The Seattle Fault is a zone of multiple shallow east–west

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s that cross the Puget Sound Lowland and through

Seattle Seattle ( ) is a seaport city on the West Coast of the United States. It is the seat of King County, Washington. With a 2020 population of 737,015, it is the largest city in both the state of Washington and the Pacific Northwest region o ...

(in the

U.S. state In the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its sove ...

Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...

) in the vicinity of Interstate Highway 90. The Seattle Fault was first recognized as a significant seismic hazard in 1992, when a set of reports showed that about 1,100 years ago it was the scene of a major

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

of about magnitude 7 – an event that entered Native American oral legend. Extensive research has since shown the Seattle Fault to be part of a regional system of faults.

Notable earthquake

First suspected from mapping of gravitational anomalies in 1965 and an uplifted marine terrace at Restoration Point (foreground in picture above), the Seattle Fault's existence and likely hazard were definitively established by a set of five reports published in ''Science'' in 1992. These reports looked at the timing of abrupt uplift and subsidence around Restoration Point and

Alki Point Alki Point is a point jutting into Puget Sound, the westernmost landform in the West Seattle district of Seattle, Washington. Alki is the peninsular neighborhood on Alki Point. Alki was the original settlement in what was to become the city of S ...

(distant right side of picture),

tsunami deposit A tsunami deposit (the term tsunamiite is also sometimes used) is a sedimentary unit deposited as the result of a tsunami. Such deposits may be left onshore during the inundation phase or offshore during the 'backwash' phase. Such deposits are u ...

s on Puget Sound, turbidity in lake paleosediments, rock avalanches, and multiple landslides around Lake Washington, and determined that all these happened about 1100 years ago (between 900 and 930 CE), and most likely due to an earthquake of magnitude 7 or greater on the Seattle Fault. ayahos3

Although the 900–930 CE earthquake was over a thousand years ago, local native legends have preserved an association of a powerful supernatural spirit – a'yahos, noted for shaking, rushes of water, and landsliding – with five locales along the trace of the Seattle Fault, including a "spirit boulder" called Psai-Yah-hus near the Fauntleroy ferry dock in West Seattle.

Geology

The Seattle Fault is the structural boundary where 50–60 million years old (early

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

)

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

of the Crescent Formation on the south has been uplifted – the Seattle Uplift – and is tipping into the Seattle Basin, where the Tertiary bedrock is buried under at least of relatively softer, lighter sedimentary strata of the younger Blakeley and Blakely Harbor formations. This has resulted in a wide zone of complex faulting, with three or more main south-dipping

s. Most of the faulting is "blind" (not reaching the surface), and generally difficult to locate because of heavy vegetation or development. Three principal strands have been identified, their location determined by high-resolution seismic reflection and aeromagnetic surveys. The northernmost strand lies nearly along Interstate 90 and then under

Lake Sammamish Lake Sammamish is a freshwater lake east of Seattle in King County, Washington, United States. The lake is long and wide, with a maximum depth of and a surface area of . It lies east of Lake Washington and west of the Sammamish Plateau, ...

. The central section of the fault zone – where it crosses the apparent location of the Olympic–Wallowa Lineament – shows marked variation in the location of the strands and of the underlying structure, but the nature and significance of this is not understood. The fault extends for approximately 70 km (43 miles) from near Fall City on the east, where it appears to be terminated by the South Whidbey Island Fault, to

Hood Canal Hood Canal is a fjord forming the western lobe, and one of the four main basins,Puget Sound faults#Question of western termination). It is the northern edge of the Seattle Uplift, of which the

Tacoma Fault The Tacoma Fault, just north of the city of Tacoma, Washington, is an active east–west striking north dipping reverse fault with approximately 35 miles (56 km) of identified surface rupture. It is believed capable of generating earthqu ...

is the southern edge. One model has the Seattle and Tacoma faults converging at depth to form a wedge, which is being popped up by approximately north–south oriented compression that ultimately derives from plate tectonics. Another model (see diagram) interprets the Seattle Uplift as a sheet of rock that is being forced up a ramp. Subsequent work suggests that the structure of the Seattle Fault may vary from east to west, with both models being applicable in different sections. A later model has part of the north-thrusting sheet forming a wedge between the sedimentary formations of the Seattle Basin and the underlying bedrock. The Seattle Fault is believed to date from about 40 million years ago (late

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...

). This is about the time that the strike-slip movement on the north-striking

Straight Creek Fault __NOTOC__ The Straight Creek Fault (SCF) is the principal north-south strike-slip fault in the state of Washington, with a minimum of 90 kilometers (54 miles) of right-lateral offset, and a major geological structure in the North Cascade mountai ...

to the east ceased, due to the intrusions of

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s. It appears that when the Straight Creek Fault became stuck the north–south compressive force that it had accommodated by strike-slip motion was transferred to the crust of the Puget Lowland, which subsequently folded and faulted, and the various blocks jammed over one another. Other scarps associated with the Seattle fault have been identified by

LIDAR Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...

-based mapping; trenching has generally shown the faulting to be more complex than was first realized. Many of the details of the Seattle Fault, including recurrence rate, remain to be resolved. A study of sediments in Lake Washington found evidence of seven large (M > 7) earthquakes in the last 3500 years. Surface scarps due to faulting are rarely observed in this area (due to topography, vegetation, and urbanization); a rare exception can be seen at Mee Kwa Mooks Park south of

. This is the site of the ''West Seattle Fault''; the prominent rise there is due to uplift on the north side of the fault.

Hazard

Port-au-Prince harbour crane after 2010 earthquake

The Seattle Fault (and the related Tacoma Fault) is not the only source of earthquake hazard in the Puget Lowland. Other faults in the near surface continental crust, such as the South Whidbey Island Fault (near Everett), and the yet to be studied Olympia Fault (near Olympia), though historically quiescent, are suspected of generating earthquakes of around magnitude 7. Others, such as the

2001 Nisqually earthquake The 2001 Nisqually earthquake occurred at on February 28, 2001 and lasted nearly a minute. The intraslab earthquake had a moment magnitude of 6.8 and a maximum Mercalli intensity of VIII (''Severe''). The epicenter was in the southern Puge ...

, originate about below Puget Sound in the

Benioff zone Benioff is a surname. Notable people with the surname include: *David Benioff (born 1970), American writer, screenwriter and television producer *Hugo Benioff (1899–1968), American seismologist and academic **Wadati–Benioff zone * Marc Benioff ...

of the subducting Juan de Fuca Plate; being so deep their energy is dissipated. And there are the infrequent but very powerful great subduction events, such as the magnitude 9

1700 Cascadia earthquake The 1700 Cascadia earthquake occurred along the Cascadia subduction zone on January 26, 1700, with an estimated moment magnitude of 8.7–9.2. The megathrust earthquake involved the Juan de Fuca Plate from mid- Vancouver Island, south along th ...

, where the entire

Cascadia subduction zone The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is a very long, sloping subduction zone where the Explorer, Juan de Fuc ...

, from Cape Mendocino to

Vancouver Island Vancouver Island is an island in the northeastern Pacific Ocean and part of the Canadian province of British Columbia. The island is in length, in width at its widest point, and in total area, while are of land. The island is the largest by ...

, slips. But the Seattle and Tacoma faults are probably the most serious earthquake threat to the populous Seattle–Tacoma area. A 2002 study of bridge vulnerability estimated that a magnitude 7 earthquake on the Seattle Fault would damage approximately 80 bridges in the Seattle–Tacoma area, whereas a magnitude 9 subduction event would damage only around 87 bridges in all of Western Washington. The same study also found that with failure of just six bridges (the minimum damage for a Benioff M 6.5 event) there could be at least $3 billion lost in business revenue alone. Subsequent retrofitting by the Washington Department of Transportation and the City of Seattle would likely reduce damage to key bridges. But there is concern that such an earthquake on the Seattle Fault would devastate unreinforced masonry (URM) buildings, of which the City of Seattle is estimated to have around a thousand, concentrated in Capitol Hill, Pioneer Square, and the International District. Seattle - Earthquake damage to Cadillac Hotel 2nd Ave S in Pioneer Square, 2001

Seattle - Earthquake damage to Cadillac Hotel 2nd Ave S in Pioneer Square, 2001

Other recent work indicates that the Seattle Fault can generate two types of earthquakes; both pose "considerable hazard" to the Seattle metropolitan region. The A.D. 900–930 earthquake is believed to be the only instance in the past 7,000 years of the type that causes a regional uplift. The other type is more localized and shallower (and therefore more damaging); at least four such events are believed to have occurred in the past 3,000 years on the west end of the fault. (The history of the central and eastern segments is not known.) Calculations based on fault length and paleoseismological studies show that the Seattle Fault can generate a very damaging magnitude 7.0 earthquake. In addition to extensive damage to unreinforced structures and structures built on fill (such as much of Seattle's Pioneer Square area, the industrial area, and the waterfront), computer modeling has shown that such earthquakes could cause a tsunami of about high on Elliott Bay. The modeling shows that such a tsunami would also inundate the industrial areas on Commencement Bay 30 miles south (Tacoma) and low-lying areas on the Puyallup River delta. There is also concern that a severe or prolonged event could cause failure of the Duwamish or

Puyallup River The Puyallup River ( ) is a river in the U.S. state of Washington. About long, it is formed by glaciers on the west side of Mount Rainier. It flows generally northwest, emptying into Commencement Bay, part of Puget Sound. The river and its tribu ...

deltas, where the main port facilities for Seattle and Tacoma are located (Harbor Island and Commencement Bay)., § 3; .

Notes

Sources

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External links

USGS Professional Paper 1560: Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest
Technical, but readable. And has many good pictures.
USGS: Reducing Earthquake Hazards in the Pacific Northwest

USGS: Earthquake ground motion movies
*
Scenario for a Magnitude 6.7 Earthquake on the Seattle Fault
Vivid and comprehensive.
Washington State's Bridge Seismic Retrofit Program

Earthquake Study: Four Vashon-Specific Scenarios
Considerations for all islands.
Puget Sound Tsunami Inundation Modeling (NOAA)

Elliott Bay inundation map (DNR)

Tsunami Hazard Map of the Elliott Bay Area (NOAA)

Additional inundation maps (DNR Publications)

Puget Sound Lidar Consortium
Use of Lidar for locating faults. {{good article Geography of Seattle Seismic faults of Washington (state) Buried rupture earthquakes