German and Turkish scientists on Tuesday said they had pinpointed an extremely dangerous seismic zone less than 20 kilometres (12 miles) from the historic heart of Istanbul.
Running under the Sea of Marmara just south of the city of some 15 million people, this segment of the notorious North Anatolian fault has been worryingly quiet in recent years, which may point to a buildup in tension, they wrote.
"The block we identified reaches 10 kilometres (about six miles) deep along the fault zone and has displayed no seismic activity since measurements began over four years ago," said Marco Bohnhoff, a professor at the German Research Centre for Geosciences (GFZ) in Potsdam, near Berlin.
"This could be an indication that the expected Marmara earthquake could originate there."
The North Anatolian fault, created by the collision of the Anatolia Plate with the Eurasia Plate, runs 1,500 kilometres (950 miles) along northern Turkey.
At the western tip of the fault, an earthquake took place in 1912 at Ganos near the Aegean Sea.
On its eastern side, a domino series of earthquakes in 1939, 1942, 1951, 1967 and 1999 displaced the stress progressively westwards, bringing it ever closer to Istanbul.
What is left now is a so-called earthquake gap under the Sea of Marmara, lying between the two fault stretches whose stress has been eased by the quakes. The "gap" itself, however, has not been relieved by an earthquake since 1766.
Seeking a more precise view of the gap, the GFZ and Istanbul's Kandilli Earthquake Observatory set up a network of seismic monitors in the eastern part of the sea.
They calculate that the Anatolian fault normally has a westward motion of between 25 and 30 millimetres (one to 1.2 inches) per year.
But this natural slippage is being blocked by a small section, about 30 km (19 miles) long, located under a chain of nine small islands known as the Princes Islands -- a popular destination for day-trippers from Istanbul.
"The seismic silence along the Princes Islands segment stands in contrast to the background activity in the broader Izmit-Marmara region," warns the study published in the journal Nature Communications.
The paper says that, conceivably, stress under the Princes Islands is being relieved "aseismically," in other words, the pressure is being eased so gradually as to be undetectable.
But this scenario is unlikely, it says.
"Our evidence indicates that this patch is locked and is therefore a potential nucleation point for another Marmara segment earthquake -- a potential that has significant natural hazards implications" for Istanbul, it warns.
The study does not make any prediction about the size of any future quake or when it could occur.
But it notes an estimate published in 2004 that found a 35-to-70 percent probability that the "gap" will be struck by an earthquake greater than magnitude seven by 2034.
Other scientists have also pointed to the possibility of several smaller "en echelon" type quakes, which may generate less ground motion but are likelier to cause tsunamis because they displace the sea floor.
The last big quakes on the North Anatolian fault in 1999 -- a 7.1-magnitude quake in Duzce and 7.4-magnitude quake in Izmit -- left some 20,000 people dead.