earth tremor vs earthquake

earth tremor and earthquake both are nouns.

Word	Noun	Adjective	Verb	Adverb
earth tremor	Yes	No	No	No
earthquake	Yes	No	No	No

As nouns, earthquake is a hypernym of earth tremor; that is, earthquake is a word with a broader meaning than earth tremor:

earth tremor: a small earthquake
earthquake: shaking and vibration at the surface of the earth resulting from underground movement along a fault plane or from volcanic activity

Other hypernyms of earth tremor include quake, seism, temblor.

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