earth tremor vs seism

earth tremor and seism both are nouns.

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

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

earth tremor: a small earthquake
seism: 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 earthquake, quake, temblor.

earth tremor (noun)	seism (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