hush up vs silence

hush up is not a noun while silence is a noun.

Word	Noun	Adjective	Verb	Adverb
hush up	No	No	Yes	No
silence	Yes	No	Yes	No

As verbs, hush up and silence are synonyms defined as:

Other synonyms of hush up include hush, quieten, shut up, still.

hush up (noun)	silence (noun)
	the absence of sound
	a refusal to speak when expected
	the trait of keeping things secret
	the state of being silent (as when no one is speaking)

hush up (verb)	silence (verb)
cause to be quiet or not talk	cause to be quiet or not talk
cover up a misdemeanor, fault, or error	keep from expression, for example by threats or pressure