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The field of plate tectonics is not that old and scientists are still learning the details of seismic geological faults. The Cascadia Subduction Zone, the eerily quiet offshore fault that threatens to trigger a magnitude 9 earthquake in the Pacific Northwest, still holds many mysteries.
A study by the University of Washington found seeps of hot, chemically distinct liquid erupting from the seabed about 50 miles off Newport, Oregon. The document, published on January 25 in Scientists progress, describes the unique underwater spring that researchers have named Pythia’s Oasis. Observations suggest the source is from water 2.5 miles below the seabed at the plate boundary, regulating the stress on the offshore fault.
The team made the discovery during a weather-related delay for a cruise aboard the RV Thomas G. Thompson. The ship’s sonar showed unexpected plumes of bubbles about three-quarters of a mile below the ocean surface. Further exploration using an underwater robot revealed that the bubbles were only a minor component of a hot, chemically distinct fluid gushing from the seafloor sediments.
“They explored in that direction and what they saw was not just methane bubbles, but water coming out of the seabed like a fire hose. That’s something I didn’t see. never seen and, to my knowledge, has never been observed before,” said co-author Evan Solomon, an associate professor of oceanography at UW who studies the geology of the sea floor.
The feature was discovered by first author Brendan Philip, who did the work as a UW graduate student and now works as a White House political adviser.
Observations from later cruises show that the fluid leaving the seabed is 9 °C (16 °F) warmer than the surrounding seawater. Calculations suggest the fluid comes directly from the Cascadia megathrust, where temperatures are estimated to be between 150°C and 250°C (300°F to 500°F).
The new seeps are not related to geologic activity at the nearby seafloor observatory the cruise was heading to, Solomon said. Instead, they occur near vertical faults that chop the massive Cascadia subduction zone. These strike-slip faults, where sections of oceanic crust and sediment slide past each other, exist because the oceanic plate strikes the continental plate at an angle, which puts pressure on the overlying continental plate.
Fluid loss from the offshore megathrust interface through these strike-slip faults is significant because it lowers fluid pressure between sediment particles and therefore increases friction between oceanic and continental plates.
“The megathrust rift zone is like an air hockey table,” Solomon said. “If the fluid pressure is high, it is as if the air is open, which means there is less friction and both plates can slip. If the fluid pressure is lower, both plates lock together, that’s when stress can build up.”
The fluid released from the fault zone is like leaking lubricant, Solomon said. That’s bad news for earthquake hazards: less lube means stress can build up to create a damaging earthquake.
It is the first known site of its kind, Solomon said. Similar fluid seep sites may exist nearby, he added, although they are difficult to detect from the ocean surface. A large fluid leak off central Oregon could explain why the northern part of the Cascadia subduction zone off Washington would be more tightly locked, or coupled, than the southern part off the coast of Washington. Oregon.
“Pythias Oasis provides a rare window into processes operating deep in the seafloor, and its chemistry suggests that this fluid originated near the plate boundary,” said co-author Deborah Kelley, professor of oceanography at UW. “This suggests that neighboring faults regulate fluid pressure and mega-thrust slip behavior along the central Cascadia subduction zone.”
Solomon has just returned from an expedition to monitor underwater fluids off the northeast coast of New Zealand. The Hikurangi subduction zone is similar to the Cascadia subduction zone but generates more frequent and smaller earthquakes which make it easier to study. But it has a different underwater structure, which means it’s unlikely to see fluid seeps like those found in the new study, Solomon said.
Brendan T. Philip et al, Fluid Sources and Overpressures in the Central Cascadia Subduction Zone Revealed by Hot, High-Rate Seafloor Seepage, Scientists progress (2023). DOI: 10.1126/sciadv.add6688
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