Columns of bubbles have risen from the seafloor, off the coast of Washington, columns of bubbles rise from the seafloor. These bubbles are actually methane which is squeezed out of the sediment and has risen up through the water. Their locations indicate what would happen in a major offshore earthquake.
“We found the first methane vents on the Washington margin in 2009, and we thought we were lucky to find them, but since then, the number has just grown exponentially,” said Paul Johnson, the lead author.
“These vents are a little ephemeral,” he added. “Sometimes they turn off-and-on with the tides, and they can move around a little bit on the seafloor. But they tend to occur in clusters within a radius of about three football fields. Sometimes you’ll go out there and you’ll see one active vent and you’ll go back to the same location and it’s gone. They’re not reliable, like the geysers at Yellowstone.”
Sediments off the coast are developed as the oceanic plate, Juan de Fuca plunges under the continental plate of North America and scrapes materials off the crust of ocean. The sediments are further heated, then deformed and finally compressed. This compression causes both the fluid and the methane gas to emerge out as bubble streams rising from the seafloor.
Bubble columns are situated mostly at the border of the flat continental shelf and the slope section where seafloor drops to the depths of the open ocean.
“Although there are some methane plumes from all depths on the margin, the vast majority of the newly observed methane plume sites are located at the seaward side of the continental shelf, at about 160 meters water depth,” said Johnson.
For understanding the reason behind the occurrence of methane bubbles, archive geologic surveys have been conducted by gas and oil companies in the 1970s and 1980s. The surveys show the fault zones in sediment where the gas and the fluid travel upward and finally emerge from the seafloor.
“Seismic surveys over the areas with methane emission indicate that the continental shelf edge gets thrust westward during a large megathrust, or magnitude-9, earthquake,” Johnson also said. “Faults at this tectonic boundary provide the permeable pathways for methane gas and warm fluid to escape from deep within the sediments.”