揺らぐ

Etymology
Formerly, with the final -ku softening to -gu. Derived from adverb, itself derived from or cognate with the imperfective form of verb  ( /  / , "to rock, to shake").

Compare.

Verb

 * 1) to waver, to sway back and forth
 * 2) to shake, to tremble
 * 3) to feel shaken