[js1138]

There are reports of superwaves in the open sea, but not necessarily related to earthquakes.

[Quix]

Short of undersea landslides

or

waves getting into a kind of multiplying-effect-sync

[for lack of a better layman's way of putting it]

WHAT OTHER causes are there?

[XRdsRev]

A TSUNAMI (SUPER-WAVE) was responsible for the destruction of one U.S. WARSHIP.

"The Loss of the U.S.S. Memphis - August 29, 1916"

http://www.compass.dircon.co.uk/Memphis.htm