tremble vs undulate

tremble

verb
  • To shake, quiver, or vibrate. 

  • To fear; to be afraid. 

noun
  • A shake, quiver, or vibration. 

undulate

verb
  • To move in wavelike motions. 

  • To cause to resemble a wave 

  • To appear wavelike. 

  • To cause to move in a wavelike motion. 

adj
  • Changing the pitch and volume of one's voice. 

  • Wavy in appearance or form. 

  • sinuous, winding up and down. 

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