Gassy Earthquakes Near Istanbul May Pose New Risks to Region

Carved within the faded wall of Istanbul’s Little Hagia Sophia is a giant crack that runs from the base of the dome toward the peak. It’s one of the many scars etched throughout the city’s ancient walls, domes and minarets that serve as a reminder of the earthquakes that continuously rattle Turkey and the surrounding region.

Istanbul — with a population of approximately 15 million — straddles one of the most active seismic fault lines on the globe. And that fault, which sits below the turquoise waters of the Sea of Marmara just south of the city, is expected to rupture in coming decades, causing a devastating earthquake.

That makes scientists eager to better understand the North Anatolian fault line. In a new study published Tuesday in Scientific Reports, Louis Géli from the French Research Institute for Marine Exploitation, IFREMER, and his colleagues showed that the fault shook nearby gas reservoirs — a result that could help scientists better assess earthquake risks.

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 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, Géli said.

Tom Parsons, a geophysicist at the Pacific Coastal and Marine Science Center who has studied the North Anatolian fault but was not involved in this study, is not so sure. He argues that while the latest results can certainly be pinned on gas movement, that does not necessarily mean the fault is locked. The waters of the Marmara Sea have hindered more complete studies of the break.

Nonetheless, the results highlight an additional hazard. Should a major earthquake hit the natural gas reservoir, it could trigger gas leaks or even explosions, argued co-author Marco Bohnhoff from the GFZ German Research Center for Geosciences. “This is a kind of seismic hazard that has so far not been considered by authorities,” he said.

Parsons agrees that because the 2011 earthquake released a large amount of underground gases, it’s likely that a larger earthquake will expel even more methane gas. He also worries a larger earthquake could spark submarine landslides within the Sea of Marmara and cause a tsunami.

In many ways, the results highlight how much is unknown, and the need for better assessment of the region’s earthquake hazards. “This is a special setting and there are no models for what would happen,” Bohnhoff said.