Abnormal methane emission concentrations detected at the northern edge of the Cascadia subduction zone

Off the coast of Washington, pillars of bubbles rise from the bottom of the sea, as if a dragon were sleeping there. But these bubbles are methane, which is squeezed out of the sediment and rises up through the water. The places where they appear provide important clues to what will happen during a major marine earthquake.

The first large-scale analysis of these gas emissions along the Washington coast reveals more than 1,700 bubble plumes, mainly grouped in a north-south strip about 30 miles (50 kilometers) offshore.

Scientists discovered the first methane emissions on the outskirts of Washington in 2009 and thought they were lucky to find them at that time. But since then this number has only grown exponentially.

The results show that gas and liquid rise through faults generated by the movement of geological plates that produce large marine earthquakes in the Pacific Northwest.

These vents are fickle, like geysers in Yellowstone.

Sometimes they turn off and on with the tides, and they can move a little along the seabed.

They are usually found in clusters within a radius of about three football fields.

The authors analyzed data from numerous research expeditions over the past decade that use modern sonar technology to map the seabed. Their new results show that more than 1,778 bubble methane plumes emerging from the waters of Washington state are grouped into 491 clusters.

The vast majority of the newly observed methane plume sections are located on the sea side of the continental shelf, at a depth of about 160 meters.

A previous UW study suggested that warming sea water could release frozen methane in this region, but further analysis showed that methane bubbles off the Pacific Northwest coast originate from sites that have been present for hundreds of years and are not associated with global warming.

To understand why methane bubbles arise here, the authors used archival geological surveys conducted by oil and gas companies in the 1970s and 1980s. they show fault zones in bottom sediments where gas and fluid migrate upward to reach the seabed.

These seismic studies in areas with methane emissions indicate that the edge of the continental shelf will be pushed to the West during a large earthquake of magnitude 9. Faults at this tectonic boundary provide permeable paths for methane gas and warm liquid to escape from deep sediments.

If this new hypothesis proves true, then it has serious implications for understanding how this subduction zone works.

A study by the University of Washington and Oregon State University was published in the journal Geophysical Research: Solid Earth.

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