... and that could mean trouble because the fault may be "locked" and storing up energy, rather than releasing energy over time.
The fault zone expected to generate the next big one lies underwater between 40 and 80 miles offshore of the Pacific Northwest coastline. Earthquake scientists have listening posts along the coast from Vancouver Island to Northern California.
But those onshore seismometers have detected few signs of the grinding and slipping you would expect to see as one tectonic plate dives beneath another, with the exception of the junctions on the north and south ends of what is formally known as the Cascadia Subduction Zone.
It is "a puzzle," according to University of Oregon geophysics professor Doug Toomey.
... The bottom line: Even with more sensitive instruments, it's still eerily quiet out there. Which leads the researchers to conclude the dangerous Cascadia fault zone is stuck — or in science-speak, it is fully "locked."
"The lack of interplate seismicity is interpreted to reflect complete healing and locking of the megathrust over three centuries after the previous great earthquake," wrote Koichiro Obana and his co-authors in the BSSA paper.
The evidence pointing to the colliding tectonic plates being completely stuck has serious implications for earthquake risk on land in the Pacific Northwest.
"If there were low levels of offshore seismicity, then we could say some strain is being released by the smaller events," Toomey said. "If it is completely locked, it means it is increasingly storing energy and that has to be released at some point."
Toomey said a big unknown is how much strain has accumulated since the plate boundary seized up, and secondly, how much more strain can build up before the fault rips and unleashes a possible magnitude-9.0 megaquake and tsunami.
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