Seismic research vessel booms in search of answers behind next Big One
Earthquake researchers are eager to tap into a wealth of new data from the fault zone off Cascadia. When Cascadia breaks, it can trigger a mega-tremor known as the “Big One”. Valuable new imagery of the geology off the coasts of Oregon, Washington State and British Columbia comes from a specialized research vessel.
The National Science Foundation’s Marcus Langseth seismic survey vessel zigzagged for nearly six weeks the length of the Cascadia submarine subduction zone from the Oregon-California border north to Vancouver Island . Research expedition left Newport, Oregon on June 1 and returned ashore in Seattle a week ago.
“It will be a tremendous asset for us to understand Cascadia,” said Co-Chief Scientist Brian Boston of Columbia University, moments after leaving the ship after 41 days at sea.
Chief scientist Suzanne Carbotte, a geophysicist at Columbia University, said the main objective was to produce a much sharper image of the submarine fault zone. Many scientists, including her, suspect that two colliding plates are stuck or “locked” together, creating a tension that will be released sooner or later in a catastrophic earthquake.
“For this expedition, we were using modern technology which is vastly superior in terms of what people can learn compared to data that had been acquired before,” Carbotte said in an interview. “The imagery we’re going to get here will be in a different class than what we have.”
The expedition used sound to probe miles below the seabed. A set of underwater air cannons directed booming acoustic pulses towards the seabed.
“It goes all the way to the seabed and then penetrates because it contains a lot of low frequencies, then it reflects, bounces off the horizons of the subsoil, very similar to a 3D CAT scan,” Carbotte said.
The reflected echoes were picked up by receivers on land, on the seabed and on a 12-kilometer-long cable towed behind the research vessel.
This kind of noisy underwater seismic survey work had the potential to confuse environmentalists. Ocean activists are concerned about disruption to protected marine mammals off the Pacific coast, primarily whales.
Senior cruise scientists said they deployed lookouts and a forward reconnaissance vessel to make sure no whales or dolphins were nearby when doing their snowshoeing underwater.
“Anytime it could become a problem that we could injure or damage a whale, we immediately stop the experiment,” Boston said. “It was mainly about protecting these species at sea and not trying to disturb them.”
Boston said closures rarely happened, less often than they thought, mostly due to encounters with humpback whales.
Harold Tobin, a professor at the University of Washington and director of the Pacific Northwest Seismic Network, is one of many enthusiastic researchers digging into the terabytes of data collected by this seismic study. Tobin came down to the Seattle Pier to greet the returning ship. He said one thing the much-improved imagery of the Cascadia Rift Zone won’t tell us is when the next Big One hits.
“It’s not so much that it will help us predict the timing directly,” said Tobin. “What he’s going to do is create that understanding and that baseline for what the defect actually looks like, and then how the changes in certain properties that we can measure – like the speed of sound waves through the ground. – if this changes over time, it could tell us new things about how the fault alters his stress.
Tobin expects many more ideas to emerge. For example, we might have a much better idea to what extent the 700 mile long offshore fault is segmented into shorter pieces with different properties.
“The reason this matters is that it could mean the difference between a magnitude 9 that rips the whole thing – or shatters the whole rift – or a series of smaller earthquakes, maybe even more earthquakes. magnitude 8, but that would have very different effects. implications for, say, the risk of a tsunami, ”said Tobin.
A better understanding of the tsunami risk could also come from 3D imaging of the upper layer of ocean sediments overlying the tectonic plate boundary. The way the seabed moves or slides in response to a strong Cascadia earthquake that begins below has a lot to do with the severity of a tsunami that could be triggered.
Tobin said he expects the first research results based on the freshly collected data to be published next year and will continue for years to come.
“We still have a lot of mysteries about the Cascadia Subduction Zone despite all these decades of study,” Tobin said.
The expedition cruise to find answers began with what Carbotte called “a lot of difficult spots”. They had problems with the balky instruments, then the fishing gear got tangled in the very long “streamer” listening cable. After that, a heavy sea completely tore the listening cable. It was quickly recovered and data collection resumed.
“I am very happy to be back on earth,” said Shoushuo Han, the expedition’s third scientific co-leader. Han said the team considered the expedition a great success because they ended up gaining over 90% of their data targets.
The National Science Foundation provided primary funding for the research cruise. NSF owns the Marcus Langseth, which is operated by Columbia University. Carbotte said the US Geological Survey was also heavily involved in this project.
The last complete tear of the Cascadia Subduction Zone took place in January 1700. The exact date and destructive power was determined from the buried forests along the Pacific Northwest and an “orphan tsunami Which ran aground in Japan.
Geologists digging in coastal marshes and offshore canyon bottoms have also found evidence of previous large earthquakes and tsunamis. Radiocarbon dating of these events allows us to estimate the recurrence interval of the Cascadia mega-earthquakes at approximately every 250 to 800 years. Since the last one struck 321 years ago, that means the populated Northwest is within the window of the next Big One.