Abstract
HOW often we use the term “terra firma”! It is used despite the fact that no square yard of the earth's surface is ever at rest; an unending train of waves, waxing and waning in amplitude, are unceasingly coursing along the earth's crust and to unknown depths. The wave period ranges between 4 and 8 sees.; the amplitude is between 1/50,000 and 1/2000 in., but with a wave-length of 8 to 16 miles. The speed of the waves is believed to be about 2 miles per sec. These microseisms have been known to seismologists for twenty years or more, and were originally thought to be air tremors. Later, the rocking of the observatory buildings in the wind was suggested as their origin, or the rocking of the ground due to the motion of trees in the vicinity; but it is now established that these disturbances are pure earth-movements travelling over long· distances. With sensitive seismographs, microseisms are easily recorded, but whilst hypotheses have not been lacking, their origin and cause still remain unknown.
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SHAW, J. Microseisms. Nature 106, 348–350 (1920). https://doi.org/10.1038/106348a0
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DOI: https://doi.org/10.1038/106348a0