File:Puget Sound faults.png, upright=1.34, The principal

Puget Sound Puget Sound ( ) is a sound of the Pacific Northwest, an inlet of the Pacific Ocean, and part of the Salish Sea. It is located along the northwestern coast of the U.S. state of Washington. It is a complex estuarine system of interconnected m ...

faults (approximate location of known extents) and other selected peripheral and minor faults. Southern tip of Vancouver Island and San Juan Islands at top left (faults not shown), Olympic Mountains at center left, Mount Rainier at lower right (near WRZ). Faults north to south: Devils Mountain, Utsalady Point, Strawberry Point, Mount Vernon Fault/Granite Falls FZ/Woods Creek, Monroe Fault, Little River,

Sequim Sequim ( ) is a city in Clallam County, Washington, United States. It is located along the Dungeness River near the base of the Olympic Mountains. The 2010 census counted a population of 6,606. Sequim lies within the rain shadow of the Olympic M ...

, Southern Whidbey Island Fault, Cherry Creek, Tokul Creek, Rattlesnake Mountain Fault Zone, Lofall, Canyon River, Frigid Creek, Saddle Mountain faults,

Hood Canal Hood Canal is a fjord forming the western lobe, and one of the four main basins,Dabob Bay, Seattle Fault Zone, Dewatto Lineament, Tacoma Fault Zone,

East Passage Narragansett Bay is a bay and estuary on the north side of Rhode Island Sound covering , of which is in Rhode Island. The bay forms New England's largest estuary, which functions as an expansive natural harbor and includes a small archipelago. S ...

, White River (extends east), Olympia Structure, Scammon Creek, Doty (extends west), Western Rainier Zone, Saint Helens Zone (extends south). Also shown:

Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, British Columbia, provincial capital of British Columbia, Canada * Victoria (mythology), Roman goddess of Victory * Victoria, Seychelle ...

(V), part of the Leech River Fault (unlabeled), and part of the Olympic–Wallowa Lineament. # # imagemap line numbers include comment lines. # line 5 # rect 272 12 336 34 scale box poly 0 36 50 58 50 70 0 48 Leech River Fault poly 108 80 132 80 140 72 180 72 184 84 383 144 383 156 250 98 108 90 Devils Mountain Fault Zone # 10 poly 186 100 208 100 245 110 245 124 216 124 210 114 Strawberry Point Fault poly 160 98 176 98 216 128 236 128 236 144 200 144 196 126 Utsalady Point Fault poly 102 92 120 102 212 160 242 190 242 206 290 252 282 262 116 112 88 94 Southern Whidbey Island Fault poly 300 202 308 196 324 224 320 234 Woods Creek Fault poly 308 174 328 206 324 216 300 176 Lake Chaplain Fault poly 328 100 254 102 324 224 300 176 308 216 290 212 248 140 256 132 Mount Vernon Fault poly 302 280 340 176 352 180 308 296 Cherry Creek Fault Zone poly 316 318 346 248 360 252 320 336 Tokul Creek Fault Zone poly 292 258 326 348 312 348 296 336 280 292 Rattlesnake Mountain Fault Zone # poly 0 144 20 152 28 142 62 152 66 166 80 171 76 184 0 160 Little River Fault rect 90 148 138 172 Sequim Fault poly 128 204 164 252 170 290 150 290 114 260 122 210 Dabob Bay Fault Zone poly 134 284 144 292 204 294 276 302 280 316 244 332 220 324 200 312 116 312 Seattle Fault Zone poly 78 342 104 300 124 280 128 280 108 300 104 310 114 324 98 350 Hood Canal Fault (questioned) poly 48 336 64 296 102 290 70 340 Saddle Mountain Faults poly 0 332 28 320 42 346 8 356 Canyon River Fault poly 28 364 64 344 70 348 52 380 28 380 Frigid Creek Fault poly 120 316 144 316 144 334 120 352 Dewatto Lineament/fault poly 126 356 156 332 204 332 204 372 228 400 192 400 148 356 Tacoma Fault Zone poly 210 352 262 360 262 376 216 376 210 364 East Passage Zone poly 272 324 320 360 383 372 383 424 256 340 264 328 Olympic–Wallowa Lineament poly 383 460 292 396 304 380 383 432 White River Fault poly 76 364 148 460 172 528 160 528 68 372 Olympia Structure (suspected fault) poly 0 536 36 536 40 520 64 520 72 536 132 536 132 548 0 548 Doty Fault poly 142 596 182 572 192 572 212 639 192 639 142 616 Saint Helens Zone poly 226 504 280 482 296 540 332 616 320 620 270 536 226 520 Western Rainier Zone circle 66 65 10

Victoria, British Columbia Victoria is the capital city of the Provinces and territories of Canada, Canadian province of British Columbia, on the southern tip of Vancouver Island off Canada's Pacific Ocean, Pacific coast. The city has a population of 91,867, and the Gre ...

circle 136 27 96

San Juan Islands The San Juan Islands are an archipelago in the Pacific Northwest of the United States between the U.S. state of Washington and Vancouver Island, British Columbia, Canada. The San Juan Islands are part of Washington state, and form the core ...

poly 383 0 383 12 376 18 360 56 340 50 362 0

Lake Shannon Lake Shannon is a long, narrow reservoir on the Baker River in Skagit County, Washington in the United States. Formed in the 1920s by the construction of an arch dam just above the town of Concrete, the lake is approximately long and averages ...

circle 42 246 45

Olympic Mountains The Olympic Mountains are a mountain range on the Olympic Peninsula of the Pacific Northwest of the United States. The mountains, part of the Pacific Coast Ranges, are not especially high – Mount Olympus is the highest at ; however, the easte ...

circle 64 456 24

Black Hills The Black Hills ( lkt, Ȟe Sápa; chy, Moʼȯhta-voʼhonáaeva; hid, awaxaawi shiibisha) is an isolated mountain range rising from the Great Plains of North America in western South Dakota and extending into Wyoming, United States. Black ...

circle 320 504 20

Mount Rainier Mount Rainier (), indigenously known as Tahoma, Tacoma, Tacobet, or təqʷubəʔ, is a large active stratovolcano in the Cascade Range of the Pacific Northwest, located in Mount Rainier National Park about south-southeast of Seattle. With a ...

poly 200 584 240 592 240 604 220 612 202 594 Riffe Lake poly 168 220 180 212 204 212 204 236 168 228 Lofall Fault The Puget Sound faults under the heavily populated

Puget Sound region The Puget Sound region is a coastal area of the Pacific Northwest in the U.S. state of Washington, including Puget Sound, the Puget Sound lowlands, and the surrounding region roughly west of the Cascade Range and east of the Olympic Mountains. ...

(Puget Lowland) of Washington state form a regional complex of interrelated seismogenic (earthquake-causing) geologic faults. These include (from north to south, see map) the: * Devils Mountain Fault * Strawberry Point and Utsalady Point faults * Southern Whidbey Island Fault (SWIF) * Rogers Belt (Mount Vernon Fault/Granite Falls Fault Zone) * Cherry Creek Fault Zone * Rattlesnake Mountain Fault Zone *

Seattle Fault The Seattle Fault is a zone of multiple shallow east–west thrust faults that cross the Puget Sound Lowland and through Seattle (in the U.S. state of Washington) in the vicinity of Interstate Highway 90. The Seattle Fault was first recognized as ...

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 ...

* Saddle Mountain Faults * Olympia structure (suspected fault) * Doty Fault * Saint Helens Zone and Western Rainier Zone

General background

Earthquake sources and hazard

The

(Puget Lowland) of western

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 ...

contains the bulk of the population and economic assets of the state, and carries seven percent of the international trade of the United States. All this is at risk of earthquakes from three sources: * A great subduction earthquake, such as the magnitude M 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 ...

, caused by slippage of 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 approximately Cape Mendocino in northern California 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 ...

in British Columbia. * Intraslab (

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 ...

) earthquakes, such as the M 6.7

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 ...

, caused by slippage or fracturing on a small part of the subducting plate at a depth of around . * Relatively shallow crustal earthquakes, generally less than deep, caused by stresses and faulting in the near-surface crustal structures. The energy released depends on the length of the fault; the faults here are believed capable of generating earthquakes as great as M 6 or 7. OFR 99-311 fig48

While the great subduction events release much energy (around magnitude 9), that energy is spread over a large area, and largely centered near the coast. The energy of the somewhat smaller Benioff earthquakes is likewise diluted over a relatively large area. The largest intra-crustal earthquakes have about the same total energy (which is about one-hundredth of a subduction event), but since they are closer to the surface they will cause more powerful shaking, and, therefore, more damage. One study of seismic vulnerability of bridges in the Seattle – Tacoma area estimated that an M 7 earthquake on the Seattle or Tacoma faults would cause nearly as much damage as a M 9 subduction earthquake. Because the Seattle and Tacoma faults run directly under the biggest concentration of population and development in the region, more damage would be expected, but all the faults reviewed here may be capable of causing severe damage locally, and disrupting the regional transportation infrastructure, including highways, railways, and pipelines. (Links with more information on various hazards can be found at

.) The Puget Sound region is not just potentially seismic, it is actively seismic. Mapping from the Pacific Northwest Seismic Network shows that the bulk of the earthquakes in western Washington are concentrated in four places: in two narrow zones under Mt. Saint Helens and Mt. Rainier, along the DDMFZ, and under Puget Sound between Olympia and approximately the Southern Whidbey Island Fault. The southern limit nearly matches the southern limit of the glaciation; possibly the seismicity reflects rebound of the upper crust after being stressed by the weight of the glacial ice.

Discovery

Thick glacial and other deposits, heavy vegetation, urban development, and a topography of sharp relief and rapid erosion obscures the surface expression of faults in this region, and has hindered their discovery. The first definite indications of most of these faults came from gravitational mapping in 1965, and their likely existence noted on mapping in 1980 and 1985. As of 1985 only the Saddle Mountain Faults had been shown to have

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

activity (since the last ice age, about 12,000 years ago). Not until 1992 was the first of the lowland faults, the

, confirmed to be an actual fault with Holocene activity, and the barest minimum of its history established. Discovery of faults has been greatly facilitated with the development of

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 ...

, a technique that can generally penetrate forest canopy and vegetation to image the actual ground surface with an unprecedented accuracy of approximately one foot (30 cm). An informa
consortium
of regional agencies has coordinated LIDAR mapping of much of the central Puget Lowland, which has led to discovery of numerous fault scarps which are then investigated by trenching (

paleoseismology Paleoseismology looks at geologic sediments and rocks, for signs of ancient earthquakes. It is used to supplement seismic monitoring, for the calculation of seismic hazard. Paleoseismology is usually restricted to geologic regimes that have ...

). Marine seismic reflection surveys on Puget Sound where it cuts across the various faults have provided cross-sectional views of the structure of some of these faults, and an intense, wide-area combined on-shore/off-shore study in 1998 (Seismic Hazards Investigation in Puget Sound, or SHIPS). resulted in a three-dimensional model of much of the subsurface geometry. Aeromagnetic surveys, seismic tomography, and other studies have also contributed to locating and understanding these faults.

Geological setting

The ultimate driver of the stresses that cause earthquakes are the motions of the

tectonic plates Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

: material from the Earth's

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

rises at spreading centers, and moves out as plates of

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...

which eventually are subducted under the more buoyant plates of

continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial'' be ...

. Western Washington lies over the

, where the Juan de Fuca Plate is subducting towards the east (see diagram, right). This is being obliquely overridden by the

North American plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Paci ...

coming out of the northeast, which has formed a bend in the subducting plate and in the

forearc Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the v ...

basin above it. This bend has distorted the subducting slab into an arch that has lifted the

and prevented them from subducting. For the past 50 million years or so (since the early

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'', ...

epoch) these have been thrust by subduction up against the

North Cascades The North Cascades are a section of the Cascade Range of western North America. They span the border between the Canadian province of British Columbia and the U.S. state of Washington and are officially named in the U.S. and Canada as the Casca ...

("fixed block" in the diagram), which sit on the North American Plate. This forms a pocket or trough – what one local geologist calls the "big hole between the mountains" – between the Cascades on the east and the

and

Willapa Hills The Willapa Hills is a geologic, physiographic, and geographic region in southwest Washington. When described as a physiographical province, the Willapa Hills are bounded by the Pacific Ocean to the west, the Columbia River to the south, the Ol ...

on the west. This pocket is catching a stream of

terranes In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...

(crustal blocks about 20 to 30 km thick) which the

Pacific plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and I ...

is pushing up the western edge of North America, and in the process imparting a bit of clockwise rotation to southwestern Washington and most of Oregon; the result has been characterized as a train wreck. These terranes were covered by the basalts of the Crescent Formation (part of

Siletzia Siletzia is a massive formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone, on the west coast of North America. It forms the basement rock under western Oregon and ...

). Folding and faulting has exposed these basalts in some places (black areas in diagram); the intervening basins have been filled by various sedimentary formations, some of which have been subsequently uplifted. Glacially deposited and shaped fill covers most of the lower elevations of

. This is the Puget Lowland. The principal effects of this complex interplay of forces on the near-surface crust underlying the Puget Lowland are: * The

basement rock In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...

of the Crescent Formation is being forced up on the southern, eastern, and northern flanks of the Olympic Mountains, and at various folds (wrinkles). * Some upper-crustal formations (such as the Western and Eastern Melange Belts, see map) have been pushed onto the older (pre-

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 ...

) basement of the North Cascades. * There is a general north or northeast directed compression within the Lowland causing folds, which eventually break to become

dip-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

(vertical movement)

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

or reverse faults. * Some

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

(horizontal) movement is expected along the peripheral faults (such as Southern Whidbey Island and Saddle Mountain faults). Further complicating this is a feature of unknown structure and origin, the Olympic–Wallowa Lineament (OWL). This is a seemingly accidental alignment of topographic features that runs roughly east-southeast from the north side of the Olympic Peninsula to the

Wallowa Mountains The Wallowa Mountains () are a mountain range located in the Columbia Plateau of northeastern Oregon in the United States. The range runs approximately northwest to southeast in southwestern Wallowa County and eastern Union County between the ...

in northeastern Oregon. It aligns with the West Coast fault and Queen Charlotte Fault system of

fault zones (similar to the

San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal) ...

in California) on the west side of

, but does not itself show any significant or through-going strike-slip movement. It is of interest here because the various strands of the Seattle Fault change orientation where they appear to cross the OWL, and various other features, such as the Rosedale monocline and Olympia structure, and a great many local topographical features, have parallel alignments. It may also be the original location of the Darrington—Devils Mountain Fault (the dashed line "X" at the top of the following map). The OWL appears to be a deep-seated structure over which the shallower crust of the Puget Lowland is being pushed, but this remains speculative.

Uplift and basin pattern

Most of these "faults" are actually zones of complex faulting at the boundaries between sedimentary basins (

synclines In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...

, "∩") and crustal uplifts ( anticlines, "∪"). There is a general pattern where most of these faults partition a series of basins and uplifts, each about 20 km wide. From the north these are (see the map at right): * Devils Mountain Fault zone (including Strawberry Point and Utsalady Point faults) : ∪ Everett Basin * Southern Whidbey Island Fault (SWIF) : ∩ "Uplift of unknown origin" (Port Ludlow) * Kingston arch (Lofall Fault) : ∪ Seattle Basin * Seattle Fault zone (approx. lines E-F) : ∩ Seattle Uplift * Tacoma Fault Zone (approx. line C) : ∪ Tacoma Basin * Olympia fault (approx. line A) : ∩ Black Hills Uplift * Doty Fault / Scammon Creek Fault (dashed lines) : ∪ Chehalis Basin The Hood Canal Fault (and its possible extensions) and Saddle Mountain faults to the west are believed to form the western boundary to all this. On the east, the Devils Mountain Fault connects with the south striking Darrington Fault (not shown) which runs to the OWL, and the Southern Whidbey Island Fault extends via the Rattlesnake Mountain Fault Zone (dashed line) to the OWL. South of the OWL a definite eastern boundary has not been found, with some indications it is indefinite. (E.g., the Olympia Fault is aligned with and appears to be the northernmost member of a set of faults between Olympia and Chehalis that may extend to the Columbia River, and there has been a suggestion that the Tacoma Fault may connect with the White River—Naches River fault on the east side of the Cascades.) The uplift and basin pattern is continued to the west and southwest by the Grays Harbor Basin, Willapa Hills Uplift, and Astoria Basin, but it is not known if these are bounded by faults in the same manner as in the Puget Sound region.

Structural models

Thrust sheet hypothesis

It is believed that all of these faults, folds, basins, and uplifts are related. According to the preeminent model, the "Puget Lowland thrust sheet hypothesis", these faults, etc., occur within a sheet of crust about 14 to 20 km deep that has separated from and is being thrust over deeper crustal blocks. Most of this thrust sheet consists of the Crescent Formation (corresponding to the Siletz River volcanics in Oregon and Metchosin Formation on Vancouver Island), a vast outpouring of volcanic

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 ...

from the

epoch (about 50 million years ago), with an origin variously attributed to a seamount chain, or continental margin rifting (see

). This "

basement A basement or cellar is one or more Storey, floors of a building that are completely or partly below the storey, ground floor. It generally is used as a utility space for a building, where such items as the Furnace (house heating), furnace, ...

" rock is covered with sedimentary deposits similar to the

Chuckanut Formation The Chuckanut Formation in northwestern Washington (named after the Chuckanut Mountains, near Bellingham), its extension in southwestern British Columbia (the Huntingdon Formation), and various related formations in central Washington (incl ...

, and more recent (typically

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

) volcanic deposits. The Seattle uplift, and possibly the Black Hills uplift, consist of Crescent Formation basalt that was exposed when it was forced up a ramp of some kind. This ramp could be either in the lower crustal blocks, or where the thrust sheet has split and one part is being forced over the next. Faults and folds may develop where the thrust sheet is being bent, or where the leading edge is thrust over softer, weaker sedimentary deposits, and breaks off and slumps. If, as this model suggests, the various faults are interconnected within the thrust sheet, there is a possibility that one earthquake could trigger others. This prospect is especially intriguing as a possible explanation of a cluster of seismic events around 1100 years ago.

Seismotectonic modeling

In the previous study seismicity, surface geology, and geophysical data were modeled in order to examine the fault structuring of the upper crust. Another model (of , USGS Open-File Report 99–0311) – not so much in competition with the first as complementing it – used seismic and other data to create a 3-D tectonic model of the whole crust; this was then analyzed using

finite element The finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest include the traditional fields of structural analysis, heat t ...

methods to determine regional geodynamic characteristics. A principal finding is that " ustal seismicity in the southern Puget Sound region appears to be controlled by a key block of Crescent Formation occurring just south of the Seattle fault." More particularly, the concentration of seismicity under Puget Sound south of the Seattle Fault is attributed to uplift of that block, bounded by the Seattle, Tacoma, and Dewatto faults on the north, south, and west (the eastern boundary is not determined), creating the Seattle Uplift. And it is suggested that the Great Seattle Quake of approximately 1,100 years ago, and other coseismic events in southern Puget Sound around that time, were a single event that affected this entire block, with a magnitude of around 8, possibly triggered by an earthquake deeper in the crust. Very little is known about the structure of the deep crust (below about ), though this and other seismic tomography studies (such as ) provide tantalizing glimpses. ---- For the following reviews the primary source of information is the U.S. Geological Survey'
Quaternary fault and fold database (QFFDB)
which includes details of discovery, a technical description, and bibliography for each fault; a specific link is provided (where available) at the end of each section.

Devils Mountain Fault

The Devils Mountain Fault (DMF) runs about 125 km (75 miles) from the town of Darrington in the Cascade foothills due west to the northern tip of

Whidbey Island Whidbey Island (historical spellings Whidby, Whitbey, or Whitby) is the largest of the islands composing Island County, Washington, in the United States, and the largest island in Washington State. (The other large island is Camano Island, ...

, and on towards

, where the DMF is believed to join the Leech River Fault, Leech River fault system at the southern end of

. At Darrington it is seen to connect with the Darrington Fault, which runs nearly south 110 km to converge with the Straight Creek Fault (SCF), and then to turn near Easton, Washington, Easton to align with the Olympic–Wallowa Lineament; together these are known as the Darrington—Devils Mountain Fault Zone (DDMFZ). The Devils Mountain Fault separates two similar but distinctive ensembles of Mesozoic (pre-

, before the dinosaurs died) or older rock. On the north is the Helena—Haystack mélange (HH mélange, purple in the diagram at right), on the south the Western and Eastern mélange belts (WEMB, blue). There are some interesting relationships here. E.g., HH mélange rock has been found in Manastash Ridge, 110 km to the south (look for the small sliver of purple near the bottom of the diagram). Also, the sedimentary

(part of the NWCS, green) north of the DMF correlates to the Suak and Roslyn Formations just north of Manastash Ridge. All this is explained by right-lateral

motion on the Straight Creek Fault, which initiated about 50 to 48 Mya (unit), Ma (millions of years ago). This is just after the terrane carrying the Olympic Mountains came into contact with the North American continent. These mélanges may have been off-shore islands or seamounts that were caught between the Olympic terrane and the North American continent, and were pushed up (obduction, obducted) onto the latter. Other similar rock has been found at the Rimrock Lake Inlier (bottom of diagram), in the San Juan Islands, and in the Pacific Coast Complex along the West Coast Fault on the west side of Vancouver Island. It appears the entire DDMFZ and Leech River fault system was pushed onto the early continental margin from an original alignment along the OWL. This is an important observation because the Strawberry Point, Utsalady Point, Southern Whidbey Island, and various other unnamed faults lying between the DDMFZ and the OWL – all of which converge at the western end of the DDMFZ – seem to be intermediate versions of the DDMFZ. Movement on the southern segment of the DDMFZ that converges with the SCF – the Darrington Fault – was, as on the SCF itself, right-lateral. And like the SCF, strike-slip motion died out between 44 and 41 MA (due to plutonic intrusions). But the western segment – the Devils Mountain Fault – has ''left''-lateral movement. This is because the Olympic terrane is moving (relative to North America) northeast; its continued clockwise rotation is akin to a giant wheel rolling up the western side of the North Cascade crystalline core. The geology also suggests that the DMF is moving obliquely up a ramp that rises to the east, possibly an ancient coastal shore. The Devils Mountain Fault is seismically active, and there is evidence of

offsets. If the entire 125 km length ruptured in a single event the resulting earthquake could be as large as magnitude 7.5. However, there are indications that the fault is segmented, which might limit rupturing and earthquake magnitude.
USGS QFFDB Fault #574, Devils Mountain Fault

Strawberry Point and Utsalady Point faults

Strands of the east-striking Devils Mountain Fault cross the northern tip of

at Dugualla Bay and north side of Ault Field (Whidbey Island Naval Air Station). Just four miles (6 km) south the city of Oak Harbor, Washington, Oak Harbor straddles several stands of the Utsalady Point Fault (UPF) as they head roughly east-southeast towards Utsalady Point at the north end of Camano Island. And in between these two the Strawberry Point Fault (SPF) skirts the south side of Ault Field, splits into various strands that bracket Strawberry Point, and then disappear (possibly ending) under the delta of the Skagit River. Both the SPF and UPF are said to be oblique-slip transpressional; that is, the faults show both horizontal and vertical slip as the crustal blocks are pressed together. These faults also form the north and south boundaries of uplifted pre-

rock, suggesting that the faults come together at a lower level, much like one model of the Seattle and Tacoma faults, but at a smaller scale. Marine seismic reflection surveys on either side of Whidbey Island extend the known length of these faults to at least 26 and 28 km (about 15 miles). The true length of the UPF is likely twice as long, as it forms the southern margin of an aeromagnetic high that extends another 25 km to the southeast. Trenching on the UPF (at a scarp identified by LIDAR) shows at least one and probably two Holocene earthquakes of magnitude 6.7 or more, the most recent one between AD 1550 to 1850, and possibly triggered by the

. These earthquakes probably caused tsunamis, and several nearby locations have evidence of tsunamis not correlated with other known quakes. While there is a bit of uplifted pre-Tertiary rock between the SPF and UPF, this does not truly fit the #pattern, uplift and basin pattern described above because of the small scale (2 km wide rather than around 20), and because the uplift here is entirely like a wedge being popped out between two nearly vertical faults, rather than being forced over a ramp such as is involved with the Seattle and Tacoma faults. Nor does this uplift delineate any significant basin between it and the Devils Mountain Fault. On the basis of marine seismic reflection surveying in the Strait of Juan de Fuca it has been suggested that the DMF, SPF, and UPF are structurally connected (at least in the segment crossing Whidbey Island).
USGS QFFDB Fault #571, Strawberry Point Fault

USGS QFFDB Fault #573, Utsalady Point Fault

Southern Whidbey Island Fault

The ''Southern Whidbey Island Fault'' (SWIF) is a significant terrane boundary manifested as an approximately four mile wide zone of complex transpressional faulting with at least three strands. Marine seismic reflection surveys show it striking northwest across the eastern end of the Strait of Juan de Fuca. Just south of

it intersects the west-striking Devils Mountain Fault (reviewed above), and either merges with it,. or crosses (and possibly truncates) it to connect with the Leech River Fault. The Leech River Fault has been identified as the northern edge of the Crescent Formation (aka Metchosin Formation, part of the

terrane that underlies much of western Washington and Oregon). Seismic tomography studies show that this portion of the SWIF marks a strong contrast of seismic velocities, such as is expected of Crescent Formation basalts in contact with the metamorphic basement rocks of the Cascades geologic province to the east. To the southeast the SWIF passes through Admiralty Inlet (past Port Townsend, Washington, Port Townsend) and across the southern part of

, crossing to the mainland between Mukilteo, Washington, Mukilteo and Edmonds, Washington, Edmonds. This section of the SWIF forms the southwestern side of the Everett Basin (see #pattern, map), which is notably aseismic in that essentially no shallow (less than 12 km deep) earthquakes have occurred there, or on the section of the SWIF adjoining it, in the first 38 years of instrumental recording. Yet it is also notable that "most seismicity in the northern Puget Sound occurs along and southwest of the southern Whidbey Island fault at typical depths of 15–27 km within the lower part of the Crescent Formation." The contrast of seismic velocities seen to the northwest is lacking in this section, suggesting that it is not the Coast Range—Cascade contact. The significance of this — whether the edge of the Crescent Formation (and implicitly of the Siletz terrane) turns southward (discussed #CRBF, below), or the metamorphic basement is supplanted here by other volcanic rock — is not known. It has been suggested that a corresponding change in the character of the SWIF may reflect a change in the direction of regional crustal strain. Prior to 2000, prominent aeromagnetic anomalies strongly suggested that the fault zone continued southeast, perhaps as far as the town of Duvall, Washington, Duvall, but this was uncertain as the SWIF is largely concealed, and the faint surface traces generally obliterated by urban development. Since 2000 studies of LIDAR and high-resolution aeromagnetic data have identified scarps near Woodinville, Washington, Woodinville which trenching has confirmed to be tectonically derived and geologically recent. Subsequent mapping shows the SWIF wrapping around the eastern end of the #pattern, Seattle Basin to merge with the Rattlesnake Mountain Fault Zone (RMFZ); the RMFZ, despite the approximately 15° bend and different context, is now believed to be the southern extension of the SWIF. Reckoned between Victoria and approximately Fall City, Washington, Fall City the length of the SWIF is around 150 km (90 miles). It has been suggested that the SWIF might extend past its intersection with the RMFZ (with only peripheral strands turning to join the RMFZ) to cross the Cascades and eventually merge with or cross the Olympic–Wallowa Lineament; a study of regional features suggests such a pattern. But detailed mapping just past the intersection shows only a complex and confused pattern of faulting, with no indication that there is, or is not, through-going faulting. Mapping of areas further east that might clarify the pattern is not currently planned. Paleoseismological studies of the SWIF are scant. One study compared the relative elevation of two marshes on opposite sides of Whidbey Island, and determined that approximately 3,000 years ago an earthquake of M 6.5–7.0 caused 1 to 2 meters of uplift. Another study identified an unusually broad band of scarps passing between Bothell, Washington, Bothell and Snohomish, Washington, Snohomish, with several scarps in the vicinity of King County's controversial Brightwater sewage treatment plant, Brightwater regional sewage treatment plant showing at least four and possibly nine events on the SWIF in the last 16,400 years. Such seismic hazards were a major issue in the siting of the plant, as it is tucked between two active strands, and the influent and effluent pipelines cross multiple zones of disturbed ground.
USGS QFFDB Fault #572, Southern Whidbey Island Fault

Rogers Belt

North of Everett is an area of parallel ridges and stream drainages oriented approximately NW-SE, evident even on non-geological maps. These ridges (part of a broader regional pattern that reflects the roots of the former ''Calkins Range'') are formed of sediments that collected in the Everett basin during the Eocene, and were subsequently folded by northeast-directed compression against the older Cretaceous and Jurassic rock to the east that bound the Puget Lowland. At the edge of this older rock is the Rogers Belt, a geologically interesting zone running from the area of Sultan, Washington, Sultan (due east of Everett) to Mount Vernon, Washington, Mount Vernon (just north of the bend in the Devils Mountain Fault). Observing these topographical features, some parallel gravity gradients, and a "very active zone of minor seismicity", William Rogers inferred in 1970 a "fault or other major structural feature". The ''Bellingham Bay—Chaplain fault zone'' was first mapped by Cheney in 1976 as running from near Chaplain Lake (north of Sultan) NNW past Bellingham Bay. Doubts on the connectivity of these faults led to abandonment of this name in 1986 when Cheney mapped the ''Mount Vernon fault'' (MVF) from near Sultan northwest past Lummi Island, Washington, Lummi Island (west side of Bellingham Bay, visible at the top of the #PLmap, map), crossing the Devils Mountain Fault (#DMF, DMF, part of the Darrington—Devils Mountain Fault Zone) near Mount Vernon. Cheney also mapped the ''Lake Chaplain Fault'', parallel and just east of the MVF, from Lake Chaplain to Granite Falls, Washington, Granite Falls. Detailed mapping of this area since 2006 has revealed a complex pattern of faults. At the northern end the right-lateral ''McMurray Fault Zone'' (MFZ) straddles Lake McMurray, just south of the Devils Mountain Fault, and is suspected of being a major bounding fault. This is located on a topographical lineament that aligns with Mount Vernon to the north, and, to the south, the city of Granite Falls and Lake Chaplain (just north of Sultan). The ''Woods Lake Fault'', running past Lake Chaplain, corresponds closest to the mapped position of the southern end of Cheney's Mount Vernon Fault. However, subsequent mapping shows that the ''Woods Creek Fault'' (WCF), a four-mile wide strip of oblique-slip and strike-slip faults just to the west and passing directly under Sultan, appears to be the more significant fault, and better aligned with Mount Vernon. Both of these faults (and some others) appear to terminate against the left-lateral ''Sultan River Fault'' at the western margin of the NNE-striking Cherry Creek Fault Zone (CCFZ; see next section). The principal zone of faulting extends from the Woods Creek Fault to the ''Granite Falls Fault Zone'' (GFFZ), slightly offset from the WCF and running under the town of Granite Falls. Although the intervening section has not been mapped, geologists believe the GFFZ connects with the McMurray FZ to the north, and forms the eastern boundary of the Everett Basin. These faults cut through the ''Western Mélange Belt'' (WMB; blue area in map), exposed from North Bend, Washington, North Bend (on Interstate 90) to Mount Vernon.. The WMB is an assemblage of Late Jurassic and Cretaceous rock (some of it as much as 166 million years old) collected in the accretionary wedge (or prism) of a subduction zone. The presence of detritus from the Idaho Batholith indicates a former location closer to southern Idaho. Some of these faults possibly developed in the Mesozoic, when these deposits were in the accretionary wedge; the cross-cutting NE and NNE-striking faults that form the various basins resulted from a subsequent change to transtension. Early Eocene igneous units in the area appear to be part of a 49- to 44- Ma magmatic belt that appeared just after the arrival of

, and possibly associated with that event. The strongly expressed topographical lineaments at the north end of the Rogers Belt pose a perplexing problem, as they show no definite offset where they are bisected by the left-lateral oblique-slip Devils Mountain Fault. The alternative, that younger faulting in the Rogers Belt has offset the DMF — Cheney argued that the MVF had offset the DMF 47 km. to the north, past Lummi Island — is contrary to the prevailing consensus that the DMF is ''not'' offset.

Cherry Creek Fault Zone

The Cherry Creek fault zone (CCFZ) was discovered in 2010 while mapping the area at the north end of the Rattlesnake Mountain fault zone (RMFZ). From a point just north of Carnation the eastern edge of the CCFZ (here it is about three-quarters of a mile wide) can be traced up Harris Creek, crossing the upper reach of Cherry Creek, eventually reaching the town of Sultan, Washington, Sultan. Here the main strand on the western edge merges with the ''Sultan River Fault'' under the Sultan River. It is projected to extend past Lake Chaplain, and perhaps to the east end of Mount Pilchuck. It is deemed a "major active or potentially active" structure. Snoqualmie Valley faults

In the crowded field of active or potentially active fault zones that have been discovered in the lower Snoqualmie Valley, the Cherry Creek fault zone is particularly notable because east of Duvall, Washington, Duvall it passes through a hotspot of active seismicity, including the 1996 5.3 Duvall earthquake. Offsets in the east–west oriented Monroe Fault (south side of the Skykomish River), earthquake focal mechanisms, and kinematic indications show that the CCFZ is a left-lateral

fault, possibly with some oblique motion (up on the eastern side). The CCFZ appears to be related to the parallel Tokul Creek fault zone to the south; both appear to be conjugate faults to the northwest-trending SWIF. The Tokul Creek Fault (TCF) strikes NNE from Snoqualmie, aligned with a possible offset of the Western Melange Belt and with a valley that cuts through to the Skykomish River; it is now believed to be of regional significance.

Rattlesnake Mountain Fault Zone

Rattlesnake Mountain is a prominent NNW trending ridge just west of North Bend, Washington, North Bend (about 25 miles east of Seattle). It is coincident with, and possibly a result of uplift on, the ''Rattlesnake Mountain Fault Zone'' (RMFZ), a band of at least eleven faults that show both dip-slip (vertical) and right-lateral strike-slip motion. (See the adjacent map. In the map above these are represented by the pair of dotted lines at the lower right. A different mountain and fault zone of the same name are located near Pasco, Washington, Pasco; se
QFFDB Fault #565
The southern end of Rattlesnake Mountain is truncated at the Olympic–Wallowa Lineament (OWL), and the faults turn easterly to merge with the OWL. The northern end of the mountain falls off where it crosses the eastern end of the

, which in turn terminates at the RMFZ; Rattlesnake Mountain forms the eastern edge of the #pattern, Seattle Uplift. The RMFZ continues NNW past Fall City and Carnation, where strands of the RMFZ have been mapped making a gentle turn of 15 to 20° west to meet the Southern Whidbey Island Fault zone (SWIF, discussed above); the RMFZ is therefore considered to be an extension of the SWIF. The relationship between these two fault zones is not entirely clear. Slippage along the SWIF would be expected to continue east-southeast until it merged with the OWL, but instead appears to be taking a shortcut ("right step") along the RMFZ. This is where the SWIF encounters the edge of the Western and Eastern Melange Belts (remnants of a mid-Cretaceous subduction zone); the RMFZ is where the Seattle Uplift is being forced against the Western Melange belt To the north the Melange Belt is manifested as the #RB, Rogers Belt, a zone of low-amplitude folding stretching from Monroe, Washington, Monroe to Mount Vernon, Washington, Mount Vernon; the apparent western edge of this zone is on-strike with the RMFZ. South of Monroe the folds of the Rogers Belt are obscured by subsequent volcanic formations, but other faults parallel to the RMFZ (e.g., the Snoqualmie Valley and Johnson's Swamp fault zones) extend the general trend of NNW faulting as far as Monroe. (Rattlesnake Mountain Fault Zone not included in QFFDB.)

Coast Range Boundary Fault

The ''Coast Range Boundary Fault'' (CRBF) is hypothesized, expected on the basis of tectonic considerations, which may correlate in part with one or more currently known faults, or may involve as yet undiscovered faulting. Simply put, the basement rock on the west side of Puget Sound does not match the basement rock on the east side. West of Puget Sound the tectonic basement of the Coast Range geologic province is the approximately 50 million year (Ma) old marine basalts of the Crescent Formation, part of the

terrane that underlies western Washington and Oregon. East of Puget Sound the basement of the Cascades province is various pre-

(older than 65 Ma) metamorphic rock. Somewhere between Puget Sound and Cascades foothills these two geological provinces come into contact. As the juxtaposition of various disparate tectonic structures in northwest Washington requires significant strike-slip movement, it is further expected that this contact will be a major fault. The northern end of the Crescent Formation (aka Metchosin Formation) has been identified as the east–west trending Leech River Fault on the southern tip of Vancouver Island. This turns and runs just south of Victoria, nearly in-line with the SWIF. Seismic tomography studies show a change in seismic velocities across the northern end of the SWIF, suggesting that this is also part of the Coast Range—Cascade contact. It therefore seems reasonable that the rest of the SWIF (and its apparent extension, the RMFZ) follows the Coast Range—Cascade contact, and (these faults being active) constitutes the CRBF. One problem with this is that the parts of the SWIF east of Puget Sound do not show the velocity contrasts that would indicate contrasting rock types. Another problem with the SWIF/RMFZ as CRBF is that a large westward step is required to connect from the RMFZ to the Saint Helens Zone (SHZ; see #SWCC, map), whereas the RMFZ turns easterly to align with the OWL. This last problem is partly solved because there is a locus of seismicity, and presumably faulting, extending from the northern end of the SHZ to the northern end of the Western Rainier Zone (see #fig48, Fig. 48), along the edge of a formation known as the ''#SWCC, Southern Washington Cascades Conductor''. However, gravity and other data suggest that near the southern tip of Whidbey Island the Crescent Formation contact may turn away from the SWIF, and may even be reentrant under north Seattle, forming the northwestern side of the Seattle Basin, and possibly connecting with the recently reported "Bremerton trend" of faulting running from the southern end of Hood Canal, through Sinclair Inlet (Bremerton), and across Puget Sound. Or the Crescent margin may simply (and quietly) just run south-southeast under Seattle to the WRZ. Other seismic tomography has tantalizingly suggested three north-striking strands under Seattle, and a fourth just east of Lake Washington. Although there is no direct evidence for any major north-striking faults under Seattle, this prospect appears to be endorsed by the geological community. How the CRBF might run north of Seattle (specifically, north of the OWL, which Seattle straddles) is unknown, and even questioned, as there is no direct evidence of such a fault. There is an intriguing view from (see Fig. 64
on-line
that the edge of the Crescent Formation offsets west along the Seattle Fault, with the Seattle Basin resulting from a gap between the main part of Siletiza and a northern block that has broken away.

Seattle Fault

The Seattle Fault is a zone of complex

and reverse faults – between lines E and F on the #pattern, map – up to 7 km wide and over 70 km long that delineates the north edge of the Seattle Uplift. It stands out in regard of its east–west orientation, depth to bedrock, and hazard to an urban population center. Seattle Fault location

The Seattle Fault was first identified in 1965 but not documented as an active fault until 1992 with a set of five articles establishing that about 1100 years ago (AD 900–930) an earthquake of magnitude 7+ uplifted Restoration Point and Alki Point, dropped West Point (the three white triangles in the Seattle Basin on the #pattern, map), caused rockslides in the Olympics, landslides into Lake Washington, and a tsunami on Puget Sound. It extends as far east as (and probably terminates at) the Rattlesnake Mountain Fault Zone (RMFZ; the southern extension of the SWIF) near Fall City, Washington, Fall City. This seems geologically reasonable, as both the SWIF and RMFZ appear to be the contact between

Crescent Formation basement of Puget Sound on the west and the older Mesozoic (pre-Tertiary) mélange belt basement rocks under the Cascades on the east.

Structure

The Seattle Fault is the most studied of the regional faults, which has led to several models of its structure, which may also be relevant to other faults. In the ''wedge'' model of a slab of rock – mainly basalts of the Crescent Formation – about 20 km thick is being pushed up a "master ramp" of deeper material; this forms the Seattle Uplift. The Seattle fault zone is where the forward edge of the slab, coming to the top of the ramp, breaks and slips into the Seattle Basin. In this model the Tacoma fault zone is primarily the result of local adjustments as the slab bends upward at the bottom of the ramp. The ''passive roof duplex'' model of , relying on seismic tomography data from the "Seismic Hazards Investigation in Puget Sound" (SHIPS) experiment, retains the thrusting slab and master ramp concepts, but interprets the Tacoma fault as a reverse fault (or back thrust) that dips north towards the south dipping Seattle fault (see diagram); as a result the Seattle Uplift is being popped up like a horst (geology), horst. While these models vary in some details, both indicate that the Seattle Fault itself is capable of a magnitude 7.5 earthquake. But if the Seattle Fault should break in conjunction with other faults (discussed #Seismotectonic modeling, above), considerably more energy would be released, on the order of ~M 8.

Question of western termination

Determination of the western terminus of the Seattle Fault has been problematic, and has implications for the entire west side of the Puget Lowland. Initially it was not specified, and rather vaguely indicated to be west of Restoration Point (i.e., west of Puget Sound). An early view was that "the Seattle Fault appears to be truncated by the Hood Canal fault ... and does not extend into the Olympic Mountains". This seems reasonable enough, as Hood Canal is a prominent physiographic boundary between the Olympic Mountains and Puget Lowlands, and believed to be the location of a major fault. Subsequent authors were confident enough to trace the fault west of Bremerton to just north of Green Mountain (the northwestern corner of the Blue Hills uplift – see "E" on the #pattern, map – a topographically prominent exposure of uplifted basalt) and just short of Hood Canal; but reluctant to map the fault further west as the distinctive aeromagnetic lineament used to locate the Seattle Fault dies out just west of Bremerton. Studies of the Seattle Fault west of Bremerton have revealed a complexity of geological structure and faulting. Several studies show that the southernmost strand of the SF, once past Green Mountain, turns southwest, towards the Saddle Mountain and Frigid Creek faults. However, the #SM, Saddle Mountain fault zone is not quite reciprocally aligned, trending more northerly to where it encounters west–east trending faults (including the ''Hamma Hamma fault zone'') that appear to be a westward extension of the Seattle Fault zone. This trend extends further north where the Pleasant Harbor lineament appears to terminate other westward extensions of the SFZ. Other studies have faults extending NW or WNW from the SF towards Dabob Bay; these are now recognized as part of the Dabob Bay fault zone. While some coherency is developing, the story is not complete: identified faults do not yet account for much of the region's seismicity. An emerging view is that the #DL, Dewatto fault marks the western edge of the relatively rigid Seattle Uplift (see #TFBmap, map). Accommodation of strain (displacement) between the Seattle Fault and the Saddle Mountain deformation zone is likely distributed across the more pliable sediments of the Dewatto Basin; this, and the greater depth to the Crescent Formation, may account for the subdued expression of the Seattle Fault west of Green Mountain..
USGS QFFDB Fault #570, Seattle Fault

Tacoma Fault Zone

The Tacoma Fault (at right, and also between lines C and D on the #pattern, Uplift and basin map, above) just north of the city of Tacoma, Washington has been described as "one of the most striking geophysical anomalies in the Puget Lowland". The western part is an active east–west striking north dipping reverse fault that separates the Seattle Uplift and the Tacoma Basin, with approximately 30 miles (50 km) of identified surface rupture. It is believed capable of generating earthquakes of at least magnitude 7, and there is evidence of such a quake approximately 1,000 years ago, possibly the same earthquake documented on the

24 miles (38 km) to the north. This is likely not coincidental, as it appears that the Tacoma and Seattle faults converge at depth (see #Structure, diagram above) in a way that north–south compression tends to force the Seattle Uplift up, resulting in dip-slip fault, dip-slip movement on both fault zones. The Tacoma Fault was first identified by as a gravitational anomaly ("structure K") running east across the northern tip of Case and Carr Inlets, then southeast under Commencement Bay and towards the town of Puyallup, Washington, Puyallup. Not until 2001 was it identified as a fault zone, and only in 2004 did trenching reveal

activity.

Scarps associated with Holocene uplift of the Tacoma fault have been traced westward to Prickett Lake (southwest of Belfair, Washington, Belfair, see map). The Tacoma fault was initially suspected of following a weak magnetic anomaly west to the Frigid Creek fault, but is now believed to connect with a steep gravitational, aeromagnetic, and seismic velocity gradient that strikes north towards Green Mountain (Blue Hills uplift). This is the Dewatto lineament, believed to result from an east-dipping low-angle thrust fault where the western flank of the Seattle Uplift has been pushed into the northwestern corner of the Tacoma Basin. It appears that the Seattle Uplift is acting as a rigid block, with the Tacoma, Dewatto, and Seattle faults being the southern, western, and northern faces. This may explain why the Seattle and Tacoma faults seem to have ruptured at nearly the same time. Interpretation of the eastern part of the Tacoma Fault is not entirely settled. Most authors align it with the strong gravitational anomaly (which typically reflects where faulting has juxtaposed rock of different density) and topographical lineament down Commencement Bay. This follows the front of the Rosedale monocline, a gently southwest-tilting formation that forms the bluffs on which Tacoma is built. On the other hand, the contrasting character of the east-striking and southeast-striking segments is unsettling, and the change of direction somewhat difficult to reconcile with the observed fault traces. Especially as seismic reflection data shows some faulting continuing east across Vashon Island and the East Passage of Puget Sound (the ''East Passage Zone'', EPZ) towards Federal Way, Washington, Federal Way and an east-striking anticline. Whether the faulting continues eastward is not yet determined. The EPZ is active, being the locale of the 1995 M 5 Point Robinson earthquake. There is evidence that the Tacoma Fault connects with the ''White River River Fault'' (WRF) via the EPZ and Federal Way, Washington, Federal Way, under the Muckleshoot Basin (see #PLmap, map), and thence to the ''Naches River Fault''. If so, this would be a major fault system (over 185 km long), connecting the Puget Lowland with the Yakima Fold Belt on the other side of the Cascades, with possible implications for both the Olympic—Wallowa Lineament (which it parallels) and geological structure south of the OWL.
USGS QFFDB Fault #581, Tacoma Fault

Dewatto Lineament

The western flank of the Seattle Uplift forms a strong gravitational, aeromagnetic, and seismic velocity gradient known as the ''Dewatto lineament''. It arises from the contrast between the denser and more magnetic basalt of the Crescent Formation that has been uplifted to the east, and the glacial sediments that have filled the Dewatto basin to the west. The Dewatto linement extends from the western end of the Tacoma fault (see map immediately above) northward towards Green Mountain at the western end of the Seattle fault. Kinematic analysis suggests that if shortening (compression) in the Puget Lowland is directed to the northeast (i.e., parallel to Hood Canal and the Saddle Mountain deformation zone) and thus oblique to the Dewatto lineament, it should be subject to both strike-slip and dip-slip forces, implying a fault. Recent geophysical modeling suggests that the Dewatto lineament is the expression of a blind (concealed), low-angle, east-dipping thrust fault, named the ''Dewatto fault''. (Originally named the Tahuya Fault.) This reflects westward thrusting of the Seattle Uplift into the Dewatto basin, a northwestern extension of the Tacoma basin. This interpretation suggests that the Seattle Uplift acts as a rigid block, and possibly explains the kinematic linkage by which large earthquakes may involve ruptures on multiple faults: the Seattle, Dewatto, and Tacoma faults represent the northern, western, and southern faces of a single block. Such interconnection also suggests a capability for larger earthquakes (> M7 for the Seattle Fault); the amount of increased risk is unknown.

Hood Canal Fault

Hood Canal marks an abrupt change of physiography between the Puget Lowland and the

to the west. Based on this and geophysical anomalies it was inferred that there is a major, active strike-slip fault zone running from the south end of Hood Canal, up Dabob Bay, and continuing north on land. This is conformable with some regional tectonic interpretations that put a major terrane boundary between the Olympics and the Puget Lowland, and imply a connection (either via the Discovery Bay Fault, or closer to Port Townsend) to the various faults in the Strait of Juan de Fuca. This boundary would be the contact where northward movement of the basement rock of the Puget Lowland against the Olympic Peninsula is accommodated; it would be expected to be a significant seismological zone. However, the Hood Canal fault has been "largely inferred" due to a paucity of evidence, including lack of definite scarps and any other signs of active seismicity. A 2001 study using high-resolution seismic tomography questioned its existence. Though a 2012 study interpreted a different variety of tomographic data as showing the Hood Canal fault, other mapping has "found no convincing evidence for the existence of this fault", considers it doubtful, depicted it "with low level of confidence", or omits it entirely. For these reasons this is now a questioned fault, and is indicated on the #PSF map, map as a dashed line. A new view is developing that the regional tectonic boundary is not under Hood Canal, but just to the west, involving the Saddle Mountain fault zone (discussed below) and associated faults. This is supported by geologically recent scarps and other signs of active faulting on the Saddle Mountain faults, and also discovery of a geophysical lineament running through Pleasant Harbor (south of Brinnon) that appears to truncate strands of the Seattle Fault. In this view Hood Canal is only a syncline (dip) between the Olympic Mountains and the Puget Lowland, and such faults as have been found there are local and discontinuous, ancillary to the main zone of faulting to the west. North of the Seattle Fault accommodation of regional movement may be along the northwest-striking ''Dabob Bay Fault Zone''.
USGS QFFDB Fault #552, Hood Canal Fault

Saddle Mountain Faults

The Saddle Mountain Faults ("East" and "West", and not to be confused with a different Saddle Mountains Fault in Adams county, eastern Washington), are a set of northeast trending reverse faults on the south-east flank of the Olympic Mountains near Lake Cushman first described in 1973 and 1975. Vertical movement on these faults has created prominent scarps that have dammed Price Lake and (just north of Saddle Mountain) Lilliwaup Swamp. The mapped surface traces are only 5 km long, but LIDAR-derived imagery shows longer lineaments, with the traces cutting Holocene alluvial traces. A recent (2009) analysis of aeromagnetic data suggests that it extends at least 35 km, from the latitude of the Seattle Fault (the Hamma Hamma River) to about 6 km south of Lake Cushman. Other faults to the south and southeast – the ''Frigid Creek Fault'' and (to the west) ''Canyon River Fault'' – suggest an extended zone of faulting at least 45 km long. Although the southwest striking Canyon River Fault is not seen to directly connect with the Saddle Mountain faults, they are in general alignment, and both occur in a similar context of Miocene faulting (where Crescent Formation strata has been uplifted by the Olympics) and a linear aeromagnetic anomaly. The Canyon River Fault is a major fault in itself, associated with a 40 km long lineament and distinct late Holocene scarps of up to 3 meters. Although these faults are west of the Hood Canal Fault (previously presumed to be the western boundary of the Puget Lowland), new studies are revealing that the Saddle Mountain and related faults connect with the Seattle fault zone. Trench studies indicate major earthquakes (in the range of M 6. to 7.8) on the Saddle Mountain faults at nearly the same time (give or take a century) as the great quake on the

about 1100 years ago (900–930 AD). Such quakes pose a serious threat to the City of Tacoma's dams at Lake Cushman, located in the fault zone, and to everyone downstream on the Skokomish River. The Canyon River Fault is believed to have caused a similar-sized earthquake less than 2,000 years ago; this is a particular hazard to the Wynoochee Dam (to the west). The history and capabilities of the Frigid Creek Fault are not known.
USGS QFFDB Fault #575, Saddle Mountain Faults

Olympia Structure

The Olympia structure – also known as the ''Legislature fault'' – is an 80 km long gravitational and aeromagnetic anomaly that separates the sedimentary deposits of the Tacoma Basin from the basalt of the Black Hills Uplift (between lines A and B on the #pattern, map). It is not known to be seismic – indeed, there is very little seismicity south of the Tacoma Basin as far as Chehalis – and not even conclusively established to be a fault. This structure is shown in the gravitational mapping of 1965, but without comment. , labelling it "structure L", mapped it from Shelton, Washington, Shelton (near the Olympic foothills) southeast to Olympia, Washington, Olympia (pretty nearly right under the state Legislature), directly under the town of Rainier, Washington, Rainier, to a point due east of the Doty Fault, and apparently marking the northeastern limit of a band of southeast striking faults in the Centralia-Chehalis area. They interpreted it as "simple folds in Eocene bedrock", though saw sufficient similarity with the Seattle Fault to speculate that this is a thrust fault. , while observing the "remarkable straight boundaries that we interpret as evidence of structural control", refrained from calling this structure a fault. (Their model of the Black Hills Uplift is analogous with their "wedge" model of the Seattle Uplift, discussed #Structure, above, but in the opposite direction. If entirely analogous, then "roof duplex" might also apply, and the Olympia Fault would be a reverse fault similar to the Tacoma Fault.) Aeromagnetic mapping in 1999 showed a very prominent anomaly (such as typically indicates a contrast of rock type); that, along with paleoseismological evidence of a major Holocene earthquake, has led to a suggestion that this structure "may be associated with faulting". One reason for caution is that a detailed gravity survey was unable to resolve whether the Olympia structure is, or is not, a fault. Although no surface traces of faulting have been found in either the Holocene glacial sediments or the basalts of the Black Hills, on the basis of well-drilling logs a fault has been mapped striking southeast from Offut Lake (just west of Rainier); it appears to be in line with the easternmost fault mapped in the Centralia—Chehalis area. A marine seismic reflection study found evidence of faulting at the mouth of Budd Inlet, just north of the Olympia structure, and aligning with faint lineaments seen in the lidar imagery. These faults are not quite aligned with the Olympia structure, striking N75W (285°) rather than N45W (315°). It is uncertain how these faults relate to the structure, and whether they are deep-seated faults, or fractures due to bending of the shallow crust. It has been speculated that the OS might connect with the seismically active Saint Helens Zone (discussed #SHZ, below), which would imply that the OS is both locked and being stressed, raising the possibility of a major earthquake. Alternately, the OS appears to coincide with a gravitational boundary in the upper crust that has been mapped striking southeast to The Dalles on the Columbia River, where there is a swarm of similarly striking faults. That Olympia and the south Sound are at risk of major earthquakes is shown by evidence of subsidence at several locations in southern Puget Sound some 1100 years ago. What is unknown is whether this was due to a great subduction earthquake, to the noted earthquake on the

about that time, or to an earthquake on a local fault (e.g., the Olympia structure); there is some evidence that there were two earthquakes over a short time period. Subsidence dated to between AD 1445 and 1655 has been reported in Mud Bay (just west of Olympia). (Not included in QFFDB.)

Doty Fault

Centralia-Chehalis District faults GM-34

The Doty Fault – the southernmost of the uplift-and-basin dividing faults reviewed here, and located just north of the Chehalis Basin – is one of nearly a dozen faults mapped in the Centralia—Chehalis coal district in 1958. While the towns of Centralia, Washington, Centralia and Chehalis, Washington, Chehalis in rural Lewis County may seem distant (about 25 miles) from Puget Sound, this is still part of the Puget Lowland, and these faults, the local geology, and the underlying tectonic basement seem to be connected with that immediately adjacent to Puget Sound. And though the faults in this area are not notably seismogenic, the southeast striking faults seem to be ''en echelon'' with the Olympia structure (fault?), and headed for the definitely active Saint Helens Zone; this appears to be a large-scale structure. The Doty fault particularly seems to have gained prominence with geologists since it was associated with an aeromagnetic anomaly, and a report in 2000 credited it capable of a magnitude 6.7 to 7.2 earthquake. The prospect of a major earthquake on the Doty Fault poses a serious hazard to the entire Puget Sound region as it threatens vital economic lifelines: At Chehalis there is but a single freeway (Interstate 5) and a single rail line connecting the Puget Sound region with the rest of the west coast; the only alternate routes are very lengthy. The Doty fault has been mapped from the north side of the Chehalis airport due west to the old logging town of Doty, Washington, Doty (due north of Pe Ell), paralleled most of that distance by its twin, the ''Salzer Creek Fault'', about half a mile to the north. Both of these are dip-slip fault, dip-slip (vertical) faults; the block between them has been popped up by compressive forces. The Doty Fault appears to terminate against, or possibly merge with, the Salzer Creek Fault at Chehalis; the Salzer Creek Fault is traced another seven miles east of Chehalis. The length of the Doty Fault is problematical: the report in 2000 gave it as 65 km (40 miles), but without comment or citation. Such a length would be comparable to the length of the Seattle or Tacoma faults, and capable of an earthquake of M 6.7. But it does not appear that there have been studies of the deeper structure of these faults, or whether there has been any recent activity. The Doty—Salzer Creek Fault does not fully fit the regional pattern of basins and uplifts bounded by faults described #pattern, above. It does bound the north side of the Chehalis basin, but the south boundary of the Black Hills Uplift is more properly the southeast striking ''Scammon Creek Fault'' that converges with the Doty—Salzer Creek Fault just north of Chehalis. In the acute angle between these is located the minor Lincoln Creek uplift, the Doty Hills, and an impressive chunk of uplifted Crescent basalt (reddish area at west edge of the map). The SE striking Scammon Creek Fault seems to be terminated by the Salzer Creek Fault (the exact relationship is not clear), with the latter continuing east for another seven miles. Yet the former is only the first of at least six more parallel southeast striking faults, which do cross the Salzer Creek Fault. These faults are: the ''Kopiah Fault'' (note the curious curve), ''Newaukum Fault'', ''Coal Creek Fault'', and three other unnamed faults. Just past them is the parallel Olympia Structure, which as a geophysical lineament has been traced to a point due east of Chehalis; these would seem to be related somehow, but the nature of that relationship is not yet known. Though these faults have been traced for only a little ways, the southeast striking anticlines they are associated with continue as far as Riffe Lake, near Mossyrock, Washington, Mossyrock. They are also on-strike with a swarm of faults on the Columbia River, bracketing The Dalles. As all of these are

and reverse faults, they probably result from northeast directed regional compression. These faults also cross the Saint Helens Zone (SHZ), a deep, north-northwest trending zone of seismicity that appears to be the contact between different crustal blocks. How they might be connected is unknown. What makes the Doty—Salzer Fault (and the short ''Chehalis Fault'' striking due east from Chehalis) stand out from the many other faults south of Tacoma is its east–west strike; the significance of this is not known. (Not included in QFFDB. See and Geologic Map for details.)

Saint Helens Zone, Western Rainier Zone

The most striking concentrations of mid-crustal seismicity in western Washington outside of Puget Sound are the ''Saint Helens Zone'' (SHZ) and ''Western Rainier Zone'' (WRZ) at the southern edge of the Puget Lowland (see seismicity map, right). Indeed, it is mainly by their seismicity that these faults are known and have been located, neither showing any surface faulting. The SHZ and WRZ lie just outside the topographical basin that constitutes the Puget Lowland (see #PLmap, image), do not participate in the #pattern, uplift and basin pattern, and unlike the rest of the faults in the Puget Lowland (which are reverse or thrust faults reflecting mostly compressive forces) they appear to be

faults; they reflect a geological context distinctly different from the rest of the Puget Lowland. In particular, to the southeast of Mount St. Helens and Mount Rainier they reflect a regional pattern of NNW oriented faulting, including the Entiat Fault in the North Cascades and the Portland Hills and related faults around Portland, Oregon, Portland (see QFFD
fault map
. Yet the SHZ and WRZ may be integral to the regional geology of Puget Sound, possibly revealing some deep and significant facets, and may also present significant seismic hazard.

The WRZ and SHZ are associated with the ''southern Washington Cascades conductor'' (SWCC), a formation of enhanced electrical conductivity lying roughly between Riffe Lake and Mounts St. Helens, Adams, and Rainier, with a lobe extending north (outlined in yellow, right). This formation, up to 15 km thick, is largely buried (from one to ten kilometers deep), and known mainly by magnetotellurics and other geophysical methods. The southwestern boundary of the SWCC, where it is believed to be in near vertical contact with the Eocene basalts of the Crescent Formation, forms a good part of the 90 km (56 mile) long SHZ. On the eastern side, where the SWCC is believed to be in contact with pre-Tertiary terranes accreted to the North American craton, matters are different. While there is a short zone (not shown) of fainter seismicity near Goat Rocks (an old Pliocene volcano) that may be associated with the contact, the substantially stronger seismicity of the WRZ is associated with the major Carbon River—Skate Mountain anticline. This anticline, or uplifted fold, and the narrower width of the northern part of the SWCC, reflects an episode of compression of this formation. Of great interest here is that both the northern lobe of the SWCC and the Carbon River anticline are aligned towards Tiger Mountain (Washington), Tiger Mountain (an uplifted block of the Puget Group of sedimentary and volcanic deposits typical of the Puget Lowland) and the adjacent Raging River anticline (see #RMFZ, map). The lowest exposed strata of Tiger Mountain, the mid-Eocene marine sediments of the Raging River formation, may be correlative with the SWCC. Does the SHZ extend north? Though the Olympia Structure (a suspected fault) runs towards the SHZ, and delineates the northern edge of an exposed section of the Crescent Formation, it appears to be an ''upper'' crustal fold, part of a pattern of folding that extends southeast to cross the Columbia River near The Dalles, and unrelated to the mid and lower crustal SHZ. It has been speculated that the SHZ might extend under the Kitsap Peninsula (central Puget Sound), possibly involved with a section of the subducting Juan de Fuca plate that is suspected of being stuck. The implications of this are not only "the possibility of a moderate to large crustal earthquake along the SHZ", but that the tectonics under Puget Sound are more complicated than yet understood, and may involve differences in the regional stress patterns not reflected in current earthquake hazard assessments.

Deeper structure

Mount St. Helens and Mount Rainier are located where their associated fault zones make a bend (see map, above).(Mt. Rainier is offset because the faults are deep and the conduits do not rise quite vertically.) These bends are located where they intercept a "subtle geological structure" of "possible fundamental importance",. a NNE striking zone (line "A" on the map) of various faults (including the Tokul Creek Fault NNE of Snoqualmie) and early-Miocene (about 24 Ma) volcanic vents and intrusive bodies (plutons and batholiths) extending from Portland, Oregon, Portland to Glacier Peak; it also marks the change in regional fault orientation noted above. This MSH-MR-GP lineament is believed to reflect a "long-lived deep-seated lithospheric flaw that has exerted major control on transfer of magma to the upper crust of southern Washington for approximately the last 25 [million years]"; it has been attributed to the geometry of the subducting Juan de Fuca plate. A parallel line ("B") about 15 miles (25 kilometers) to the west corresponds to the ''western'' limit of a zone of seismicity stretching from the WRZ to southwest of Portland. Curiously, the extension of line "B" north of the OWL is approximately the ''eastern'' limit of Puget Sound seismicity, the rest of southwestern Washington and the North Cascades being relatively aseismic (see the seismicity map, above). This line may also mark the northwestern boundary of the SWCC. North of the RMFZ it follows a topographical lineament that can be traced to Rockport (on Hwy. 20); it includes the Cherry Creek Fault Zone NNE of Carnation, location of the 1965 Duvall earthquake. Between the Cherry Creek and parallel Tokul Creek faults is a contact between formations of the Western Melange Belt. The zone between these two lines, reflecting changes in regional structure, seismicity, fault orientation, and possibly the underlying lithospheric structure, appears to be a major structural boundary in the Puget Lowland. Also intersecting at Mount St. Helens is a NE (045°) trending line (red) of Pleistocene (about 4 Ma) plug domes and a topographic lineament (followed in part by Highway 12). This line is the southernmost of a band of NE trending faults and topographical lineaments that extend from the Oregon coast into the North Cascades. A similar line aligns with the termination of the WRZ, SHZ, an
Gales Creek Fault Zone
(northwest of Portland), with faulting along the upper Nehalem River on the Oregon coast, and a topographical contrast at the coast (between Neahkahnie Mountain and the lower Nehalem River valley) distinct enough to be seen on the seismicity map above (due west of Portland). Other similar lineaments (such as from Astoria, Oregon, Astoria to Glacier Peak) align with various topographical features and changes in fault orientation. These lineaments have been associated with possible zones of faulting in the crust and subducting plate. These features suggest that the southern Puget Lowland is influenced by the deep crust and even the subducting Juan de Fuca plate, but the details and implications are not yet known.

Other faults

Actual

There are numerous other faults (or fault zones) in the Puget Lowland, and around its edges, sketchily studied and largely unnamed. These are usually fairly short, and not believed to be significantly seismogenic. However, most seismic activity is not associated with any known fault. Seismicity sometimes occurs in zones, such as has been observed under Mercer Island, or from downtown Seattle towards Kirkland but whether particular zones reflect undiscovered faults, or might be the source of damaging earthquakes, is generally unknown. Ongoing mapping is revealing more faults. E.g., mapping along the Rattlesnake Mountain Fault Zone has revealed a complex network of active or potentially active faults across (and likely beyond) the lower Snoqualmie Valley, including the Cherry Creek Fault Zone, scene of the 1996 M 5.3 Duvall earthquake. The ''San Juan Island'' and ''Leach River'' faults crossing the southern end of

are significant and undoubtedly connected with the Darrington—Devils Mountain and Southern Whidbey Island faults, and certainly of particular interest to the residents of Victoria, B.C. But their significance to the Puget Sound area is unknown. The ''Little River Fault'' (see th
QFFDB, Fault 556
is representative of an extensive zone of faults along the north side of the Olympic Peninsula and in the Strait of Juan de Fuca (likely connected with the fault systems at the south end of Vancouver Island, se

, but these lie west of the crustal blocks that underlie the Puget Lowland, and again their possible impact on the Puget Sound region is unknown. One of these faults, the ''Sequim Fault Zone'' (striking east from the town of Sequim, Washington, Sequim), crosses Discovery Bay (and various possible extensions of the #HCF, Hood Canal Fault) and bounds the Port Ludlow Uplift ("uplift of unknown origin" on the #pattern, map); it appears to extend to the Southern Whidbey Island Fault.. An ''Everett Fault'', running east-northeast along the bluffs between Mukilteo, Washington, Mukilteo and Everett, Washington, Everett – that is, east of the SWIF and at the southern edge of the Everett Basin – has been claimed, but this does not appear to have been corroborated. A ''Lofall Fault'' has been reported on the basis of marine seismic reflection surveying, but has not been confirmed by trenching. This fault seems to be associated with the Kingston arch anticline, and part of the #pattern, uplift and basin pattern, but shortened because of the geometry of the SWIF. It is not notably seismogenic. Although the largely unstudied ''White River Fault'' (WRF) appears to lie just outside the Puget Lowland, it may actually connect under the Muckleshoot Basin to the East Passage Zone and the

(#PLmap, map). This would pose significantly greater seismic hazard than currently recognized, especially as the White River Fault is believed to connect with the ''Naches River Fault'' that extends along Highway 410 on the east side of the Cascades towards Yakima. The Straight Creek Fault is a major structure in the

, but has not been active for over 30 million years. Various other faults in the North Cascades are older (being offset by the Straight Creek Fault) and are unrelated to the faults in Puget Sound.

Conjectured

A ''Puget Sound Fault'' running down the center of Puget Sound (and Vashon Island) was once proposed, but seems to have not been accepted by the geological community. A ''Coast Range Boundary Fault'' (CRBF, discussed #CRBF, above) was inferred on the basis of differences in the basement rock to the west and east of Puget Sound (the Crescent Formation—Cascadia core contact), and arbitrarily mapped at various locations including Lake Washington; north of the OWL this is now generally identified, with the Southern Whidbey Island Fault. Where it might run south of Seattle is not known; an argument has been made that it runs beneath Seattle but this is still conjectural. Study of surface deformation suggests possible unmapped faults near Federal Way, running between Sumner and Steilacoom, and south of Renton..

Notes

Sources

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External links

Preliminary Atlas of Active Shallow Tectonic Deformation in the Puget Lowland, Washington (USGS Open-File Report 2010-1149)
Maps of the region's faults, with an overview.
USGS Quaternary fault and fold database
Technical descriptions and bibliographies.
The Pacific Northwest Seismic Network
All about earthquakes and geologic hazards of the Pacific Northwest.
The Origin of Puget Sound
Short, but good.
Geologic map of northwestern Washington (GM-50).Geologic map of southwestern Washington (GM-34).Various maps from Washington DNR.Aeromagnetic anomaly maps (USGS OFR 99-514).
{{Faults Seismic faults of Washington (state)