Seismometer

Seismometer (in Greek seismos = earthquake and metero = measure) are used by seismologists to measure and record seismic waves. By studying seismic waves, geologists can map the interior of the Earth, and measure and locate earthquakes and other ground motions. The term seismograph is usually interchangeable, but seismometer seems to be a more common useage.

Basic principles

Seismometers have:

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• A frame securely affixed to the earth. The foundation is critical, and often the most expensive part of a seismic station.
• An inertial mass suspended in the frame by some method, using springs or gravity to establish a steady-state reference position.
• A damper system to prevent long term oscillations in response to an event.
• A means of recording the motion of the mass relative to the frame. Early seismometers used optics, or motion-amplifying mechanical linkages, while modern instruments use electronic amplification of signals generated by position or motion sensors.

Passing seismic waves move the frame, while the mass tends to stay in a fixed position due to its inertia. The seismometer measures the relative motion between the frame and the suspended mass.

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Professional seismic observatories usually have instruments measuring three axes, north-south, east-west, and up-down. Seismologists generally prefer a vertical seismograph if only one instrument is available.

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A professional station is often mounted on bedrock with an uncracked connection to a continental plate. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Amateur, or less exotic instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site should always be surveyed for ground noise with a temporary installation before pouring the pier and laying conduit.

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