avert vs head off

avert

verb
  • To ward off, or prevent, the occurrence or effects of. 

  • To turn aside or away. 

head off

verb
  • To avoid the undesirable consequences of; prevent. 

  • To start out. 

  • To intercept. 

  • To turn away from the wind. 

  • To begin moving away. 

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