let up vs peter out

let up

verb
  • To cease or stop. 

  • To cease or stop (doing something intense). 

peter out

verb
  • Synonym of peter (“originally (mining), of a vein of ore: to be depleted of ore; now (generally), to diminish to nothing; to dwindle, to trail off”) 

How often have the words let up and peter out occurred in a corpus of books? (source: Google Ngram Viewer )