The finding follows a series of aftershocks that occurred in the weeks after an earthquake struck the western part of the Marmara Sea on July 25, 2011. Typically, those smaller rumbles are a good omen. They suggest that the fault is releasing pent-up energy. But when a fault line is motionless, scientists have to worry because it might be locked and could rupture suddenly.

But when Dr. Géli and his colleagues analyzed those aftershocks they noticed something rather odd. The tremors did not occur at the same depth as the main earthquake deep within the hard bedrock, but at shallower depths within the sea’s muddy sediment. Such a surprise could mean that a mechanism other than tectonic stress was at play.

The researchers now blame underground gases. When the larger quake hit a nearby gas reservoir, it released gases that moved upward and triggered weaker quakes. The finding suggests that the North Anatolian fault line is not directly responsible for the aftershocks, which could mean that it is far more at risk of a major quake than previously assumed, Dr. Géli said.