earthquake vs seaquake

earthquake and seaquake both are nouns.

Word	Noun	Adjective	Verb	Adverb
earthquake	Yes	No	No	No
seaquake	Yes	No	No	No

As nouns, seaquake is a hyponym of earthquake; that is, seaquake is a word with a more specific, narrower meaning than earthquake:

earthquake: shaking and vibration at the surface of the earth resulting from underground movement along a fault plane or from volcanic activity
seaquake: an earthquake at the sea bed

Other hyponyms of earthquake include seismic disturbance, shock, earth tremor, microseism, tremor, submarine earthquake.

earthquake (noun)	seaquake (noun)
shaking and vibration at the surface of the earth resulting from underground movement along a fault plane or from volcanic activity	an earthquake at the sea bed
a disturbance that is extremely disruptive