Yea... the fault between the tectonic plates goes right up next to the Bosporus.
Istanbul's had earthquakes before, mind; the Hagia Sophia must've suffered at least ten and is still standing (Early Byzantine architects were AMAZING, that city's walls weren't breached from 500ish to 1453). This one's overdue, though, and so could be vaaaairy nasty.
I really want to go to Istanbul sometime...