From AMS Glossary
A collective term for small motions in the earth that are unrelated to an earthquake and that have a period of 1.0–9.0 s.
They are caused by a variety of natural and artificial agents. Certain types of microseisms seem to be closely correlated with pressure disturbances and can be used to locate such disturbances, especially in the case of tropical cyclones. In addition, traffic, industrial activities, and wind flexure of trees and tall structures can create microseisms.