The Collaborative International Dictionary of English v.0.48:

Seaquake \Sea"quake`\ (s[=e]"kw[=a]k`), n.
   A quaking of the sea.
   [1913 Webster] Sear

WordNet (r) 3.0 (2006):

seaquake
    n 1: an earthquake at the sea bed [syn: seaquake, submarine
         earthquake]