Seattle's biggest quake threat may be closer to home
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The earthquake danger most Seattleites think about isn't necessarily the one most likely to hit soonest, new research suggests.
Why it matters: While those fearing an earthquake apocalypse typically worry about the well-documented Cascadia subduction zone, a new study shows smaller faults branching from the Seattle fault zone rupture much more frequently and closer to home.
What they found: The research published this month in the Geological Society of America focused on shallow fault strands within the Seattle fault zone running from Bainbridge Island toward Bellevue, per the authors.
- Ruptures within the Seattle zone over the past millennia have been dominated by these smaller branching strands, which have slipped about every 350 years, the study found.
- The most recent of those events likely occurred in the early 19th century, based on radiocarbon dating and tree-ring analysis.
- By contrast, the Cascadia subduction zone — the source of the feared "Big One" — ruptures every 550 years or so and last slipped around 1700.
The findings suggest the Seattle fault zone is more complex than previously understood — and may have produced more earthquakes than scientists had previously documented, Harold Tobin, director of the Pacific Northwest Seismic Network, told Axios.
What they're saying: These secondary fault ruptures hit "pretty close to home," Stephen Angster, a U.S. Geological Survey geologist and the study's lead author, told Phys.org, and are likely to be more destructive for Seattle than a Cascadia event.
Reality check: Tobin said the study doesn't suddenly make the Seattle zone newly dangerous.
- "We knew the Seattle fault is active," Tobin said. "That means it's had earthquakes in the past and it can have earthquakes in the future."
- But the findings do reinforce the need to strengthen older buildings — especially unreinforced brick and stone structures — because even moderate shallow earthquakes can cause significant structural damage to masonry.
State of play: The findings come as Seattle prepares to mark the 25th anniversary of the 2001 Nisqually earthquake later this month — and amid a renewed push to retrofit vulnerable masonry buildings.
- City officials say they've made progress toward a mandatory retrofit program for unreinforced masonry (URM) buildings, which are especially vulnerable in shallow quakes.
- Within the last few years, the city approved Resolution 32111 and adopted the 2021 Seattle Existing Building Code to reduce retrofit requirements and lower construction costs for qualifying unreinforced masonry buildings, per Bryan Stevens of the Seattle Department of Construction and Inspections.
- Seattle has also lowered permit fees and is a leader in the statewide "Fix the Bricks" campaign to push for seismic upgrades, he told Axios.
Yes, but: A state bill that would have funded development of financial incentives for retrofits failed to advance this session, slowing efforts to make upgrades more affordable.
The bottom line: Seattle's greatest earthquake threat may not come from the Big One but from a shallower break under the heart of the city that scientists are still working to understand.