Seismometer | Universe Today

Written by Jerry Coffey

A seismometer is a a device that is made to measure and record the motions of the ground, especially those of seismic waves generated by earthquakes and nuclear explosions. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources. Seismograph is another term. 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 distinguishes them from a seismoscope, which merely indicates that motion has occurred, perhaps with some simple measure of how large it was.

Inertial seismometers have several parts of note. A mass, usually called the inertial mass, that can move relative to the instrument frame, but is attached to it by a spring that will hold it fixed relative to the frame if there is no motion, and also damp out any motions once the motion of the frame stops and a A means of recording the motion of the mass relative to the frame, or the force needed to keep it from moving. 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 seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop. The motion of the mass relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is recorded digitally. In other systems the mass is allowed to move, and its motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is often used in the geophones used in seismic surveys for oil and gas.

There is a great deal more information on early types of seismometers on this website. Here on Universe Today there is a great article on the use of seismometers on the Moon. Astronomy Cast has a good episode to help you understand the basics about Earth and its building blocks.

Filed under: Astronomy

Related stories on Universe Today

• LRO Spots Apollo 14 Booster Crash Site on Moon
• Watch Out for Moonquakes
• GPS Can Predict Tsunamis
• Rosetta Finally Launches
• What's Up this Week: January 29 – February 4, 2007

Tags: geophone, seismometer, sesimoscope