lead up to vs usher in

lead up to

verb
  • To set in motion; to act as a causal or preparatory event or sequence of events. 

  • To follow or mark a path toward. 

  • To precede in time. 

usher in

verb
  • To begin something with preparatory material; to introduce. 

  • To announce the arrival of something; to herald. 

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