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97-415: The Puget Sound faults under the heavily populated Puget Sound region (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: The Puget Sound region (Puget Lowland) of western Washington contains the bulk of the population and economic assets of the state, and carries seven percent of

194-470: A 3-D tectonic model of the whole crust; this was then analyzed using finite element methods to determine regional geodynamic characteristics. A principal finding is that "[c]rustal 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

291-639: 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

388-619: A high-rise central business district . The satellite cities are primarily suburban , featuring a small downtown core and a small industrial area or port . The suburbs consist mostly of residences, strip malls , and shopping centers. The region is also home to numerous ports. The two largest and busiest are the Port of Seattle and Port of Tacoma , which, if combined, comprise the third largest container port in North America after Los Angeles/Long Beach and New York/New Jersey. As defined by

485-466: 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 30 km or 19 miles), though this and other seismic tomography studies (such as Ramachandran 2001 ) provide tantalizing glimpses. For the following reviews the primary source of information is the U.S. Geological Survey's Quaternary fault and fold database (QFFDB) , which includes details of discovery,

582-533: A seamount chain, or continental margin rifting (see Siletzia ). This " basement " rock is covered with sedimentary deposits similar to the Chuckanut Formation , and more recent (typically Miocene ) 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

679-702: 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 from the Eocene epoch (about 50 million years ago), with an origin variously attributed to

776-635: A technical description, and bibliography for each fault; a specific link is provided (where available) at the end of each section. 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 , and on towards Victoria, British Columbia , where the DMF is believed to join the Leech River fault system at

873-517: Is a bit of uplifted pre-Tertiary rock between the SPF and UPF, this does not truly fit the 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

970-739: 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 . It is characterized by a complex array of saltwater bays, islands, and peninsulas carved out by prehistoric glaciers. Poet Robert Sund called the Puget Sound region "Ish River country", owing to its numerous rivers with names ending in "ish", such as

1067-471: 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. 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

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1164-587: Is due to the Aleutian Low coming in from the northwest, and variation in winter rain from year to year is mostly due to variation in the strength of the Aleutian Low (strong = dry and weak = wet). The dry season (May-Sep) is caused by a subtropical high moving north from California. The driest part of the region is in the rain shadow of the Olympic Mountains around Sequim and Whidbey Island , receiving 40-75 cm of precipitation per year. The wettest part

1261-457: Is especially intriguing as a possible explanation of a cluster of seismic events around 1100 years ago. 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 Stanley, Villaseñor & Benz 1999 , USGS Open-File Report 99–0311) – not so much in competition with the first as complementing it – used seismic and other data to create

1358-709: Is in the foothills of both the Cascade and the Olympic Mts, and on the west side of the Sound, receiving 125-1000+ cm of precipitation per year. North Pacific Oak Woodland is one of the principal plant associations of the Puget Trough, where many of the soils are well drained mesic. Counties of the Puget Sound region: In addition, the San Juan Islands (all of San Juan County plus a few islands belonging to Whatcom County ) are often considered part of

1455-632: 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 Mount Saint Helens and Mount Rainier, along the DDMFZ, and under Puget Sound between Olympia and approximately the Southern Whidbey Island Fault. The southern limit nearly matches

1552-704: 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 , 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 which trenching has confirmed to be tectonically derived and geologically recent. Subsequent mapping shows

1649-413: Is various pre- Tertiary (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. Puget Sound region The Puget Sound region

1746-519: The Boeing Company became established in the area. During World War II the Puget Sound area became a focus for the war industry, with Boeing producing many of the nation's heavy bombers and the ports of Seattle, Bremerton and Tacoma available for shipbuilding. The most important yards in the Sound during World War II were Seattle-Tacoma Shipbuilding 's Seattle and Tacoma yards, also known as Todd Pacific , Todd Seattle and Todd Tacoma and

1843-466: The Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust . The interior of the main continental landmass includes an extensive granitic core called a craton . Along most of the edges of this craton are fragments of crustal material called terranes , which are accreted to the craton by tectonic actions over a long span of time. Much of North America west of

1940-663: The Duwamish , Samish , Sammamish , Skokomish , Skykomish , Snohomish , and the Stillaguamish . The ish ending is from Salishan languages and means "people of". The Puget Sound region was formed by the collision and attachment of many terranes ("microcontinents") to the North American Plate between about 50 to 10 million years ago. About 15,000 years ago during the Vashon Glaciation ,

2037-801: The Fifteen-Twenty fracture zone around 16°N. On the northerly boundary is a continuation of the Mid-Atlantic Ridge called the Gakkel Ridge . The rest of the boundary in the far northwestern part of the plate extends into Siberia . This boundary continues from the end of the Gakkel Ridge as the Laptev Sea Rift , on to a transitional deformation zone in the Chersky Range , then the Ulakhan Fault between it and

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2134-710: The Gonâve microplate , and the parallel Puerto Rico Trench running north of Puerto Rico and the Virgin Islands and bounding the Puerto Rico–Virgin Islands microplate , are also a part of the boundary. The rest of the southerly margin which extends east to the Mid-Atlantic Ridge and marks the boundary between the North American plate and the South American plate is vague but located near

2231-788: The Miocene period and are still geologically active, creating earthquakes and volcanoes. The Yellowstone hotspot is most notable for the Yellowstone Caldera and the many calderas that lie in the Snake River Plain , while the Anahim hotspot is most notable for the Anahim Volcanic Belt in the Nazko Cone area. For the most part, the North American plate moves in roughly a southwest direction away from

2328-555: The North American plate coming out of the northeast, which has formed a bend in the subducting plate and in the forearc basin above it. This bend has distorted the subducting slab into an arch that has lifted the Olympic Mountains and prevented them from subducting. For the past 50 million years or so (since the early Eocene epoch) these have been thrust by subduction up against the North Cascades ("fixed block" in

2425-991: The Okhotsk microplate , and finally the Aleutian Trench to the end of the Queen Charlotte Fault system (see also: Aleutian Arc ). The westerly boundary is the Queen Charlotte Fault running offshore along the coast of Alaska and the Cascadia subduction zone to the north, the San Andreas Fault through California , the East Pacific Rise in the Gulf of California , and the Middle America Trench to

2522-645: The Oregon Country , and became U.S. territory when the 1846 Oregon Treaty was signed. After arriving along the Oregon Trail , many settlers wandered north to what is now Washington and settled the Puget Sound area. The first non-indigenous settlement was New Market (now known as Tumwater ) in 1846. In 1853 Washington Territory was formed from part of Oregon Territory . In 1888 the Northern Pacific railroad line reached Puget Sound, linking

2619-887: The Puget Sound Navy Yard . They produced a significant portion of destroyers and escort carriers . Smaller operations included Winslow , Associated Shipbuilders and the Lake Washington Shipyard . Since 1995, Puget Sound has been recognized as an American Viticultural Area by the Alcohol and Tobacco Tax and Trade Bureau . The urban region designated the Puget Sound Region is centered on Seattle and consists of nine counties , two urban center cities and four satellite cities making up what has been dubbed "Pugetopolis." Both urban core cities have large industrial areas and seaports plus

2716-834: The Rocky Mountains is composed of such terranes. The southern boundary with the Cocos plate to the west and the Caribbean plate to the east is a transform fault , represented by the Swan Islands Transform Fault under the Caribbean Sea and the Motagua Fault through Guatemala . The parallel Septentrional and Enriquillo–Plantain Garden faults running through Hispaniola and bounding

2813-518: The Seattle–Tacoma combined statistical area (CSA), which encompasses most of the Puget Sound region. The population of this wider region is 4,269,349—almost two-thirds of Washington's population—as of 2012 . The Seattle CSA is the 12th largest CSA , and the 13th largest primary census statistical area in the country. The additional metropolitan and micropolitan areas included are: A state-run ferry system, Washington State Ferries , connects

2910-537: The U.S. Census Bureau and the Office of Management and Budget , the Seattle metropolitan area is officially the Seattle–Tacoma–Bellevue, WA metropolitan statistical area (MSA) and consists of: Based on commuting patterns, the adjacent metropolitan areas of Olympia , Bremerton , and Mount Vernon , along with a few smaller satellite urban areas, are grouped together in a wider labor market region known as

3007-549: 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

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3104-620: 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 . 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. In

3201-488: The CCFZ is a left-lateral strike-slip 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

3298-440: 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 Holocene 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. Strands of

3395-626: 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). 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

3492-709: The Farallon plate. The boundary along the Gulf of California is complex. The gulf is underlain by the Gulf of California Rift Zone , a series of rift basins and transform fault segments from the northern end of the East Pacific Rise in the mouth of the gulf to the San Andreas Fault system in the vicinity of the Salton Trough rift/ Brawley seismic zone . It is generally accepted that a piece of

3589-461: The MVF, from Lake Chaplain to 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

3686-599: The Mid-Atlantic Ridge at a rate of about 2.3 centimeters (~1 inch) per year. At the same time, the Pacific plate is moving to the northwest at a speed of between 7 and 11 centimeters (~3-4 inches) per year. The motion of the plate cannot be driven by subduction as no part of the North American plate is being subducted, except for a small section comprising part of the Puerto Rico Trench ; thus other mechanisms continue to be investigated. One study in 2007 suggests that

3783-461: 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 strike-slip motion on the Straight Creek Fault , which initiated about 50 to 48 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

3880-448: The North American plate was broken off and transported north as the East Pacific Rise propagated northward, creating the Gulf of California. However, it is as yet unclear whether the oceanic crust between the rise and the mainland coast of Mexico is actually a new plate beginning to converge with the North American plate, consistent with the standard model of rift zone spreading centers generally. A few hotspots are thought to exist below

3977-469: The North American plate. The most notable hotspots are the Yellowstone (Wyoming), Jemez Lineament (New Mexico), and Anahim (British Columbia) hotspots. These are thought to be caused by a narrow stream of hot mantle convecting up from the Earth's core–mantle boundary called a mantle plume , although some geologists think that upper mantle convection is a more likely cause. The Yellowstone and Anahim hotspots are thought to have first arrived during

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4074-437: 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

4171-422: 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

4268-442: The Olympic terrane and the North American continent, and were pushed up ( 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

4365-416: The Puget Sound region was covered by a lobe of the Cordilleran Ice Sheet . The glacier that covered the area was about 3,000 feet (900 m) thick within the vicinity of Seattle. By the time Captain George Vancouver found the Sound in 1792, early native people had already been there for over 5,000 years. Logging started as early as 1853. In the 1880s logging railroads cut their way into Puget Sound. 1886

4462-444: 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.) 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,

4559-429: 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

4656-402: 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. 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

4753-463: 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

4850-455: The SWIF forms the southwestern side of the Everett Basin (see 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

4947-406: 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 ) and across the southern part of Whidbey Island , crossing to the mainland between Mukilteo and Edmonds . This section of

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5044-438: The SWIF wrapping around the eastern end of the 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 the length of the SWIF is around 150 km (90 miles). It has been suggested that the SWIF might extend past its intersection with

5141-405: 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

5238-462: The Saddle Mountain Faults had been shown to have Holocene activity (since the last ice age, about 12,000 years ago). Not until 1992 was the first of the lowland faults, the Seattle Fault , 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 , a technique that can generally penetrate forest canopy and vegetation to image

5335-418: 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

5432-420: 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

5529-400: The Seattle Uplift is being forced against the Western Melange belt To the north the Melange Belt is manifested as the Rogers Belt , a zone of low-amplitude folding stretching from Monroe to 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

5626-441: 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 Seattle Fault . The Puget Sound region

5723-404: The Skykomish River; it is now believed to be of regional significance. Rattlesnake Mountain is a prominent NNW trending ridge just west of 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

5820-399: The St. Helens fire burned 300,000 acres (1,200 km ). Mount Rainier National Park started in 1899. The 1902 Yacolt Burn burned 600,000 acres (2,400 km ). Olympic National Park was established in 1938. George Vancouver explored Puget sound in 1792. Vancouver claimed it for Great Britain on June 4, 1792, naming it for one of his officers, Lieutenant Peter Puget . It became part of

5917-474: 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- Tertiary rock, suggesting that

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6014-454: The actual ground surface with an unprecedented accuracy of approximately one foot (30 cm). An informal 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 ). Marine seismic reflection surveys on Puget Sound where it cuts across the various faults have provided cross-sectional views of

6111-417: 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 ; see 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

6208-419: The area appear to be part of a 49- to 44- Ma magmatic belt that appeared just after the arrival of Siletzia , 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

6305-479: The area of Sultan (due east of Everett) to 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

6402-402: 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 Siletzia terrane that underlies western Washington and Oregon. East of Puget Sound the basement of the Cascades province

6499-418: 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 (west side of Bellingham Bay, visible at the top of the map ), crossing the Devils Mountain Fault ( DMF , part of the Darrington—Devils Mountain Fault Zone) near Mount Vernon. Cheney also mapped the Lake Chaplain Fault , parallel and just east of

6596-449: 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 it passes through a hotspot of active seismicity, including the 1996 M L 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

6693-437: 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 Olympic Mountains and Willapa Hills on the west. This pocket is catching a stream of terranes (crustal blocks about 20 to 30 km thick) which the Pacific plate is pushing up the western edge of North America, and in

6790-441: 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 Chuckanut Formation (part of

6887-400: The east-striking Devils Mountain Fault cross the northern tip of Whidbey Island 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 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

6984-586: The eastern end of the Seattle Fault , which in turn terminates at the RMFZ; Rattlesnake Mountain forms the eastern edge of the 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

7081-643: The eastern end of the Strait of Juan de Fuca. Just south of Victoria, British Columbia 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 Siletzia terrane that underlies much of western Washington and Oregon). Seismic tomography studies show that this portion of

7178-467: 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

7275-556: The greater Puget Sound area. Prominent islands: Urban centers: Satellite cities: Other principal cities: Military bases: 47°36′N 122°24′W  /  47.6°N 122.4°W  / 47.6; -122.4 North American plate It extends eastward to the seismically active Mid-Atlantic Ridge at the Azores triple junction plate boundary where it meets the Eurasian plate and Nubian plate . and westward to

7372-473: The international trade of the United States. All this is at risk of earthquakes from three sources: 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

7469-535: The larger islands to the Washington mainland, as well as both sides of the sound, allowing cars and people to move about the greater Puget Sound region. The region has a Csb ( warm summer mediterranean ) climate with some areas in the far east (western foothills of the Cascades) having an oceanic ( Cfb ) climate if the 30 millimetres (1.2 in) isohyet is used. The wet season is from October to April and

7566-449: 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

7663-466: The lower elevations of Puget Sound . This is the Puget Lowland. The principal effects of this complex interplay of forces on the near-surface crust underlying the Puget Lowland are: 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

7760-521: 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 below ), or the metamorphic basement is supplanted here by other volcanic rock –

7857-546: The map at right): 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

7954-705: 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

8051-515: The motions of the tectonic plates : material from the Earth's mantle rises at spreading centers , and moves out as plates of oceanic crust which eventually are subducted under the more buoyant plates of continental crust . Western Washington lies over the Cascadia subduction zone , where the Juan de Fuca plate is subducting towards the east (see diagram, right). This is being obliquely overridden by

8148-666: The north side of the Olympic Peninsula to the Wallowa Mountains in northeastern Oregon. It aligns with the West Coast fault and Queen Charlotte Fault system of strike-slip fault zones (similar to the San Andreas Fault in California) on the west side of Vancouver Island , but does not itself show any significant or through-going strike-slip movement. It is of interest here because the various strands of

8245-419: 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 and Snohomish , with several scarps in

8342-502: 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 ). 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

8439-410: The region to eastern states. For a long period Tacoma was noted for its large smelters where gold, silver, copper and lead ores were treated. Seattle was the primary port for trade with Alaska and the rest of the country and for a time possessed a large shipbuilding industry. The region around eastern Puget Sound developed heavy industry during the period including World War I and World War II , and

8536-406: 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

8633-410: The shallower crust of the Puget Lowland is being pushed, but this remains speculative. Most of these "faults" are actually zones of complex faulting at the boundaries between sedimentary basins ( synclines , "∩") 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

8730-492: 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

8827-554: The south. On its western edge, the Farallon plate has been subducting under the North American plate since the Jurassic period. The Farallon plate has almost completely subducted beneath the western portion of the North American plate, leaving that part of the North American plate in contact with the Pacific plate as the San Andreas Fault. The Juan de Fuca , Explorer , Gorda , Rivera , Cocos and Nazca plates are remnants of

8924-407: 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 1700 Cascadia earthquake . These earthquakes probably caused tsunamis, and several nearby locations have evidence of tsunamis not correlated with other known quakes. While there

9021-678: The southern end of Vancouver Island . 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 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- Tertiary , before

9118-542: 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. 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

9215-428: 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. The ultimate driver of the stresses that cause earthquakes are

9312-714: 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 (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

9409-527: The vicinity of King County's controversial 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. 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

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