Tectonic stress isn't the only cause of earthquakes. New research suggests small quakes can be triggered by natural gas rising underground.

Researchers measured such an earthquake beneath the Maramara Sea, not far from Istanbul, Turkey.

In the wake of an earthquake, the distribution of tectonic stress is altered. As a result, quakes are typically followed by a series of small quakes, or after-shocks. But when researchers analyzed seismic data from the 5.1 earthquake that shook the western part of the Maramara Sea in July of 2011, they found very few aftershocks near the quake's origins.

"Instead, we recorded a lot of tremors at very shallow depths below the seafloor," Marco Bohnhoff, a scientists at the GFZ German Research Center for Geosciences, said in a news release. "This was quite surprising, because these layers consist of soft sediment that typically deforms aseismically under tectonic stress and does not make abrupt movements typical for earthquakes."

When researchers took a close look at how stress was redistributed in the wake of the original quake, they realized a nearby natural gas reservoir had come under increased pressure as a result of the tectonic reshuffling. The added pressure caused gas to migrate upwards, triggering a series of small earthquakes closer to the surface.

Scientists can't be certain of the exact mechanisms that triggered the quakes. The rising gas could have caused water-filled cavities in the rock to oscillate or triggered the slippage of small shear fractures.

The research, published Tuesday in the journal Scientific Reports, could help scientists better understand the multitude of variables and underground processes that determine a region's tectonic behavior.

"The seismic hazard and risk for the metropolitan region of Istanbul does not necessarily change as a result of the new findings," said Bohnhoff. "But they must be included in various earthquake scenarios to make them more realistic. In this way, we also highlight an aspect hitherto completely ignored by the public."

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