Well, I cant say that, talking earthquakes, Istanbul is the securest city on earth. However, the subject beeing known by the government, and by all local authorities, new constructions are closely watched and all built according to very strict criterias. I would recommend new developments which are very much secured to the deadliest earthquake.

On the other hand, one other city that is not too much under the spots, but that I think should be, is Ankara, which is one the 2nd most populated city in Turkey, and of course the capital city of Turkey. (and no earthquake risks...)