The Seattle Fault represents one of the most significant seismic hazards in the Pacific Northwest — not because of its size or slip rate, but because of its location. Running directly beneath one of the most densely populated metropolitan areas on the West Coast, a major earthquake on the Seattle Fault would produce intense shaking precisely where millions of people live and work.
For the science behind how faults produce earthquakes, see what causes earthquakes. For current earthquake activity in Washington, visit our Washington earthquake tracker.
Geography: Location and Extent
Fault Trace
The Seattle Fault Zone is a complex system of interconnected thrust faults that extends approximately 70 km (43 miles) east-west across the Puget Sound region:
- Western extent: Extends offshore into Puget Sound west of Bainbridge Island
- Bainbridge Island: The fault runs along the northern edge of the island
- Downtown Seattle: Passes directly beneath the city's central business district
- Mercer Island: Crosses beneath or near the island in Lake Washington
- Bellevue: Continues beneath the eastern suburbs
- Eastern extent: Extends toward the Cascade Range foothills near Issaquah
The fault is not a single structure but a zone of multiple parallel and branching fault strands spanning approximately 5–7 km north-south.
Relationship to Regional Fault Systems
The Seattle Fault is one of several crustal faults in the Puget Lowland, a structural depression between the Cascade Range and the Olympic Mountains. Other significant faults in the region include:
| Fault | Location | Hazard |
|---|---|---|
| Seattle Fault | Beneath Seattle | M7+ capable; tsunami potential |
| South Whidbey Island Fault | North of Seattle | M6–7 capable |
| Tacoma Fault | Beneath Tacoma | M7+ capable |
| Devils Mountain Fault | Northern Puget Sound | Less well characterized |
| Olympia Fault | Near state capital | M6–7 capable |
These crustal faults operate independently of the Cascadia Subduction Zone, which lies offshore and is capable of M9+ megathrust earthquakes.
Geology: How the Seattle Fault Works
Tectonic Setting
The Seattle Fault is a south-dipping thrust fault formed by north-south compression of the Puget Lowland crust. This compression is caused by the convergence of the Juan de Fuca Plate beneath the North American Plate along the Cascadia Subduction Zone. As the crust is squeezed, faults like the Seattle Fault accommodate the shortening through reverse (thrust) motion.
In a thrust fault, the upper block (hanging wall) moves up and over the lower block (footwall). On the Seattle Fault, this means the block south of the fault is being pushed up and northward over the block to the north.
Fault Geometry
The Seattle Fault dips southward at approximately 45–60 degrees, meaning that:
- The surface trace (where the fault intersects the surface) runs roughly east-west
- The fault plane extends southward at depth beneath south Seattle
- Earthquake rupture would cause the ground south of the fault to uplift relative to the north
This geometry is critical for understanding the hazard: the fault is shallow (rupturing at depths of 5–15 km), which concentrates strong shaking near the surface.
The 900 CE Earthquake
Discovery
The Seattle Fault was largely unknown until the late 1980s and 1990s when researchers discovered dramatic geological evidence of a large prehistoric earthquake. Key discoveries included:
- Sudden coastal uplift: Bainbridge Island and the south shore of Puget Sound show evidence of abrupt uplift of 5–7 meters approximately 1,100 years ago
- Submarine landslides: Lake Washington sediments record massive underwater landslides triggered by violent shaking
- Tsunami deposits: Sandy layers in coastal marshes around Puget Sound were deposited by tsunami waves
- Tree-ring dating: Drowned forests and landslide deposits were dated to approximately 900–930 CE using radiocarbon and tree-ring analysis
Event Characteristics
Based on geological evidence, seismologists estimate the ~900 CE earthquake had the following characteristics:
| Parameter | Estimate | Evidence |
|---|---|---|
| Magnitude | M7.0–7.5 | Uplift amount, rupture length |
| Surface rupture | ~30–50 km | Extent of documented uplift |
| Vertical displacement | 5–7 m | Raised beaches on Bainbridge Island |
| Tsunami height | 2–5 m | Tsunami deposits in marshes |
| Shaking duration | 20–40 seconds | Typical for M7+ shallow thrust |
| Landslides | Extensive | Submarine and terrestrial landslide evidence |
Impacts
The earthquake caused dramatic landscape changes still visible today:
- Raised beaches: The south shore of Bainbridge Island was lifted above the waterline, creating a raised platform
- Landslides: Massive rock avalanches occurred in the Cascade foothills
- Tsunami waves: Waves swept across Puget Sound, depositing sand in coastal marshes
- Subsidence: Areas north of the fault subsided, flooding some coastal areas
If a similar earthquake occurred today, it would devastate the Seattle metropolitan area.
Seismic Hazard
Earthquake Probability
The Seattle Fault is considered a significant seismic hazard by the USGS and the Pacific Northwest Seismic Network. Key hazard factors:
- Recurrence interval: Estimated 1,000–3,000 years between major earthquakes
- Time since last event: Approximately 1,100 years (~900 CE)
- Current status: Within the estimated recurrence window; could rupture at any time
- No creep detected: The fault appears fully locked, accumulating strain
Shaking Intensity
Because the Seattle Fault is directly beneath the city, shaking would be extremely intense:
- Very strong shaking (MMI VIII–IX): Expected throughout downtown Seattle, Bellevue, and nearby communities
- Severe shaking (MMI X): Possible in areas directly above the fault rupture
- Duration: 20–40 seconds of strong shaking
- Directivity: Depending on rupture direction, shaking could be amplified to the east or west
The shallow depth (5–15 km) means seismic waves have less distance to attenuate before reaching the surface, resulting in stronger ground motion than deeper earthquakes of similar magnitude.
Tsunami Hazard
Unlike the Cascadia Subduction Zone (which would generate ocean tsunamis), the Seattle Fault would generate tsunamis within Puget Sound:
- Source: Seafloor displacement and submarine landslides in Puget Sound
- Arrival time: Minutes to tens of minutes — very little warning time
- Wave heights: Estimated 2–5 meters along Puget Sound shorelines
- Affected areas: Seattle waterfront, Bainbridge Island, Alki Beach, Tacoma, and all Puget Sound coastal areas
- Warning: Local tsunamis provide much less warning time than distant tsunamis
Estimated Impacts of a Future Earthquake
A study by the Washington State Department of Natural Resources modeled the effects of a M7.2 Seattle Fault earthquake:
| Impact | Estimate |
|---|---|
| Fatalities | 1,600+ |
| Injuries | 24,000+ |
| Buildings damaged | 9,600 major damage; 39,000 moderate damage |
| Economic losses | $50+ billion |
| Displaced households | 130,000+ |
| Bridge closures | Numerous, including I-90 floating bridge |
| Liquefaction | Widespread in fill areas (SoDo, Duwamish, Harbor Island) |
| Fire following earthquake | Significant risk in dense areas |
Critical Infrastructure
The Seattle Fault's location beneath the urban core means critical infrastructure is directly exposed:
| Infrastructure | Vulnerability | Notes |
|---|---|---|
| I-90 floating bridge | High | Crosses Lake Washington near fault trace |
| SR 520 floating bridge | Moderate | North of fault but subject to strong shaking |
| Downtown Seattle buildings | Variable | Mix of seismically retrofitted and vulnerable structures |
| Seattle-Tacoma International Airport | Moderate | Subject to liquefaction hazards |
| Port of Seattle | High | Built on fill; liquefaction and tsunami vulnerable |
| Water and sewer systems | High | Numerous crossings and old infrastructure |
Monitoring
Seismic Monitoring
The Seattle Fault is monitored by the Pacific Northwest Seismic Network (PNSN), operated by the University of Washington in partnership with the USGS:
- Dense network of seismograph stations throughout Puget Sound region
- Real-time earthquake detection and location
- ShakeAlert earthquake early warning system coverage
- Continuous monitoring for any signs of fault activity
Geodetic Monitoring
GPS stations and InSAR (satellite radar) measurements track crustal deformation across the Puget Lowland, although the Seattle Fault's slow slip rate (~1–2 mm/year estimated) makes it difficult to measure directly.
Research Drilling
The U.S. Geological Survey and partners have conducted drilling studies to directly sample and analyze the Seattle Fault at depth, improving understanding of fault geometry and behavior.
Preparedness
Given the high hazard posed by the Seattle Fault, residents and businesses in the Puget Sound region should:
- Prepare an emergency kit: Supplies for 72+ hours without outside assistance
- Secure your home: Strap water heaters, secure bookshelves, know how to shut off gas
- Know your hazard zone: Understand if you are in a liquefaction or tsunami zone
- Sign up for ShakeAlert: Receive earthquake early warning notifications
- Practice Drop, Cover, Hold On: The proven protective action during shaking
For detailed preparedness guidance, see what to do during an earthquake and earthquake emergency plan.