Scientists uncovered evidence that a large earthquake in 1945 released more than seven million cubic metres of methane into the North Arabian Sea.
The discovery exposes a natural source of greenhouse gas emissions that has not been considered before, they claim.
As a greenhouse gas, methane is 20 times more potent than carbon dioxide, but less abundant in the atmosphere.
Enormous quantities of methane are locked in icy structures called hydrates on the floors of the continental shelves surrounding the Earth's land masses.
An estimated 1000 to 5000 gigatonnes of carbon is trapped in methane hydrates - more than the total amount released by burning fossil fuels each year.
Analysis of sediment cores taken from the North Arabian Sea in 2007 revealed chemical signs of large-scale methane release.
A search of historical records showed that a magnitude 8.1 earthquake had occurred nearby in 1945.
"Based on several indicators, we postulated that the earthquake led to a fracturing of the sediments, releasing the gas that had been trapped below," said lead scientist Dr David Fischer, from the University of Bremen in Germany.
A conservative estimate indicated that roughly 7.4 million cubic metres of methane had escaped, said the researchers writing in the journal Nature Geoscience.
"There are probably even more sites in the area that had been affected by the earthquake," Dr Fischer added.
Methane hydrates have been suggested as a promising energy source, but mining them is expensive and risky.
The hydrates help to stabilise the ocean floor, and there is a chance that disturbing them will trigger land slips and tsunamis.
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