Seismometer
From open-encyclopedia.com - the free encyclopedia.
A seismometer is an instrument for measuring earthquakes and other ground motions.
The principle can be shown by imagining a giant stationary pendulum, with a stylus on the bottom. As the earth starts to move, the heavy mass of the pendulum has the inertia to stay still in the non-earth frame of reference. The result is that the stylus scratches a pattern corresponding with the earth's movement.
Modern instruments are, of course, electronic. They come in two forms analog and digital. Analog instruments come in three main varieties: short period, long period and broad-band. The short and long period measure velocity and are very sensitive however they 'clip' or go off-scale for ground motion that is strong enough to be felt by people.
The modern broad-band seismometer (so called because of the capacity to record a very broad range of frequencies) consists of a small 'proof mass', confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is then recorded.
Accelerometers output this directly as acceleration (remembering Newton's F=ma), but broad-band seismometers use an integrating circuit to output velocity.
Another type of seismometer is a digital strong-motion seismometer. A strong-motion seismometer measures acceleration and outputs acceleration; this can be mathematically integrated later to give velocity though. One drawback of strong-motion seismometers is they are not as sensitive to ground motions as an analog instrument but they do stay on scale during the strongest of seismic shaking.
Seismometers spaced in an array can be used to precisely locate, in three dimensions, the source of the earthquake, using the time it takes for seismic waves to propagate away from the epicentre, the point of fault rupture. Seismometers are also used to detect underground nuclear test explosions.
External Links
Excellent history of seismometers http://neic.usgs.gov/neis/seismology/history_seis.html
Nice Java applet of old damped-mass seismometer http://www.ifg.tu-clausthal.de/java/seis/sdem_app-e.html
es:Sismómetro
