A seismometer is an instrument used to measure seismic (ground motion) activity caused by earthquakes, volcanic eruptions, the use of explosives or other forces.

The fundamental sensing principle for the seismometer is based on the differential motion between a free mass (which tends to remain at rest) and a supporting structure anchored in the ground (which moves with the vibrating Earth) to record seismic waves. This same principle is used in accelerometers.

The four basic types of seismic waves are:

• two preliminary body waves that travel through the Earth and
• two that travel only at the surface (L waves).

Combinations, reflections, and diffractions of these waves produce an infinite variety of other types.

Early seismometers were seismographs that physically recorded seismic movements on graph paper. Instead of these mechanical analog instruments, today’s digital seismometers can use microelectromechanical systems (MEMS) accelerometers. Examples of commercially available seismometers include the STS-1 and EpiSensor.

In addition to accurately recording both small local earthquakes and large distant ones, the STS-1 electronic seismometer can also record earth tides and the sonic boom produced by the Space Shuttle as it flies into Edwards Air Force Base.

Kinemetrics EpiSensor Model FBA ES-T triaxial surface package seismometer is useful for many types of earthquake recording applications. It consists of three EpiSensor force balance accelerometer modules mounted orthogonally in a single package. With full scale recording ranges of ± 0.25 to ± 4g that are user selectable, the EpiSensor provides on-scale recording of earthquake motions even at near-fault locations and in a wide variety of structure types.

Kinemetrics EpiSensor seismometer. Image courtesy of Kinemetrics.