The columns of bubbles are especially pronounced off Alki Point in West Seattle and near the ferry terminal in Kingston, Washington, according to a in the January issue of Geochemistry, Geophysics, Geosystems.

“There’s methane plumes all over Puget Sound,” said lead author , a 91̽��professor of oceanography. “Single plumes are all over the place, but the big clusters of plumes are at Kingston and at Alki Point.”

Previous 91̽��research had found methane bubbling up from the outer coasts of Washington and Oregon. The bubbles in Puget Sound were first discovered by surprise in 2011, when the UW’s global research vessel, the RV Thomas G. Thompson, had kept its sonar beams turned on as it returned to its home port on the 91̽��campus. The underwater images created by the soundwaves showed a distinct, persistent bubble plumes as the vessel rounded the Kingston ferry terminal.

Since then, the team analyzed sonar data collected during 18 cruises on the UW’s smaller research vessel, the RV Rachel Carson. Methane plumes were seen from Hood Canal to offshore of Everett to south of the Tacoma Narrows. At Alki, the bubbles rise 200 meters, about the height of the Space Needle, to reach the ocean’s surface.

“Off Alki, every 3 feet or so there’s a crisp, sharp hole in the seafloor that’s 3-5 inches in diameter,” Johnson said. “There are holes all over the place, but there aren’t bubbles or fluid coming out of all of them. There’s occasionally a burst of bubbles, and then another one 50 feet away that has a new burst of bubbles.”

This research video shows bubbles emerging from the seafloor about 200 meters (650 feet) deep. It was recorded Oct. 25, 2020, about 1 mile offshore from Seattle’s Alki Point. Credit: Paul Johnson/91̽��

The study is an early step toward exploring the release of methane from estuaries, or places where saltwater and freshwater meet, a subject more widely studied in Europe. Though only a small amount of natural methane is released compared to human sources, understanding how the greenhouse gas cycles through ecosystems becomes increasingly important with climate change.

“In order to understand methane in the atmosphere and control the human sources, we have to know the natural sources,” Johnson said.

The two persistent fields of bubble plumes occur above geologic faults: for the Alki bubbles, located above a branch of the Seattle Fault, and for the Kingston bubbles, above the South Whidbey Fault. It’s likely that the bubbles are connected to the underlying geology, Johnson said.

Questions remain about the bubbles’ origins. One initial hypothesis, that the bubbles might be coming from the Cascadia Subduction Zone, was not supported by preliminary data. The gas bubbles don’t show the same distinctive chemistry as nearby hot springs and deep wells that connect to this geologic feature deep underground.

Humans also don’t seem responsible. Puget Sound has in the past been a dumping ground for waste or sediment, but vigorous tides sweep that material out into the open ocean, Johnson said. Sewer outflows, gas lines and freshwater storm drains also don’t match the plumes’ locations.

Instead, a biological source of methane beneath the seafloor seems likely, Johnson said. The source may be in the dense clay sediment deposited after the last Ice Age, when glaciers first carved out the Puget Sound basin. The methane seems to be biological in origin, and the bubbles also support methane-eating bacterial mats in the surrounding water.

Jerry (Junzhe) Liu, a senior in oceanography, helped to analyze the data and participated in a 2019 cruise that contributed data.

“I’m interested in two seemingly parallel fields: fault zones and air-sea interactions for climate,” Liu said. “This project covers all the way from below the seafloor to above the ocean’s surface.”

In follow-up work, scientists used underwater microphones this fall to eavesdrop on the bubbles. , an associate professor at the 91̽�� Bothell, is analyzing the sound that bubbles make when they are emitted. The team also hopes to go back to Alki Point with a remotely operated vehicle that could place instruments inside a vent hole to fully analyze the emerging fluid and gas.

Co-authors of the paper are , an engineer in 91̽��oceanography; Chenyu (Fiona) Wang, a former 91̽��undergraduate; , a 91̽��associate professor of oceanography; , a 91̽��affiliate assistant professor of oceanography and researcher at the Pacific Northwest National Laboratory; Susan Merle and Sharon Walker at the National Oceanic and Atmospheric Administration; and Tamara Baumberger at Oregon State University. The research was funded by the National Science Foundation.

For more information, contact Johnson at paulj@uw.edu.

The , from the 91̽�� and Oregon State University, was recently published in the Journal of Geophysical Research: Solid Earth.

The first large-scale analysis of these gas emissions along Washington’s coast finds more than 1,700 bubble plumes, primarily clustered in a north-south band about 30 miles (50 kilometers) from the coast. Analysis of the underlying geology suggests why the bubbles emerge here: The gas and fluid rise through faults generated by the motion of geologic plates that produce major offshore earthquakes in the Pacific Northwest.

“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 lead author , a 91̽��professor of oceanography.

“These vents are a little ephemeral,” Johnson 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.”

The authors analyzed data from multiple research cruises over the past decade that use modern sonar technology to map the seafloor and also create sonar images of gas bubbles within the overlying water. Their new results show more than 1,778 methane bubble plumes issuing from the waters off Washington State, grouped into 491 clusters.

“If you were able to walk on the seafloor from Vancouver Island to the Columbia River, you would never be out of sight of a bubble plume,” Johnson said.

The sediments off the Washington coast are formed as the Juan de Fuca oceanic plate plunges under the North American continental plate, scraping material off the ocean crust. These sediments are then heated, deformed and compressed against the rigid North American plate. The compression forces out both fluid and methane gas, which emerges as bubble streams from the seafloor.

The bubble columns are located most frequently at the boundary between the flat continental shelf and the steeply sloped section where the seafloor drops to the abyssal depths of the open ocean. This abrupt change in slope is also a tectonic boundary between the oceanic and continental plates.

“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,” Johnson said.

A previous from the 91̽��had suggested that warming seawater might be releasing frozen methane in this region, but further analysis showed the methane bubbles off the Pacific Northwest coast arise from sites that have been present for hundreds of years, and are not related to global warming, Johnson said.

Instead, these gas emissions are a long-lived natural feature, and their prevalence contributes to the continental shelf area being such productive fishing grounds. Methane from beneath the seafloor provides food for bacteria which then produce large quantities of bacterial film. This biological material then feeds an entire ecological chain of life that enhances fish populations in those waters.

“If you look online at where the satellite transponders show where the fishing fleet is, you can see clusters of fishing boats around these methane plume hotspots,” Johnson said.

To understand why the methane bubbles occur here, the authors used archive geologic surveys conducted by the oil and gas companies in the 1970s and 1980s. The surveys, now publicly accessible, show fault zones in the sediment where the gas and fluid migrate upward until emerging 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 said. “Faults at this tectonic boundary provide the permeable pathways for methane gas and warm fluid to escape from deep within the sediments.”

The location of these faults could potentially provide new understanding of the earthquake hazard from the Cascadia Subduction Zone, which last ruptured more than 300 years ago. If the seafloor movement during a subduction-zone earthquake occurs closer to shore, and a major component of this motion occurs within the shallower water, this would generate a smaller tsunami than if the seafloor motion were entirely in deep water.

“If our hypothesis turns out to be true, then that has major implications for how this subduction zone works,” Johnson said.

Co-author at Oregon State University is also the lead author of an upcoming paper that will provide a similar inventory of methane bubble plumes off Oregon’s coast. Other co-authors are at Oregon State University, former 91̽��oceanography doctoral student , and former 91̽��oceanography undergraduate students and . The study was funded by the National Science Foundation’s GeoPRISM program.

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For more information, contact Johnson at 206-543-8474 or paulj@uw.edu.

Researchers analyzing data from ocean bottom seismometers off the Washington-Oregon coast tied a series of underwater landslides on the Cascadia Subduction Zone, 80 to 161 kilometers (50 to 100 miles) off the Pacific Northwest coast, to a 2012 magnitude-8.6 earthquake in the Indian Ocean – more than 13,500 kilometers (8,390 miles) away. These underwater landslides occurred intermittently for nearly four months after the April earthquake.

Previous research has shown earthquakes can trigger additional earthquakes on other faults across the globe, but the new study shows earthquakes can also initiate submarine landslides far away from the quake.

“The basic assumption … is that these marine landslides are generated by the local earthquakes,” said , an oceanographer at the 91̽�� and lead author of the published in the Journal of Geophysical Research: Solid Earth, a journal of the American Geophysical Union.

“But what our paper said is, ‘No, you can generate them from earthquakes anywhere on the globe.’”

The new findings could complicate sediment records used to estimate earthquake risk. If underwater landslides could be triggered by earthquakes far away, not just ones close by, scientists may have to consider whether a local or a distant earthquake generated the deposits before using them to date local events and estimate earthquake risk, according to the study’s authors.

The submarine landslides observed in the study are smaller and more localized than widespread landslides generated by a great earthquake directly on the Cascadia margin itself, but these underwater landslides generated by distant earthquakes may still be capable of generating local tsunamis and damaging underwater communications cables, according to the study authors.

A happy accident

The discovery that the Cascadia landslides were caused by a distant earthquake was an accident, Johnson said.

Scientists had placed ocean bottom seismometers off the Washington-Oregon coast to detect tiny earthquakes, and also to measure ocean temperature and pressure at the same locations. When Johnson found out about the seismometers at a scientific meeting, he decided to analyze the data the instruments had collected to see if he could detect evidence of thermal processes affecting seafloor temperatures, such as methane hydrate formation.

Johnson and his team combined the seafloor temperature data with pressure and seismometer data and video stills of sediment-covered instruments from 2011-2015. Small variations in temperature occurred for several months, followed by large spikes in temperature over a period of two to 10 days. They concluded these changes in temperature could only be signs of multiple underwater landslides that shed sediments into the water. These landslides caused warm, shallow water to become denser and flow downhill along the Cascadia margin following the 8.6-magnitude Indian Ocean earthquake on April 11, 2012, causing the temperature spikes.

The Cascadia margin runs for more than 1,100 kilometers (684 miles) off the Pacific Northwest coastline from north to south, encompassing the area above the underlying subduction zone, where one tectonic plate slides beneath another.

Steep underwater slopes hundreds of feet high line the margin. Sediment accumulates on top of these steep slopes. When the seismic waves from the Indian Ocean earthquake reached these steep underwater slopes, they jostled the thick sediments piled on top of the slopes. This shaking caused areas of sediment to break off and slide down the slope, creating a cascade of landslides all along the slope. The sediment did not fall all at once so the landslides occurred for up to four months after the earthquake, according to the authors.

The steeper-than-average slopes off the Washington-Oregon coast, such as those of Quinault Canyon, which descends 1,420 meters (4,660 feet) at up to 40-degree angles, make the area particularly susceptible to submarine landslides. The thick sediment deposits also amplify seismic waves from distant earthquakes. Small sediment particles move like ripples suspended in fluid, amplifying the waves.

“So these things are all primed, ready to collapse, if there is an earthquake somewhere,” Johnson said.

Disrupting the sediment record

The new finding could have implications for tsunamis in the region and may complicate estimations of earthquake risk, according to the study’s authors.

Subduction zones like the Cascadia margin are at risk for tsunamis. As one tectonic plate slides under the other, they become locked together, storing energy. When the plates finally slip, they release that energy and cause an earthquake. Not only does this sudden motion give any water above the fault a huge shove upward, it also lowers the coastal land next to it as the overlying plate flattens out, making the shoreline more vulnerable to the waves of displaced water.

Submarine landslides increase this risk. They also push ocean water out of the way when they occur, which could spark a tsunami on the local coast, Johnson said.

Scientists also use underwater sediment records to estimate earthquake risk. By drilling sediment cores offshore and calculating the age between landslide deposits, scientists can create a timeline of past earthquakes used to predict how often an earthquake might occur in the region in the future and how intense it could be.

An earthquake off the Pacific Northwest would create submarine landslides all along the coast from British Columbia to California. But the new study found that a distant earthquake might only result in landslides up to 20 or 30 kilometers (12 to 19 miles) wide. That means when scientists take sediment cores to determine how frequent local earthquakes occur, they may not be able to tell if the sediment layers arrived on the seafloor as a result of a distant or local earthquake.

Johnson says more core sampling over a wider range of the margin would be needed to determine a more accurate reading of the geologic record and to update estimates of earthquake risk.

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For more information, contact Johnson at 206-543-8474 or johnson@ocean.washington.edu.

Additional co-authors: Joan Gomberg at 206-616-5581 or gomberg@uwgs.gov; Susan Hautala at 206-543-0596 or hautala@uw.edu; and Marie Salmi at 206-616-5821 or maries3@uw.edu.

A new , to be published in the September issue of , may shake the research community’s confidence in what the sediment record can say about past earthquakes. The lead author is , a 91̽��affiliate professor of Earth and space sciences and a geologist with the U.S. Geological Survey.

The report focuses on the Cascadia subduction zone — a giant active submarine fault that slants eastward beneath the Pacific coast of southern British Columbia, Washington, Oregon and northern California. Studies in the past three decades have provided increasingly specific estimates of Cascadia earthquake sizes and repeat times, which affect public safety through seismic provisions in building design and tsunami limits on evacuation maps.

At issue is not whether the Cascadia subduction zone produces enormous earthquakes, Atwater said. What the report asks instead is how much geologists can say, with confidence, about the history of those earthquakes going back thousands of years.

Sediment cores collected over the past two decades near the foot of the continental slope, about 100 miles off Washington’s coast and on seafloor sloping to nearly two miles deep, are among a broader set of cores that underpin influential estimates of Cascadia earthquake size and recurrence in 2012.

The new report questions the geologic basis for these estimates, not by collecting newer samples, but by looking at a larger suite of cores collected and first analyzed in the 1960s and 1970s. Authors visited the 91̽��Libraries and dusted off oceanography doctoral dissertations submitted more than four decades ago.

The new study draws on sediment core log notes handwritten by co-author Bobb Carson (’71), and another 91̽��alumnus, William Barnard (’73), during research cruises off the Washington coast. Both were graduate students with Dean McManus, a 91̽��emeritus professor of oceanography. Other samples were collected by co-author , then a graduate student at Oregon State University and now a scientist at the University of California, Santa Cruz.

Those Nixon-era sediment cores were originally collected for research unrelated to earthquakes. The scientists were interested in tracing – beds of sand and mud laid down by bottom-hugging, sediment-driven currents that infrequently emerged from submarine canyons onto the deep ocean floor. Not until a 1990 by a Canadian government geologist would turbidites be reinterpreted as clues to Cascadia earthquake history.

The new study asks how well geologists have managed to read that evidence. Images of the marine sediment layers collected in 2012 by co-authors , a 91̽��professor of oceanography, and current 91̽��oceanography graduate student suggest that in some of the deep-sea canyons along the Washington coast, shaking would not send the sediment flowing as the current models predict.

The historical samples confirmed that this can happen.

“Few earthquakes managed to register at Bobb [Carson]’s core sites offshore of northern Washington,” Atwater said. “For seismology it would be great if earthquakes and turbidites were to correspond one for one. But Bobb’s geology makes clear that it’s not nearly that simple.”

Carson said he was surprised to be contacted about his 91̽��doctoral work.

“Wouldn’t anyone be?” said Carson, who went on to become a professor and dean before retiring from Pennsylvania’s Lehigh University. “Usually you think the data will be used once in a paper, and people will refer to it for four or five years, but you don’t expect it to be resurrected after 45 years.”

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For more information, contact Atwater at 206-553-2927 or atwater@uw.edu.

This article was adapted from a USGS   See also a from UC Santa Cruz.