sever vs split off

sever

verb
  • To disconnect; to disunite; to terminate. 

  • To make a separation or distinction; to distinguish. 

  • To cut free. 

  • To suffer disjunction; to be parted or separated. 

split off

verb
  • To break away from a group or mass. 

How often have the words sever and split off occurred in a corpus of books? (source: Google Ngram Viewer )