How we record earthquakes
Seismographs
During an earthquake, vibrations caused by the breakage of rock along a fault zone radiate outward from the point of rupture. The instrument used to record and measure these vibrations is called a seismograph.
Traditional seismographs consisted of a sensing element, called a seismometer, an amplifier, and a hardcopy display unit often using photographic or heat-sensitive paper. The visual record produced by a seismograph is called a seismogram. In modern seismographs, the display is replaced or augmented with a digitizer and either local digitial storage (eg., removable disks) or a telemetry system using radio, telephone or the Internet to send the digital data stream to a central recording and analysis site. EarthquakesCanada owns and operates the Canadian National Seismograph Network and several special deployments, all of which are monitored from its data centers located in Ottawa, Ontario and Sidney, British Columbia.
How Seismometers Work
To determine the motion of the earth during an earthquake, ground motion must be measured against something that remains relatively fixed (i.e., not affected by the shaking). In a seismometer, the fixed object consists of a mass suspended on springs within a case. During an earthquake, the mass remains still while the case around it moves with the ground shaking. Most modern seismometers work electromagnetically. A large permanent magnet is used for the mass and the outside case contains numerous windings of fine wire. Movements of the case relative to the magnet generate small electric signals in the wire coil.
Earthquake waves decrease in strength as they travel through the earth. High-frequency waves attenuate most severely; consequently, seismographs designed for monitoring local earthquakes must respond to a different frequency of ground motion from those used for recording distant earthquakes. Instruments sensitive to seismic waves that vibrate several times per second, called short period seismographs, are used to record local earthquakes, during which the waves reaching the seismograph are still very rapid and close together. Long period seismographs respond to lower frequency waves and are used to record distant events. Modern broadband seismographs perform both functions.
Some short period seismographs magnify ground motion several hundred thousand times. Such sensitive high-gain instruments can detect ground far movements too small to be felt by a human being. In the case of large earthquakes nearby, the ground motion may exceed the recording capacity of seismographs. To record the signals from large local earthquakes accurately, a third type of low-gain, Strong motion seismograph is needed. Strong motion seismographs apply minimal magnification (less than 100x), and are generally sensitive to ground acceleration. Traditional strong motion instruments would not operate continuously, but only when triggered by strong ground movement, and would record only until the ground motion returned to an imperceptible level. Modern digital strong motion recorders are now replacing analog (photographic paper) recorders, and some have the option for continuous telemetry.
To completely characterize the earth's movement, the motion must be measured in three perpendicular directions. Consequently, seismographs often employ three sensors, recording in each of the north-south, east-west and vertical (up and down) directions.
Suggested Reading
- "The Amateur Scientist", Scientific American, July 1957 and July 1979: Basic principles and how to build a simple seismograph.
- Hodgson, John. Earthquakes and Earth Structure. New Jersey, Prentice Hall, 1964, p. 60-69: How seismographs work and interpretation of seismograms.