Understanding the hazard potential of the Seattle Fault zone: It’s “pretty close to home”
A new study in GSA Bulletin seeks to constrain how often and where faults cause earthquakes under the heart of Seattle
Geological Society of America
Contributed by Rudy Molinek, GSA Science Communication Fellow
Boulder, Colo., USA: In the Pacific Northwest, big faults like the Cascadian subduction zone located offshore, get a lot of attention. But big faults aren’t the only ones that pose significant hazards, and a new study in the journal GSA Bulletin investigates the dynamics of a complex fault zone that runs right under the heart of Seattle.
“My job as a paleoseismologist,” says Dr. Stephen Angster, a research geologist at the U.S. Geological Survey’s Earthquake Science Center in Seattle and lead author of the new study, “is to figure out when and how often these local faults rupture, which would help us predict roughly when we come in the window of the next potential rupture.”
The study focuses on the east-west trending Seattle Fault Zone, or SFZ, which cuts through Bainbridge Island and Seattle. Geologists have known for a while that the main fault appears to rupture on time-scales greater than 5,000 years, though it’s only in recent years that geologists have begun to map out smaller secondary faults within the SFZ. However, the tools geologists use to calculate earthquake hazards don’t commonly include these smaller, secondary faults, and Angster hopes that learning more about them could help understand their hazard potential.
“When we generate the National Seismic Hazard Model for the U.S., we leave out these shorter faults because they don’t meet the minimum requirement for length and thus are considered to have a low magnitude potential,” says Angster. “In the case of the SFZ, we don’t fully understand the rupture dynamics at depth, but they're rupturing more frequently and pretty close to home.”
The SFZ helps accommodate strain, or deformation, that’s the result of squeezing of the Earth’s crust from Portland, Oregon, to Vancouver, British Columbia. Strain accumulates constantly, but is released only periodically through earthquakes. Of the total strain in the region, the SFZ takes up about 15%. Additionally, the fact that geologists can’t directly see the faults on Earth’s surface, makes it harder to study their dynamics.
Instead, Angster and his colleagues use techniques that give clues into the subsurface, like surveys that measure small magnetic variations of the underlying bedrock. They can also look for evidence of past surface-rupturing earthquakes by closely examining high-resolution lidar images, which allow them to see through the thick tree canopy and find scarps that formed when the ground was displaced during a past rupture. Then, they dig trenches across the scarps to figure out how long ago the ruptures occurred and how big they were.
The team then documented the histories of two newly discovered secondary faults in the SFZ and found that secondary faults are rupturing there about every 350 years—far more often than the main fault.
“The surface ruptures from earthquakes within the SFZ have been dominated within the last 2500 years by these secondary fault events,” says Angster.
The most recent rupture appears to have been in the nineteenth century, according to radiocarbon and tree-ring dating of trees that diedafter an earthquake. Going forward, Angster and his colleagues hope to refine their understanding of these secondary faults and determine how much hazard they pose to the four million residents of the Seattle area.
“The thing about the Seattle fault is that in the Cascadia event, we’ll shake pretty hard and long when it happens,” says Angster, “but it's likely not going to be as destructive for Seattle as a major event on the Seattle fault. I think we're still trying to wrap our heads around the size and the potential of these smaller faults and the relationship between main fault rupture and these more frequent, smaller ruptures.”
GSA Bulletin:: https://doi.org/10.1130/B38333.1
About the Geological Society of America
The Geological Society of America (GSA) is a global professional society with more than 18,000 members across over 100 countries. As a leading voice for the geosciences, GSA advances the understanding of Earth's dynamic processes and fosters collaboration among scientists, educators, and policymakers. GSA publishes Geology, the top-ranked “geology” journal, along with a diverse portfolio of scholarly journals, books, and conference proceedings—several of which rank among Amazon’s top 100 best-selling geology titles.
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A new study in GSA Bulletin seeks to constrain how often and where faults cause earthquakes under the heart of Seattle
Geological Society of America
Contributed by Rudy Molinek, GSA Science Communication Fellow
Boulder, Colo., USA: In the Pacific Northwest, big faults like the Cascadian subduction zone located offshore, get a lot of attention. But big faults aren’t the only ones that pose significant hazards, and a new study in the journal GSA Bulletin investigates the dynamics of a complex fault zone that runs right under the heart of Seattle.
“My job as a paleoseismologist,” says Dr. Stephen Angster, a research geologist at the U.S. Geological Survey’s Earthquake Science Center in Seattle and lead author of the new study, “is to figure out when and how often these local faults rupture, which would help us predict roughly when we come in the window of the next potential rupture.”
The study focuses on the east-west trending Seattle Fault Zone, or SFZ, which cuts through Bainbridge Island and Seattle. Geologists have known for a while that the main fault appears to rupture on time-scales greater than 5,000 years, though it’s only in recent years that geologists have begun to map out smaller secondary faults within the SFZ. However, the tools geologists use to calculate earthquake hazards don’t commonly include these smaller, secondary faults, and Angster hopes that learning more about them could help understand their hazard potential.
“When we generate the National Seismic Hazard Model for the U.S., we leave out these shorter faults because they don’t meet the minimum requirement for length and thus are considered to have a low magnitude potential,” says Angster. “In the case of the SFZ, we don’t fully understand the rupture dynamics at depth, but they're rupturing more frequently and pretty close to home.”
The SFZ helps accommodate strain, or deformation, that’s the result of squeezing of the Earth’s crust from Portland, Oregon, to Vancouver, British Columbia. Strain accumulates constantly, but is released only periodically through earthquakes. Of the total strain in the region, the SFZ takes up about 15%. Additionally, the fact that geologists can’t directly see the faults on Earth’s surface, makes it harder to study their dynamics.
Instead, Angster and his colleagues use techniques that give clues into the subsurface, like surveys that measure small magnetic variations of the underlying bedrock. They can also look for evidence of past surface-rupturing earthquakes by closely examining high-resolution lidar images, which allow them to see through the thick tree canopy and find scarps that formed when the ground was displaced during a past rupture. Then, they dig trenches across the scarps to figure out how long ago the ruptures occurred and how big they were.
The team then documented the histories of two newly discovered secondary faults in the SFZ and found that secondary faults are rupturing there about every 350 years—far more often than the main fault.
“The surface ruptures from earthquakes within the SFZ have been dominated within the last 2500 years by these secondary fault events,” says Angster.
The most recent rupture appears to have been in the nineteenth century, according to radiocarbon and tree-ring dating of trees that diedafter an earthquake. Going forward, Angster and his colleagues hope to refine their understanding of these secondary faults and determine how much hazard they pose to the four million residents of the Seattle area.
“The thing about the Seattle fault is that in the Cascadia event, we’ll shake pretty hard and long when it happens,” says Angster, “but it's likely not going to be as destructive for Seattle as a major event on the Seattle fault. I think we're still trying to wrap our heads around the size and the potential of these smaller faults and the relationship between main fault rupture and these more frequent, smaller ruptures.”
GSA Bulletin:: https://doi.org/10.1130/B38333.1
About the Geological Society of America
The Geological Society of America (GSA) is a global professional society with more than 18,000 members across over 100 countries. As a leading voice for the geosciences, GSA advances the understanding of Earth's dynamic processes and fosters collaboration among scientists, educators, and policymakers. GSA publishes Geology, the top-ranked “geology” journal, along with a diverse portfolio of scholarly journals, books, and conference proceedings—several of which rank among Amazon’s top 100 best-selling geology titles.
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DOI
2018 Kīlauea earthquake may have stalled fault’s slow slip for decades
The magnitude 6.9 earthquake that took place in 2018 on the south flank of Kīlauea on the Island of Hawaiʻi may have stalled episodes of periodic slow slip along a major fault underlying the volcano, according to a new study by scientists at the U.S. Geological Survey.
Since the rupture, slow slip events have stopped on this portion of the fault, and it may take almost 60 years to rebuild the amount of stress that was released during the earthquake on these slow-slip patches, the researchers report in the Bulletin of the Seismological Society of America.
The researchers also observed no signs of viscoelastic relaxation—a flow of the lower crust and upper mantle in response to stress changes—after the earthquake.
These features of the 2018 earthquake can be useful for understanding and evaluating earthquake hazards on this part of Kīlauea, said Ingrid Johanson at the USGS Hawaiian Volcano Observatory and colleagues.
The May 4, 2018 earthquake occurred in the middle of a massive eruption of the volcano’s East Rift Zone. The earthquake ruptured part of Kīlauea’s décollement fault, where the volcanic pile meets the underlying oceanic crust.
The region is densely instrumented with Global Navigation Satellite System (GNSS) stations, which measure changes in land levels to create an overall picture of how the Earth’s crust moves and deforms before, during and after earthquakes and eruptions.
Johanson and colleagues used these data to look at where fault slip took place during and after the earthquake, to compare with areas of slip from past earthquakes and past slow-slip events. Compared to the sudden rupture of an earthquake, slow slip seismic events can release energy over weeks and months.
The 2018 earthquake ruptured through patches of the fault where slow slip events have occurred somewhat regularly over the past two decades. But these events have not been observed since the 2018 earthquake.
The researchers calculate that the earthquake may have released almost 60 years’ worth of stress built up along the fault in these patches, and that the resulting “stress shadow” may keep these events from happening again for decades.
“Assuming steady loading from fault creep, we calculate that [the slow slip fault regions] could be in stress shadow for 56 years, plus or minus 3 years,” said Johanson.
“However, loading on the décollement fault is strongly connected to volcanic activity, so a change in volcanic activity could push the fault out of the stress shadow early and we shouldn’t rule out the possibility of a through-going rupture in the same location within the projected 56 years,” she added.
One of the events that the researchers were hoping to study during the 2018 event was postseismic viscoelastic relaxation, which offers a unique chance to observe crust and mantle deformation, the researchers noted.
“So although the 2018 earthquake occurred within the first week of an historic eruption and caldera collapse and one day after lava first erupted from a fissure in Leilani Estates,” Johanson said, “it was a priority to capture the deformation pattern following the earthquake and to make sure that additional portions of the flank were instrumented.”
However, Johanson and colleagues found minimal afterslip on the fault and no signs of viscoelastic deformation.
The researchers say it’s possible that the dip of the décollement fault is so shallow that it remains within the brittle upper portion of the crust and might not transfer stress into the lower crust and upper mantle to cause deformation.
“Given its rupture area, it is likely that the 1975 magnitude 7.7 Kalapana earthquake also remained in the brittle portion of the crust, as may future events in this size category,” said Johanson. “However, it is hypothesized that the 1868 magnitude 7.9 Great Kaʻū earthquake ruptured the décollement all the way below Mauna Loa. If another rupture occurred to this depth, then it might put enough stress into the lower crust to generate a [viscoelastic] response.”
The 2018 earthquake originated on the same part of the fault as the 1975 Kalapana and other historic Kīlauea south flank earthquakes. It’s possible that magma intrusion through this part of the fault was responsible for these earlier earthquakes, as appears to be the case for the 2018 earthquake.
“This emphasizes that monitoring volcanic activity is key for understanding stress accumulation and earthquake activity on Kīlauea’s south flank and highlights how connected magmatic and tectonic processes are in Hawaiʻi,” said Johanson.
The magnitude 6.9 earthquake that took place in 2018 on the south flank of Kīlauea on the Island of Hawaiʻi may have stalled episodes of periodic slow slip along a major fault underlying the volcano, according to a new study by scientists at the U.S. Geological Survey.
Since the rupture, slow slip events have stopped on this portion of the fault, and it may take almost 60 years to rebuild the amount of stress that was released during the earthquake on these slow-slip patches, the researchers report in the Bulletin of the Seismological Society of America.
The researchers also observed no signs of viscoelastic relaxation—a flow of the lower crust and upper mantle in response to stress changes—after the earthquake.
These features of the 2018 earthquake can be useful for understanding and evaluating earthquake hazards on this part of Kīlauea, said Ingrid Johanson at the USGS Hawaiian Volcano Observatory and colleagues.
The May 4, 2018 earthquake occurred in the middle of a massive eruption of the volcano’s East Rift Zone. The earthquake ruptured part of Kīlauea’s décollement fault, where the volcanic pile meets the underlying oceanic crust.
The region is densely instrumented with Global Navigation Satellite System (GNSS) stations, which measure changes in land levels to create an overall picture of how the Earth’s crust moves and deforms before, during and after earthquakes and eruptions.
Johanson and colleagues used these data to look at where fault slip took place during and after the earthquake, to compare with areas of slip from past earthquakes and past slow-slip events. Compared to the sudden rupture of an earthquake, slow slip seismic events can release energy over weeks and months.
The 2018 earthquake ruptured through patches of the fault where slow slip events have occurred somewhat regularly over the past two decades. But these events have not been observed since the 2018 earthquake.
The researchers calculate that the earthquake may have released almost 60 years’ worth of stress built up along the fault in these patches, and that the resulting “stress shadow” may keep these events from happening again for decades.
“Assuming steady loading from fault creep, we calculate that [the slow slip fault regions] could be in stress shadow for 56 years, plus or minus 3 years,” said Johanson.
“However, loading on the décollement fault is strongly connected to volcanic activity, so a change in volcanic activity could push the fault out of the stress shadow early and we shouldn’t rule out the possibility of a through-going rupture in the same location within the projected 56 years,” she added.
One of the events that the researchers were hoping to study during the 2018 event was postseismic viscoelastic relaxation, which offers a unique chance to observe crust and mantle deformation, the researchers noted.
“So although the 2018 earthquake occurred within the first week of an historic eruption and caldera collapse and one day after lava first erupted from a fissure in Leilani Estates,” Johanson said, “it was a priority to capture the deformation pattern following the earthquake and to make sure that additional portions of the flank were instrumented.”
However, Johanson and colleagues found minimal afterslip on the fault and no signs of viscoelastic deformation.
The researchers say it’s possible that the dip of the décollement fault is so shallow that it remains within the brittle upper portion of the crust and might not transfer stress into the lower crust and upper mantle to cause deformation.
“Given its rupture area, it is likely that the 1975 magnitude 7.7 Kalapana earthquake also remained in the brittle portion of the crust, as may future events in this size category,” said Johanson. “However, it is hypothesized that the 1868 magnitude 7.9 Great Kaʻū earthquake ruptured the décollement all the way below Mauna Loa. If another rupture occurred to this depth, then it might put enough stress into the lower crust to generate a [viscoelastic] response.”
The 2018 earthquake originated on the same part of the fault as the 1975 Kalapana and other historic Kīlauea south flank earthquakes. It’s possible that magma intrusion through this part of the fault was responsible for these earlier earthquakes, as appears to be the case for the 2018 earthquake.
“This emphasizes that monitoring volcanic activity is key for understanding stress accumulation and earthquake activity on Kīlauea’s south flank and highlights how connected magmatic and tectonic processes are in Hawaiʻi,” said Johanson.
Journal
Bulletin of the Seismological Society of America
Bulletin of the Seismological Society of America
DOI
Method of Research
Observational study
Observational study
Subject of Research
Not applicable
Not applicable
Article Title
Rupture into Slow‐Slip Fault Regime During the 2018 6.9 Island of Hawai‘i Earthquake is Followed by Modest Postseismic Slip
Rupture into Slow‐Slip Fault Regime During the 2018 6.9 Island of Hawai‘i Earthquake is Followed by Modest Postseismic Slip
Article Publication Date
3-Feb-2026
3-Feb-2026
Study reveals the extent of rare earthquakes in deep layer below Earth’s crust
Stanford University
image:
A map showing continental mantle earthquakes across the globe. (Image credit: Axel Wang)
view moreCredit: Axel Wang
Stanford researchers have created the first-ever global map of a rare earthquake type that occurs not in Earth’s crust but in our planet’s mantle, the layer sandwiched between the thin crust and Earth’s molten core. The new map will help scientists learn more about the mechanics of mantle earthquakes, in turn opening a window into the complexities and triggers for all earthquakes.
As reported in a study published Feb. 5 in Science, continental mantle earthquakes occur worldwide but are clustered regionally, particularly in the Himalayas in southern Asia and the Bering Strait between Asia and North America, south of the Arctic Circle. Through analysis of these deep quakes, scientists expect to gain unique insights into the crust-mantle boundary and the behavior of the upper mantle – the source of volcanic magma that partially drives tectonic plate movements.
“Until this study, we haven’t had a clear global perspective on how many continental mantle earthquakes are really happening and where,” said lead study author Shiqi (Axel) Wang, a former PhD student in the lab of geophysics professor Simon Klemperer at the Stanford Doerr School of Sustainability. “With this new dataset, we can start to probe at the various ways these rare mantle earthquakes initiate.”
Continental mantle quakes are too deep to cause much shaking or danger at Earth’s surface. But their distinctive origins stand to advance multiple fields of Earth science, which could in turn improve understanding of risks from common, shallower earthquakes.
“Although we know the broad strokes that earthquakes generally happen where stress releases at fault lines, why a given earthquake happens where it does and the main mechanisms behind it are not well grasped,” added Klemperer, senior study author. “Mantle earthquakes offer a novel way to explore earthquake origins and the internal structure of Earth beyond ordinary crustal earthquakes.”
Above and below the Moho
Unlike Earth’s cold, brittle crust, the mantle is a warm, semisolid zone of dense rock about 1,800 miles thick that comprises the bulk of our planet’s interior. The boundary between the crust and the mantle is known as the Mohorovičić discontinuity, also called “the Moho.”
For decades, seismologists and geophysicists have debated whether the viscous mantle could support significant seismic activity. The points of origin of most continental earthquakes measure to depths of roughly 6 to 18 miles, which is squarely above the Moho and thus in the crust. Noted exceptions are subduction zones, where denser oceanic plates dive under lighter crustal plates, sometimes unleashing quakes from hundreds of miles down. Yet sensor measurements have at times pointed to far deeper, sub-Moho hypocenters under continental landmasses, away from subduction zones, even 50 miles below the Moho.
Based on accumulating evidence, most researchers over the last 10 years have accepted that rare earthquakes do originate in the mantle, perhaps roughly 100 times less often than crustal quakes. But identifying them unequivocally has proved challenging due to a lack of data.
To distinguish mantle earthquakes from crustal earthquakes, Wang and Klemperer developed a method for comparing two types of seismic waves. These vibrations, generated by earthquakes and other phenomena, reverberate throughout Earth as if our planet were a rung bell.
The two wave types are called Sn or “lid” waves, a type of shear wave that travels across the top of the mantle known as the “lid,” and Lg waves, which are high-frequency undulations that bounce around easily through the crust. The ratio of the waves’ sizes determines their origins.
“Our approach is a complete game-changer because now you can actually identify a mantle earthquake purely based on the waveforms of earthquakes,” said Wang.
Rarities galore
By poring over data from seismic monitoring stations worldwide and incorporating other critical information such as crustal thickness, the researchers whittled an initial set of over 46,000 earthquakes down to 459 identified continental mantle earthquakes since 1990.
The total number is conservative, the researchers said. A good deal more mantle quakes would likely be captured by expanding sensor networks, particularly in remote areas such as the Tibetan Plateau that ranges north from the dramatic uplift of the Himalayas. Klemperer has spent much of his career researching seismicity in this geographically isolated region. His initial exposure to the notion of continental mantle earthquakes there eventually informed his student, Wang, to pursue the topic further.
With a wealth of mantle-emanating temblors now on the books, plus their reliable method for identifying future quakes, Wang and Klemperer plan to delve more into the particulars of these rare events. Some appear to be aftershocks spawned by propagating seismic waves from crustal earthquakes. Others might burst forth from the heat-driven convection of the mantle itself as it recycles subducted slabs of Earth’s crust.
Looking ahead, the Stanford researchers anticipate a much fuller picture of Earth’s hidden subterranean workings coming to light.
“Continental mantle earthquakes might be part of an inherently interconnected earthquake cycle, both from the crust and also the upper mantle,” said Wang. “We want to understand how these layers of our world function as a whole system.”
This research was supported by the National Science Foundation.
Journal
Science
Article Title
Continental mantle earthquakes of the world
Article Publication Date
5-Feb-2026
