The Collaborative International Dictionary of English v.0.48:

Microseism \Mi"cro*seism\, n. [Micro- + Gr. ? an earthquake, fr.
   ? to shake.]
   A feeble earth tremor not directly perceptible, but detected
   only by means of specially constructed apparatus. --
   Mi`cro*seis"mic, Mi`cro*seis"mic*al, a.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

microseism
    n 1: a small earthquake [syn: tremor, earth tremor,
         microseism]