Seismometers

Before technological advances in the last few decades, to record seismic signals we developed and deployed many different kinds of seismometers. In the 1960's, as part of an effort to verify underground nuclear test threshold treaties, two seismometer models became standard for global earthquake analyses, the world-wide network long-period and short-period instruments. These instruments have been replaced by "broad-band" instruments that can detect ground motions over large ranges, or "bands", of ground-vibration periods. However, since much of our historical data are recorded on the older "narrow-band" short-period and long-period instruments, they remain important sources of data. 

Seismometer Response Curves

Seismometers are usually designed to record signals over a specified range of frequencies (or periods) so it is convenient to discuss instruments based on the range of vibration frequencies that they can detect. Thus one way to characterize seismometers is to describe the range of vibration frequencies that they can detect. A plot of the amplification as a versus frequency is called an instrument response. An instrument is sensitive to vibrations at frequencies for which the "response" curve is relatively large. Five sample instrument response curves are shown below. The frequency of is shown along the horizontal axis, the equivalent period (period = 1/frequency) is shown along the top horizontal axis. The vertical axis shows the ground-motion amplification factor.

To characterize an instrument, what's really important is the range of amplitudes, not the specific amplification, which is usually adjusted depending on the location of the seismometer. I have used numbers around one to illustrate the differences between the response curves for different instruments but actual amplification factors are usually much larger than those shown.

The broad band instrument senses most frequencies equally well; the long-period and short period instruments are called "narrow" band, because they preferentially sense frequencies near 1/(15 s) and 1 hertz respectively. The yellow region is the low end of the frequency range audible to most humans (we can hear waves around 20 hertz to 20,000 hertz). The left panel is a comparison of a modern broadband seismometer response and the classic World-Wide Standard Seismic Network (WWSSN) long- and short-period instruments. The same broad-band response is shown in the right panel, to compare the response with a special short-period instrument, the Wood-Anderson, and an accelerometer. The Wood-Anderson short-period instrument was the one that Charles Richter used to develop his magnitude scale for southern California. The accelerometer is an instrument designed to record large amplitude and high-frequency shaking near large earthquakes. Those are the vibrations that are important in building, highway, etc. design.

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.