Istanbul, a megapolis of 15 million-plus inhabitants, was struck by a powerful earthquake today, sparking fear in schools and workplaces. One death as a result of a heart attack was reported after the 5.8 magnitude tremblor struck at 1:59 p.m., causing high-rises to sway and a minaret to crumble. At least 43 people were said to have been injured in the earthquake.

Cellular telephone networks were down for over two hours after the earthquake struck, sowing further panic as residents were unable to reach loved ones.

The Kandilli Observatory and Earthquake Research Institute placed the depth of the tremor at 12.6 kilometers (7.8 miles) and its epicenter 70 kilometers (40 miles) west of Istanbul in the Sea of Marmara. It was felt in neighboring provinces including Tekirdag and Bursa. 

Coming on the heels of a 4.6 magnitude quake that hit off the coast of Silivri in Istanbul province earlier this week, today’s earthquake triggered fears of an impending “big one.” Experts have warned that Istanbul is at risk of a major earthquake ever since a 7.6 magnitude quake struck the nearby city of Izmit in 1999, killing over 17,000 people including hundreds in Istanbul.