Source : Webster's Revised Unabridged Dictionary (1913)

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}, {*seis"mic*al}, a.

Source : WordNet®

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