seism vs earth tremor

seism and earth tremor both are nouns.

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

As nouns, earth tremor is a hyponym of seism; that is, earth tremor is a word with a more specific, narrower meaning than seism:

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

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

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