wide networks of seismometers, which have allowed geophysicists to learn more about the interior of the earth than ever before.

Rules of spelling are anything but

Its important to remember that seismometer, seismology, and all the other seismo words are a part of that vast majority of words which are exceptions to the 'i before e except after c' rule.

Back to seismology glossary.

Seis*mom"e*ter (?), n. [Gr. an earthquake + -meter.] Physics

An instrument for measuring the direction, duration, and force of earthquakes and like concussions.


© Webster 1913.

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