earthquake vs seism

earthquake and seism both are nouns.

Word	Noun	Adjective	Verb	Adverb
earthquake	Yes	No	No	No
seism	Yes	No	No	No

As nouns, earthquake and seism are synonyms defined as:

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

Other synonyms of earthquake include quake, temblor.

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