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SEATTLE — With the Cascadia Subduction Zone parked off the coast and shallow faults lurking under most major cities, the Puget Sound area already faces a daunting array of seismic scenarios. A new study adds another: the possibility of a one-two earthquake punch.

Using state-of-the-art tree ring and radiocarbon dating methods, researchers found the most recent major earthquake on the Seattle Fault wasn't a solo act. The Saddle Mountain Fault, which slices across the Olympic Peninsula near Lake Cushman, ruptured at about the same time.

The team also was able to zero in on the date with stunning precision, narrowing it to a six-month window between the fall of 923 A.D.  and the spring of the following year — almost exactly 1,100 years ago.

Their results were published Wednesday in the journal Science Advances.

The project, which spanned more than five years and included divers with underwater chain saws to sample trees drowned by the quakes, is a scientific tour de force nearly unprecedented in seismology, said Harold Tobin, director of the Pacific Northwest Seismic Network at the University of Washington. To help nail down the date, the team used a new approach that detects traces of ancient solar storms captured in tree rings.

But the findings add a new worst-case possibility to the seismic threats facing a region that's home to 4 million people, said Tobin, who did not participate in the study. A seismic double whammy would be much more damaging than any single quake, especially to old brick and concrete buildings, and vulnerable bridges and infrastructure. It's a scenario that hasn't been factored into hazard maps, building codes and emergency planning — but it needs to be, Tobin added.

"The chance in any given year is not high, and there's no reason to freak out because of this study," Tobin said. "But it underscores that these are things that we need to be prepared for."

A 2005 analysis estimated a relatively modest magnitude 6.7 quake on the Seattle Fault could kill 1,600 people, destroy nearly 10,000 buildings and cause up to $50 billion in economic losses. If the Seattle and Saddle Mountain faults ruptured simultaneously, the new study estimates, the resulting quake would clock in at magnitude 7.8 — nearly 40 times more powerful — and affect a much bigger area.

The exact sequence of the ancient quakes remains unclear, said study co-author Morgan Page, of the U.S. Geological Survey.

Based on statistical analysis of global quake sequences, she calculates a 3-to-1 likelihood the faults ruptured simultaneously. But it's also possible two quakes were separated by hours, days or even months. In that case, the study estimates their magnitudes at 7.3 to 7.5.

Seismologists used to categorize most clustered quakes as aftershocks, and assumed they occurred on the same fault, said John Cassidy, a senior seismologist for Natural Resources Canada who was not involved in the project. But over the past several years, more has been learned about how a slip on one fault can put stress on others. February's devastation in Turkey was caused by a powerful quake that triggered the rupture of an adjacent fault nine hours later.

Shallow quakes near urban areas can be especially nasty because they are so close to the surface. Shaking would be far more intense than from Washington's most recent big earthquake — the magnitude 6.8 Nisqually event in 2001, which originated more than 20 miles underground.

"An earthquake on the Seattle Fault is ... likely to be much, much more catastrophic," Tobin said.

Fortunately, shallow fault quakes are extremely rare in Washington. The seismic mayhem in 923-924 A.D. appears to be the most intense episode since the end of the last ice age 16,000 years ago, the researchers report. Geologists still don't have extensive quake histories for most of the dozen or so faults in the area, but most seem to pop off every thousand to several thousand years, Tobin said. The USGS estimates about a 5% chance of a major Seattle Fault quake in the next 50 years.

But the quake that struck Morocco earlier this month occurred in an area where earthquakes also are rare, and nothing as powerful had ever been recorded, Tobin pointed out.

The new research builds on more than 30 years of geological discoveries. Field work in the early 1990s uncovered multiple clues left by the Seattle Fault quake, which uplifted Alki Point and the southeastern tip of Bainbridge Island as much as 20 feet and spawned a tsunami that swamped the site of the West Point Treatment Plant. It also triggered landslides on Mercer Island that sent old-growth forests sliding into Lake Washington, where some are still standing upright in nearly 300 feet of water.

The Saddle Mountain quake broke the ground with a 24-foot-tall scarp that blocked a drainage and created Price Lake, drowning trees as the water rose.

Preserved in oxygen-poor cold water, those submerged trees from Lake Washington, Price Lake and other sites were the key to the new research.

Previous radiocarbon analysis already had dated the Seattle Fault quake to sometime between 900 and 930 A.D. Intriguing hints from several other faults, including Saddle Mountain, Olympia and Tacoma, suggested they also ruptured around then. But conventional methods yielded possible age ranges that spanned centuries at those other sites.

"There was too much uncertainty," said co-author Brian Sherrod, a USGS scientist who has spent most of his career investigating faults across the state.

A chance encounter at a scientific conference between Sherrod and Bryan Black, a leading dendrochronologist from the University of Arizona, set the multiyear odyssey in motion.

"It's a big puzzle. That's why it took so long," said Sherrod. "To me, the story is these new techniques Bryan and his colleagues bring to the picture."

Tree rings can yield much more precise dates than radiocarbon, Black explained. But the samples have to be in good condition, with bark and extensive ring sequences.

One of the most daunting tasks was rounding up dozens of cores and wood slices collected over the years by other researchers. Eventually, Black corralled usable samples from 47 Douglas firs at six sites where trees were drowned by the earthquakes. Some of the samples recorded nearly three centuries of history.

The team had to collect new samples from Price Lake, where stumps of the quake-killed Douglas firs remain visible. USGS divers struggled in the murky water to slice usable samples with intact bark from the bases of the trees. "It would take up to an hour to cut a single wedge from a tree, because they were working with almost zero visibility," said Black, lead author of the study.

To anchor the tree rings in time, he used a reference sequence from 1,300-year-old firs collected on Vancouver Island in the early 1990s — so the date of the outermost ring is known.

Black sanded each sample multiple times, including with micron-level diamond grit sheets called lapping films. "It's sanded to the point where you can see the individual cells within the wood under a microscope," he said.

As he lined up his samples and compared them with the reference trees, the answer was unmistakable.

The validity of the date was confirmed through radiocarbon analysis of individual tree rings, looking for evidence of cosmic timestamps called Miyake events. Named for the Japanese physicist who discovered them, the events represent spikes of radiation from solar flares or exploding stars centuries or millennia ago. Luckily for dendrochronology, spikes in cosmic radiation generate spikes in atmospheric levels of carbon-14, the isotope whose slow decay is the clock that anchors radiocarbon dating.

Even luckier for Black and his colleagues, a Miyake event occurred in the year 774 A.D. and showed up in their Douglas fir samples, providing an absolute benchmark from which to count.

"Boom," he said. "Everything came together, and we saw that these trees all died with the last completely formed ring being the year 923."

The mystery of exactly when the Seattle Fault ruptured and whether other faults were involved has been around for more than 30 years — and Black said he can now see why. "This has been the most complicated and challenging dating project in my career."

And he and Sherrod aren't done. They're on the hunt for submerged trees linked to the Tacoma and Olympia faults, to find out whether the earthquake outburst 1,100 years ago was even more extensive.

In the meantime, Seattle is once again trying to tackle the hazard posed by old brick buildings, also called unreinforced masonry (URM), which are among the most dangerous places to be in an earthquake. More than 1,000 URM apartment buildings, offices, churches and meeting halls are scattered across the city, and efforts to require retrofits have stalled repeatedly.

The latest development is a new rule that simplifies and standardizes the requirements for retrofits, said URM program manager Amanda Hertzfeld. The next step will be a voluntary retrofit program that could be in place by next spring. The goal is to eventually mandate upgrades, Hertzfeld said, but there's no timeline yet.

WASHINGTON — President Joe Biden on Wednesday signed a landmark memorandum ordering federal agencies to do their part to restore salmon runs in the Columbia and Snake rivers and honor the U.S. government's treaty obligations to Northwest tribes.

"It is time for a sustained national effort to restore healthy and abundant native fish populations in the Basin," Biden said in the document, adding that it is his administration's policy to work with Congress, tribes, states, local governments and other stakeholders "to pursue effective, creative, and durable solutions" to help salmon, steelhead and other native fish recover.

Coming less than a week after the administration committed $200 million over 20 years to reintroduce salmon above Chief Joseph and Grand Coulee dams as part of a settlement agreement with Upper Columbia tribes, Wednesday's move was hailed by tribal leaders and environmental groups.

"We commend President Biden for his commitment to salmon recovery and focusing the full power and scope of the federal government on this issue," Corinne Sams, chair of the Columbia River Inter-Tribal Fish Commission, said in a statement. "He has sent a clear message throughout the federal government that business as usual is no longer acceptable. Never before has the federal government issued a Presidential Memorandum on salmon. This is historic."

The directive gives all relevant federal agencies — including the Interior Department, Bonneville Power Administration, Army Corps of Engineers and others — 120 days to review their programs affecting native fish. If any of those programs are not consistent with the legal responsibilities the federal government has under treaties it signed with Northwest tribes in 1855, which guaranteed Indigenous people the right to fish in all their "usual and accustomed" places, the agencies must align them with Biden's stated goal of restoring salmon to abundance.

Chief James Allan, chairman of the Coeur d'Alene Tribe, said in a statement that the tribe is encouraged by Biden's commitment and plans to hold his administration to its word.

"We have said for years that there are many things the federal agencies can do to help these fish populations but they have flat out ignored us or refused to act," he said. "Hopefully this marks a change in federal policy for the better, and we will continue pushing for full accountability and recovery."

The Nez Perce Tribe, in a statement, commended the president's pledge to respect the 1855 treaties, in which the Nez Perce and other tribes ceded some 60,000 square miles of what are now Washington, Oregon and Idaho in exchange for hunting and fishing rights.

"By publicly acknowledging that healthy and abundant salmon runs are essential, we know the Biden Administration is prioritizing the needs of the Northwest and working to uphold our Treaty," the tribe said. "We are relying on these Federal Agencies to take the necessary, urgent actions to restore salmon populations in the Columbia Basin. We are committed to working with the Biden Administration in partnership as we move forward."

Biden's memorandum directs the White House Council on Environmental Quality and the Office of Management and Budget to develop an "intergovernmental partnership" between the federal government, the Columbia Basin tribes and the states of Washington, Idaho, Oregon and Montana.

Biden's move drew mixed reactions from Congress, where the region's lawmakers largely agree on efforts to help salmon recover but have clashed — not necessarily along party lines — over proposals to breach four dams on the Lower Snake River.

Sen. Patty Murray, D-Wash., welcomed the memorandum and pledged to use her position as chair of the Senate Appropriations Committee to continue investing in salmon recovery.

"I'm really glad President Biden and his administration are taking salmon recovery and Tribal treaty rights seriously and working from every angle to restore fish populations in the Columbia River Basin, while meeting the region's resiliency needs," Murray said in a statement. "Salmon are absolutely essential for our environment, our economy, and Pacific Northwest Tribes — and ensuring we are making real federal investments in salmon recovery has long been a top priority for me."

Rep. Dan Newhouse, a Central Washington Republican and a vocal opponent of dam breaching, accused the Biden administration of using the announcement as a smokescreen "to give the perception that residents and stakeholders are being heard" while planning to breach the dams. The administration has so far avoided taking a public stance on the deeply contentious issue.

"While there may not be explicit recommendations to breach the Lower Snake River Dams in this memorandum, that is the goal of this Administration," Newhouse said in a statement. "This announcement is bureaucracy at its worst and the fact remains that these dams are vital to our economy, our efforts to reduce carbon emissions, and the ability to send our commodities overseas."

Rep. Cathy McMorris Rodgers, a Spokane Republican who also opposes removing the dams, expressed measured optimism about Biden's order.

"While I'm pleased to see the Biden administration finally acknowledge the irreplaceable benefits the Columbia-Snake River System provides to Eastern Washington and the entire Pacific Northwest, they cannot continue to ignore the science and facts," she said in a statement, pointing to recent improvements in certain salmon runs. "Our mitigation efforts are leading to positive results that we can — and will — build on if this administration is willing to work together to achieve measurable and defined shared goals."

Biden nodded to the benefits dams provide — in the form of hydropower, irrigation and barge transportation — committing to "secure a clean and resilient energy future for the region" and "support local agriculture and its role in food security domestically and globally." At the same time, he pledged to invest in the communities that depend on the dams "to enhance resilience to changes" in the dams' operation, which suggests support for a long-term plan proposed by Rep. Mike Simpson, R-Idaho Falls, to make dam removal viable.

Along with Biden's memorandum, the Interior Department announced $3.6 million in new funding for tribal fish hatcheries in the region. That adds to hundreds of millions in previously announced funding for hatcheries, removing culverts and other barriers to fish migration, habitat cleanup and more.

Todd Myers, environmental director at the Washington Policy Center, a think tank that opposes dam removal, said "there's a lot to like" about all of those investments, although he wants to see lower overall federal spending to reduce the nation's budget deficit.

Myers said he hopes the 120-day review process will produce recommendations such as culling the seals and sea lions that feast on salmon and whose numbers have exploded in recent decades, partly as a result of legislation Congress passed in 1972. On the other hand, he worries the review could result in the administration endorsing dam breaching.

Kurt Miller, executive director of Northwest RiverPartners, an association of public and cooperative power utilities that opposes dam breaching, called Wednesday's announcement "bittersweet."

"Northwest RiverPartners applauds the Biden Administration for going on record as recognizing the unique and essential role the region's hydroelectric dams, including the Lower Snake River Dams, play in helping us meet our clean energy, climate, economic, and salmon recovery objectives," Miller said in a statement. "Unfortunately, the Memorandum released today builds on and extends a flawed process that has denied affected stakeholders and the public a meaningful role."

Conservation and fishing groups, which largely support dam breaching as the centerpiece of a broader range of actions to help salmon recover, welcomed Biden's declaration while emphasizing that it didn't meet all their demands.

"With this directive, the President is sending a clear message to the Bonneville Power Administration, Army Corps of Engineers and other relevant agencies and leaders within the federal government that business-as-usual is no longer acceptable," Tanya Riordan, policy and advocacy director of Washington-basd Save Our wild Salmon, said in a statement.

Mitch Cutter, salmon and steelhead conservation associate at the Idaho Conservation League, said in a statement, "We applaud President Biden and his Administration for saying the right things, now it's time to do the right things."

A coalition of environmental groups  engaged in ongoing litigation with the federal government over salmon in the Columbia Basin applauded the announcement while making clear that it didn't fully resolve the issues in the long-running legal fight.

"We're heartened by the commitment the Biden Administration is demonstrating in this Memorandum to honor obligations to Tribal Nations and to restore Columbia River salmon to a healthy abundance," Amanda Goodin, an attorney at Earthjustice involved in the litigation, said in a statement. "Now we need to finish the job. NOAA Fisheries has already concluded that the best and only certain way to recover Snake River salmon to a healthy abundance is to breach the four Lower Snake River dams."

In August, the coalition and the Biden administration agreed to extend a deadline to reach a settlement in that litigation until the end of October. At least one more major announcement affecting salmon in the Columbia Basin is expected by then.