Some examples of adverbs modifying other adverbs would be the following:
Abdullah finished his test somewhat hastily. (To what extent did Abdullah finish his test hastily?)
The restaurant is almost fully booked. (To what extent is the restaurant fully booked?)
Isadora jogs quite frequently. (To what extent does Isadora jog frequently?)
Cheyenne is often sharply dressed. (To what extent is Cheyenne sharply dressed?)