A feeble earth tremor not directly perceptible, but detected only by means of specially constructed apparatus.

A vibration of the Earth that is unrelated to earthquake activity - instead it is caused by wind, moving trees, ocean waves or human activity.

a small earthquake

A motion in the Earth that is unrelated to an earthquake. It is caused by a variety of natural and artificial agents, for example wave action, wind, traffic and industrial noise.

A feeble oscillatory disturbance of the earth's crust, detectable only by very sensitive seismographs. Certain types of microseisms seem to be closely correlated with pressure disturbances. See microbarm.

A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds. It is caused by a variety of natural and artificial agents.

Weak, almost continuos background seismic waves or Earth noise that can be detected only by seismographs often caused by surf, ocean waves, wind, or human activity.

a faint, recurrent earth tremor lasting from 1 to about 9 seconds, detectable only with a seismograph. Microseisms are not related to earthquakes, but are due to atmospheric disturbances. [AHDOS