Seismic instruments are used to measure low-frequency ground motion caused by earthquakes. They detect the seismic waves created by subsurface ruptures and convert ground motions into electronic signals which are suitable for transmission.
Seismometers are spring-mounted seismic instruments which resemble a pendulum. They detect seismic waves and magnify ground movement by mechanical, electronic, electromagnetic, or mechanical-optical methods. Seismometer outputs are transmitted to seismographs, seismic instruments that produce seismographs and seismograms. A seismograph records the movement of the earth. Typically, the output of a seismograph is proportional to ground velocity; however, special seismographs called accelerographs provide outputs that are proportional to ground acceleration.
A seismogram is a paper document that displays summary information about the magnitude and location of an earthquake. There are four major types of seismograms: tectonic-like earthquakes; shallow, volcanic earthquakes; surface events; and harmonic tremors.
Seismic instruments such as seismographs measure earthquakes with different degrees of sensitivity. There are five are five basic types of galvanometric seismographs, each of which belongs to a lettered class. Class A seismographs are short-period devices with a maximum sensitivity in the period range 0.1 to 1.0 second. Class B seismographs are long-period devices with high-yield magnification for periods longer than 100 seconds. Class C seismographs feature constant magnification over durations from 1 to 10 seconds. Class D seismographs are characterized by a uniform to velocity over a relatively wide range of times. Class E seismographs have a seismometer and galvanometer with identical periods. Non-galvanometric seismographs include mechanical devices that are designed to record high-frequency motion with very low magnification.
Seismic instruments can be divided into short-period and long-period devices. Short-period products are designed for passive listening at frequencies greater than 1 Hz during short-term deployments for activities such as experiments. Long-period seismic instruments are also suitable for passive listening at greater than 1 Hz, but for longer deployments that involve continuous monitoring.