earth tremor vs tremor

earth tremor and tremor both are nouns.

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

As nouns, earth tremor and tremor are synonyms defined as:

Other synonyms of earth tremor include microseism.

earth tremor (noun)	tremor (noun)
a small earthquake	an involuntary vibration (as if from illness or fear)
	a small earthquake
	shaking or trembling (usually resulting from weakness or stress or disease)

earth tremor (verb)	tremor (verb)
	shake with seismic vibrations