Seismic Signals

Much like sunlight contains different colors that can be split apart with a prism, seismic waves contain many different "frequencies" that we can record with specially "tuned" seismometers. The idea is completely analogous with light and sound

Seismic Light Sound
Short-period Blue Treble
Long-period Red Bass

 

The frequency is an indication of how quickly the signal oscillates. In the case of seismology it represents how many times the ground vibrates through a cycle each second. Seismic signals generally are a composite of vibrations spanning many different frequencies.

The range of ground motions that are interesting to seismologists is very large because the process of earth deformation occurs at many different rates and scales.

The above figure is modified from a figure in Aki and Richards (1980). The amplitude range of interesting signals in earthquake studies as a function of frequency compared with a similar range of physical dimensions of some common items. Since I am comparing the "spectral" amplitude as a function of frequency with physical dimensions of the common items, the analogy is not perfect, but the range of variation in size is well represented. Distance from the earthquake is represented by an upper-case Delta. We usually specify large distances over Earth's surface in units of degrees and 1 degree = 111.19 km.

The large range of amplitudes we are interested in exists because we are interested in all the processes occurring in Earth, from small rock fractures that form in mines to the great earthquakes that occur each year. The amount of energy released by these different processes is enormous, and the large range of interesting amplitudes reflects this.

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