aftershock vs earth tremor

aftershock and earth tremor both are nouns.

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

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

aftershock: a tremor (or one of a series of tremors) occurring after the main shock of an earthquake
earth tremor: a small earthquake

Other hypernyms of aftershock include microseism, tremor.

aftershock (noun)	earth tremor (noun)
a tremor (or one of a series of tremors) occurring after the main shock of an earthquake	a small earthquake