The small area between the Mediterranean Sea and the Black Sea has two straits, with the Sea of Marmara between them. So this quake was quite close to Istanbul. A 5.8 quake is substantial, even by the standards of a country with excellent building codes and mostly modern buildings. I doubt Turkey's codes are that good, or if they are, not consistently enforced. And Istanbul has quite a stock of very old buildings. I would not be surprised if the death toll is substantial.