It was a magnitude 5.8 earthquake, but nobody felt it. That’s because this quake took about 50 days to shake itself out. Occurring a few kilometres south of Istanbul, this “slow earthquake”, which took place during summer 2016, could be a sign that the dangerous North Anatolian fault is reawakening.

Geologists know that strain is travelling from east to west across Turkey (caused by Asia ploughing into Europe). Large earthquakes have sequentially released strain along the east-west trending North Anatolian fault, with the most recent being the devastating magnitude 7.6 Izmit quake in 1999, which killed more than 17,000 people. Istanbul and the surrounding area are next in line, posing a severe threat to the 15 million inhabitants.

Scientists reported in the journal Earth and Planetary Science Letters, that they had spotted the slow quake using strain-metre data from a borehole in the region. Had it released its energy in one go it would have been equivalent to a magnitude 5.8 quake. Although it released strain in the immediate vicinity, the wider impact isn’t clear. “It may have brought other parts of the fault closer to failure, but we need more data to get the full picture,” said the paper’s lead author, Patricia Martínez-Garzón.