The new research indicates the worst-case scenario of frequent 923-style events is less likely than some scientists thought, said Harold Tobin<\/a>, a geophysicist at the University of Washington and head of the Pacific Northwest Seismic Network<\/a>, who was not involved in either study. But researchers also found that \u201cthe less worse, but still bad scenarios\u201d are more likely than previously thought.<\/p>\n Meet the Seattle Fault<\/p>\n \u201cFor a fault that has had so much attention, there\u2019s so much we still don\u2019t know.\u201d<\/p>\n The Seattle fault zone is a thrust fault system that stretches about 75 kilometers (46 miles) from the foothills of the Cascades east of Seattle to the Hood Canal, which runs along the shores of the Olympic Peninsula to the city\u2019s west, passing under Seattle along the way.<\/p>\n Geologists began rigorously exploring the fault system in the early 1990s, intrigued by gravitational anomalies, uplifted marine terraces<\/a> (stair-step geological formations along coastlines), and evidence of a roughly 1,000-year-old tsunami<\/a>. All these features hinted at a major, shallow earthquake on a local fault zone\u2014likely the 923 event.<\/p>\n But \u201cfor a fault that has had so much attention, there\u2019s so much we still don\u2019t know,\u201d said Elizabeth Davis<\/a>, an earthquake geologist at the University of Washington who led the Geology study.<\/p>\n The most pressing questions are how big quakes on the fault get, how often they hit, and, ultimately, what risks the fault poses to people who live in the Puget Sound area.<\/p>\n \u201cIt takes some real geologic sleuthing to get at those tough questions,\u201d Tobin said.<\/p>\n Biggest Seattle Fault Quakes Are Rare<\/p>\n Davis focused on the activity of the main fault, which can generate the biggest quakes in the Seattle fault zone complex. It was responsible for the 923 quake. But the existing record went back only about 5,000 years.<\/p>\n \u201cWe just don\u2019t know what the recurrence interval for these big quakes is,\u201d Davis said. \u201cWe wanted to lengthen the record.\u201d<\/p>\n To do so, Davis and her collaborators turned to marine terraces, the oldest of which date back to the end of the last ice age about 11,000 years ago. The quake in 923 raised terraces by about 8 meters (26 feet), and scientists wanted to look for similar-scale uplift in terraces all around the sound.<\/p>\n The researchers mapped more than 150 terraces around Puget Sound and measured their depths. After accounting for regional slopes, they estimated uplift over time that could have been caused by quakes.<\/p>\n They found that in that 11,000-year period, only the 923 event generated significant uplift. Thick sediment mantles could mask smaller events but not 923-scale quakes, Davis said.<\/p>\n Estimating true recurrence intervals requires knowing the timing of multiple events. But the finding is \u201cnot bad news,\u201d she said. It provides some evidence that the recurrence interval is likely not shorter than about 5,000 years.<\/p>\n \u201cThat could give us more of a buffer between now and when the next big one like that will happen,\u201d said Stephen Angster<\/a>, a U.S. Geological Survey geologist who led the GSA Bulletin study.<\/p>\n Smaller, Damaging Quakes Are More Frequent<\/p>\n Angster\u2019s work focused on Seattle\u2019s secondary faults, which are smaller, mostly blind faults (those not visible at the surface) capable of generating damaging earthquakes. Previous work had shown that one of these secondary faults generated a magnitude 6.7 earthquake, highlighting the risk they pose. Angster wanted to explore rupture histories of these secondary faults, particularly whether they could rupture independently from the main fault.<\/p>\n The researchers used a suite of paleoseismic tools<\/a>, including magnetic data, field and lidar mapping, trenches dug across faults, and geochronology. They studied two newly identified secondary faults that have orientations similar to the main fault.<\/p>\n They found three new earthquakes to add to the region\u2019s seismic history, including the oldest and youngest events in the known record, which were around 11,000 years ago and in the early 1800s, respectively. The earthquakes appear to be evidence of ruptures that occurred independently of the main fault, suggesting that the smaller\u2014but still dangerous\u2014secondary faults should be considered in hazard modeling.<\/p>\n With that lengthened record and the addition of three quakes, the recurrence interval the researchers found was about every 350 years over the past 2,500 years. This timing refined the previous estimate of every several hundred years.<\/p>\n There also appears to be an increase in activity over the past 2,000 years.<\/p>\n \u201cMaybe we should be paying attention to that,\u201d Angster said.<\/p>\n What Happens Next<\/p>\n \u201cThere are other earthquakes that aren\u2019t as big but that occur more frequently. Those might not be as catastrophic, but it would be a very bad scenario for Seattle\u201d if such events occurred.<\/p>\n \u201cThese are both carefully done studies,\u201d Tobin said. \u201cWe now have evidence that the 923 event was the biggest in 11,000 years. But there are other earthquakes that aren\u2019t as big but that occur more frequently. Those might not be as catastrophic, but it would be a very bad scenario for Seattle\u201d if such events occurred.<\/p>\n It\u2019s still to be determined whether the risk from secondary faults will be incorporated into the National Seismic Hazard Model<\/a>, which includes the 923 quake but not smaller ones along the Seattle fault zone. The secondary faults were left out in previous efforts because they are shorter than the minimum length required to be included and because of uncertainties in their potential rupture magnitude.<\/p>\n \u2014Rebecca Dzombak (@rdzombak.bsky.social<\/a>), Science Writer<\/p>\n Citation:\u00a0Dzombak, R. (2026), On the Seattle Fault, the biggest quakes aren\u2019t the most likely,\u00a0Eos, 107, https:\/\/doi.org\/10.1029\/2026EO260114<\/a>. Published on 14 April 2026.<\/p>\n![\"A](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/04\/seattle-fault-1024x575.png\")
The Seattle fault zone runs east-west under the city and the surrounding Puget Sound. Credit: Washington Geological Survey (Washington Department of Natural Resources)<\/a><\/p>\n