let up vs relent

let up

verb
  • To cease or stop (doing something intense). 

  • To cease or stop. 

relent

verb
  • To slacken; to abate. 

  • To give in or be swayed; to become less hard, harsh, or cruel; to show clemency. 

noun
  • Stay; stop; delay. 

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