The Seismometer is an instrument designed to measure the motions of the ground. This includes seismic waves generated by earthquakes, nuclear explosions, and other seismic sources. Records of these activities allow seismologists to map the interior of the Earth, and locate and measure the size of the different sources.

There are also seismographs, which is sometimes used in place of the word seismometer. However, a seismograph is the older instrument in which the measuring and recording of ground motion were combined. In modern instruments they are separate functions.

They are both different from seismoscopes since they create a continuous record of ground motion. Seismoscopes only indicate that motion has occurred. The seismometer is composed of a weight that can move relative to the instrument frame. They also have a means of recording the motion of the mass relative to the frame.

Motion on the ground moves the frame; however, the mass tends not to move because of inertia. The motion between the frame and the mass can be measured to determine the motion of the ground.

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 instruments used optical levers to amplify the small motions involved.

Modern instruments use electronics. Through the use of an electronic feedback loop the newer systems keep the mass nearly motionless. In some systems the weight is allowed to move, this design is often used in the geophones used in seismic surveys for oil and gas. Professional observatories have instruments that measure three axis: north-south, east-west, and the vertical. The vertical is most often used because it is less noisy and gives better records of some seismic waves.

A solid foundation is critical for seismic reading. Sometimes they are mounted on bedrock, while others might be in boreholes to avoid thermal effects. Others are put in insulated enclosure on small buried piers of un-reinforced concrete. These modern instruments use sensors, amplifiers, and recording devices covering a wide range of frequencies. There is some distortion within the signals in these machines however the professionally designed systems have carefully characterized frequency transforms.

The instruments come in three main varieties: short period, long period and broadband. Velocity is measured using the short and long period; however they go off-scale when ground motion is strong enough to be felt by people.

There are digital and analog output seismographs. The analog require analog recording equipment and sometimes analog-to-digital converter. Digital seismographs can be simply input to a computer.
The broadband seismograph records in a broad range of frequencies. It consists of a small “proof mass”, confined by electrical forces and driven by sophisticated electronics. Through the use of a feedback the mass is held in place then the amount of force necessary to hold it back is recorded.

Most seismographs measure the ground motion using the distance sensor. The voltage generated in a sense coil measures the velocity of the ground. The current provides a sensitive, accurate measurement of the force between the mass and frame, thus measuring directly the ground’s acceleration.

Even a small wind can cause problems with the buoyancy of the seismographs. That is why many of them are sealed in rigid gas-tight enclosures. Since the Earth’s magnetic field moves most seismographs are made with moving parts that interact minimally with magnetic fields. Many are also constructed of low expansion materials so that temperature does not affect the reading.

The seismometer can also determine the center of the earthquake by taking readings on the seismic waves.