Your vivid memory of what you were doing when you learned about the attack on the world trade center is an example of

The answer is "it is an example of flashbulb memory".

A flashbulb memory refers to an extraordinarily clear 'depiction' existing apart from everything else and conditions in which a bit of astonishing and considerable  news was heard. The expression "flashbulb memory" proposes the amazement, aimless brightening, detail, and quickness of a photo; however flashbulb memories are just to some degree unpredictable and are a long way from finish.