Level 1 — Absolute Beginner

Earthquakes can shake the ground hard. People in the city of Seattle in the United States are afraid of a big earthquake one day.

Under the city, there are big cracks in the rock. We call these cracks 'faults.' Scientists thought there was one big fault under Seattle.

Now scientists have found small cracks called 'secondary faults' near the big one. These small cracks shake the ground every 350 years on average.

The big fault wakes up only once in a long, long time. But the small cracks wake up much more often. Scientists tell people to be ready in case of a quake.

earthquake

a strong shaking of the ground caused by movement deep in the earth

ground

the surface of the earth that we walk on

shake

to move quickly from side to side

city

a very big town

Seattle

a big city in the United States, near the Pacific Ocean

fault

a long break in the rock under the earth

scientist

a person who studies how the world works

ready

prepared, so you know what to do

Level 2 — Elementary

A new study has changed what scientists know about earthquakes near the U.S. city of Seattle, in the state of Washington. The research was led by the U.S. Geological Survey and the University of Southern California and was published on May 18, 2026.

Scientists have long known about a large crack in the rock under the city, called the Seattle Fault Zone. Big earthquakes on that main fault happen rarely — about once in a thousand years.

However, the new study shows that there is a hidden network of smaller faults next to the main one. These smaller faults are called 'secondary' or 'splay' faults. They shake the ground roughly every 350 years on average, which is much more often than the master fault.

The most recent rupture probably happened during the 1800s. The team studied old trees, dug deep trenches across the ground, and used special airborne cameras and magnetic sensors to find the hidden faults. About four million people live in the Seattle area, so the news is important for building rules and earthquake preparation.

study

a careful piece of research about a particular subject

researcher

a person who works to discover new information

geological

related to the study of rocks and the earth

fault zone

an area where many connected faults run together

rupture

the act of breaking, used here for the moment when a fault slips and an earthquake happens

splay fault

a smaller branching fault that comes off a larger main fault

trench

a long, narrow hole dug into the ground

magnetic sensor

a tool that measures small changes in the earth's magnetic field

Level 3 — Intermediate

A team from the U.S. Geological Survey's Earthquake Hazards Program and the University of Southern California Department of Earth Sciences has reported, in a paper published in the Bulletin of the Seismological Society of America and amplified through a ScienceDaily release on May 18, 2026, that previously unmapped secondary splay faults of the Seattle Fault Zone appear to rupture every ~350 years on average — far more often than the once-per-millennium recurrence interval estimated for the master fault. The finding rests on a multi-method paleoseismic campaign combining airborne lidar bare-earth scarp identification, ground-truth trenching, optically stimulated luminescence and radiocarbon dating, and a fresh high-resolution aeromagnetic survey flown over the central Puget Lowland in summer 2025.

The Seattle Fault Zone is a roughly east-west striking, south-vergent reverse-thrust system that crosses the southern Seattle metropolitan area at shallow seismogenic depth. The single best-known historical event is the AD 900–930 earthquake — magnitude approximately 7.0 — which uplifted Restoration Point and Alki Point and triggered a tsunami in Puget Sound recorded in indigenous oral histories. Until now, hazard models from the USGS National Seismic Hazard Maps have treated that AD 900–930 event as the canonical recurrence anchor on the system.

The new work identifies at least five hitherto-unmapped scarps in the Bainbridge Island–West Seattle–Bellevue corridor with offsets ranging from 30 cm to 1.8 m, dated via stacked OSL and radiocarbon to four discrete rupture clusters: ~1810–1880 CE, ~1500 CE, ~1150 CE, and ~900 CE. The recurrence-interval analysis yields a Bayesian-estimated mean of 348 years (95% credible interval 240–490 years) on the secondary system as a whole, with the most recent surface rupture having occurred during the 19th century — almost certainly before the regional seismograph network came online but possibly recorded in Skykomish and Snohomish oral traditions describing 'great shakings' of forested ridges.

The hazard implication is non-trivial. While the inferred secondary-fault earthquakes are smaller (M 6.0–6.6 in the team's slip-area scaling), they rupture shallowly and very close to dense urban infrastructure: downtown Seattle, the SR-99 tunnel, the Sound Transit Link light rail, two major hospital complexes and the SeaTac International Airport approach all sit within 10 kilometres of at least one newly mapped scarp. The authors recommend that the USGS update its 2023 National Seismic Hazard Maps to add the secondary system as an active source and that the City of Seattle and Bellevue revisit unreinforced-masonry retrofit prioritisation accordingly.

paleoseismic

concerning the geologic record of past earthquakes preserved in offset landforms and sediment layers

lidar

a remote-sensing technique that maps surface topography with high resolution using pulsed laser beams

optically stimulated luminescence

Level 4 — Advanced

A team from the U.S. Geological Survey's Earthquake Hazards Program (Pacific Northwest Region) and the University of Southern California Department of Earth Sciences has reported, in a Bulletin of the Seismological Society of America paper posted on May 14 and amplified through a ScienceDaily release on May 18, 2026, that previously unmapped secondary splay faults of the Seattle Fault Zone have ruptured at a Bayesian-estimated mean interval of approximately 348 years over the past 2,500 years — roughly threefold more frequent than the once-per-millennium recurrence interval long inferred for the master south-vergent reverse-thrust system. The finding rests on an integrated paleoseismic campaign combining airborne lidar bare-earth scarp identification at 1-metre horizontal resolution, ground-truth trenching at five sites across Bainbridge Island, West Seattle and Bellevue, stacked optically-stimulated-luminescence and AMS radiocarbon dating of 38 stratigraphically-bracketed samples, and a 2,400-line-km high-resolution aeromagnetic survey flown over the central Puget Lowland aboard a Cessna 208B Caravan in July–August 2025.

The Seattle Fault Zone is a roughly east-west-striking, south-vergent reverse-thrust system crossing the southern Seattle metropolitan area at shallow seismogenic depth (4–8 km top-of-rupture in most reconstructions). The single best-known historical event is the AD 900–930 earthquake — moment magnitude approximately 7.0 — which uplifted Restoration Point on Bainbridge Island by 7 metres, raised Alki Point and West Point, and triggered a Puget Sound tsunami with run-up indicators preserved at Cultus Bay, Mowitch Beach and Discovery Park and recorded in Snohomish and Suquamish oral traditions of a 'shaking of the ground' followed by 'a wave that climbed the trees.' Until the present work, the 2023 USGS National Seismic Hazard Maps treated that AD 900–930 event as the canonical recurrence anchor on the system, with the secondary splay set explicitly classified as 'characterised but not separately enumerated.'

The new work identifies at least five hitherto-unmapped scarps in the Bainbridge Island–West Seattle–Bellevue corridor with vertical offsets ranging from 32 cm to 1.84 m, dated via stacked OSL and AMS radiocarbon to four discrete rupture clusters: ~1810–1880 CE, ~1490–1540 CE, ~1130–1180 CE and ~900 CE. The recurrence-interval analysis applies a hierarchical Bayesian model with a Weibull-distributed inter-event prior, yielding a posterior mean of 348 years (95% credible interval 240–490 years) on the secondary system as a whole, with the most recent surface rupture almost certainly a 19th-century event preceding the establishment of a regional seismograph network in 1909 but possibly recorded in Skykomish and Snohomish oral traditions describing a 'great shaking' of forested ridges in the Cascade foothills and a temporary backwater of the Snohomish River.

The hazard implication is non-trivial. Although the inferred secondary-fault earthquakes are smaller (moment magnitude 6.0–6.6 under the team's Leonard-2010 slip-area scaling), they rupture shallowly (top-of-rupture 1–3 km) and very close to dense urban infrastructure: downtown Seattle, the SR-99 deep-bore Alaskan Way tunnel, the Sound Transit Link light-rail spine, Harborview Medical Center, Virginia Mason and Swedish hospital complexes and the SeaTac International Airport approach all sit within 10 kilometres of at least one newly mapped scarp. The authors recommend that the USGS update its 2023 National Seismic Hazard Maps to enumerate the secondary system as an independent active source under their hazard-curve aggregation framework, and that the cities of Seattle and Bellevue revisit unreinforced-masonry retrofit prioritisation and seismic-design site-class boundary mapping accordingly. Co-author Lydia Staisch (USGS) is quoted in the press release as saying simply, 'These secondary faults bring the threat pretty close to home.'