Earthquakes can rip open sub-sea pockets of methane, a highly potent greenhouse gas, according to a study by German and Swiss scientists published on Sunday in the Journal Nature Geoscience. Quake-caused methane should be added to the list of heat-trapping carbon emissions that affect the world’s climate system, although the scale of this contribution remains unclear.
And here a part of their abstract in which they actually describe how quakes actually form cracks into the ground trough which gas goes. Pretty close to hydraulic fracturing (man-made):
Our seismic reflection data suggest that co-seismic shaking fractured gas-hydrate-bearing sediments, creating pathways for the free gas to migrate from a shallow reservoir within the gas hydrate stability zone into the water column. […] We conservatively estimate that 3.26×108 mol of methane have been discharged from the seep site since the earthquake. We therefore suggest that hydrocarbon seepage triggered by earthquakes needs to be considered in local and global carbon budgets at active continental margins.