Warm liquid spewing from Oregon seafloor comes from Cascadia fault,
could offer clues to earthquake hazards

Date:
April 11, 2023
Source:
University of Washington
Summary:
Oceanographers discovered warm, chemically distinct liquid shooting
up from the seafloor about 50 miles off Newport. They named the
unique underwater spring 'Pythia's Oasis.' Observations suggest
the spring is sourced from water 2.5 miles beneath the seafloor at
the plate boundary, regulating stress on the offshore subduction
zone fault.


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FULL STORY ==========================================================================
The field of plate tectonics is not that old, and scientists continue to
learn the details of earthquake-producing geologic faults. The Cascadia Subduction Zone -- the eerily quiet offshore fault that threatens to
unleash a magnitude- 9 earthquake in the Pacific Northwest -- still
holds many mysteries.


==========================================================================
A study led by the University of Washington discovered seeps of warm, chemically distinct liquid shooting up from the seafloor about 50 miles
off Newport, Oregon. The paper, published Jan. 25 in Science Advances, describes the unique underwater spring the researchers named Pythia's
Oasis. Observations suggest the spring is sourced from water 2.5 miles
beneath the seafloor at the plate boundary, regulating 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 beneath the ocean's
surface. Further exploration using an underwater robot revealed the
bubbles were just a minor component of warm, chemically distinct fluid
gushing from the seafloor sediment.

"They explored in that direction and what they saw was not just methane bubbles, but water coming out of the seafloor like a firehose. That's
something that I've never seen, and to my knowledge has not been
observed before," said co-author Evan Solomon, a UW associate professor
of oceanography who studies seafloor geology.

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 policy
advisor.

Observations from later cruises show the fluid leaving the seafloor is
9 degrees Celsius (16 degrees Fahrenheit) warmer than the surrounding
seawater.

Calculations suggest the fluid is coming straight from the Cascadia
megathrust, where temperatures are an estimated 150 to 250 degrees Celsius
(300 to 500 degrees Fahrenheit).

The new seeps aren't related to geologic activity at the nearby seafloor observatory that the cruise was heading toward, Solomon said. Instead,
they occur near vertical faults that crosshatch the massive Cascadia
Subduction Zone. These strike-slip faults, where sections of ocean crust
and sediment slide past each other, exist because the ocean plate hits
the continental plate at an angle, placing stress on the overlying
continental plate.

Loss of fluid from the offshore megathrust interface through these
strike-slip faults is important because it lowers the fluid pressure
between the sediment particles and hence increases the friction between
the oceanic and continental plates.

"The megathrust fault zone is like an air hockey table," Solomon
said. "If the fluid pressure is high, it's like the air is turned on,
meaning there's less friction and the two plates can slip. If the fluid pressure is lower, the two plates will lock -- that's when stress can
build up." Fluid released from the fault zone is like leaking lubricant, Solomon said.

That's bad news for earthquake hazards: Less lubricant means stress can
build to create a damaging quake.

This is the first known site of its kind, Solomon said. Similar fluid
seep sites may exist nearby, he added, though they are hard to detect
from the ocean's surface. A significant fluid leak off central Oregon
could explain why the northern portion of the Cascadia Subduction Zone,
off the coast of Washington, is believed to be more strongly locked,
or coupled, than the southern section off the coast of Oregon.

"Pythias Oasis provides a rare window into processes acting deep in
the seafloor, and its chemistry suggests this fluid comes from near
the plate boundary," said co-author Deborah Kelley, a UW professor of oceanography. "This suggests that the nearby faults regulate fluid
pressure and megathrust slip behavior along the central Cascadia
Subduction Zone." Solomon just returned from an expedition to monitor sub-seafloor fluids off the northeast coast of New Zealand. The Hikurangi Subduction Zone is similar to the Cascadia Subduction Zone but generates
more frequent, smaller earthquakes that make it easier to study. But
it has a different sub-seafloor structure meaning it's unlikely to have
fluid seeps like those discovered in the new study, Solomon said.

The research off Oregon was funded by the National Science
Foundation. Other co-authors are Theresa Whorley, who did the work as a UW doctoral student and now works as an environmental consultant in Seattle;
Emily Roland, a former UW faculty member now at Western Washington
University; Masako Tominaga at Woods Hole Oceanographic Institution;
and Anne Tre'hu and Robert Collier at Oregon State University.

* RELATED_TOPICS
o Earth_&_Climate
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========================================================================== Story Source: Materials provided by University_of_Washington. Original
written by Hannah Hickey. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Brendan T. Philip, Evan A. Solomon, Deborah S. Kelley, Anne
M. Tre'hu,
Theresa L. Whorley, Emily Roland, Masako Tominaga, Robert
W. Collier.

Fluid sources and overpressures within the central Cascadia
Subduction Zone revealed by a warm, high-flux seafloor seep. Science
Advances, 2023; 9 (4) DOI: 10.1126/sciadv.add6688 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/04/230411105851.htm

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