Here's why:
* Gerund: A gerund is a verb form ending in "-ing" that functions as a noun. In this case, "ringing" is the gerund.
* Gerund Phrase: A gerund phrase is a gerund plus any modifiers or complements that function as a noun. In this case, "of alarm" modifies the gerund "ringing".
The entire phrase "ringing of alarm" acts as the subject of the sentence, performing the action of "jolting".