happen vs occur

happen

verb
  • To occur or take place. 

  • To do or occur by chance or unexpectedly. 

  • To encounter by chance. 

occur

verb
  • To happen or take place. 

  • To present or offer itself. 

  • To be present or found. 

  • To come or be presented to the mind; to suggest itself. 

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