Wiktionary
n. (context geology English) Any of the continuous background movements of the Earth's surface and interior, having a period of 1 to 9 seconds, not associated with any seismic activity
Wikipedia
Microtremor is a low amplitude (in the order of micrometres) ambient vibration of the ground caused by man-made or atmospheric disturbances. The term Ambient Vibrations is now preferred to talk about this phenomenon. Observation of microtremors can give useful information on dynamic properties of the site such as predominant period and amplitude. Microtremor observations are easy to perform, inexpensive and can be applied to places with low seismicity as well, hence, microtremor measurements can be used conveniently for seismic microzonation. More detailed information on the shear wave velocity profile of the site can be obtained from microtremor array observation.
Usage examples of "microtremor".
Continual microtremors from the subterranean activities made a hash of any attempt at fine monitoring.
There was a hint of anxiety in the microtremors of the technician's voice.