silence vs hush up

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

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

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

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

silence (noun)	hush up (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)

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