Puget Sound faults
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:
- Devils Mountain Fault
- Strawberry Point and Utsalady Point faults
- Southern Whidbey Island Fault (SWIF)
- Seattle Fault
- Tacoma Fault
- Hood Canal Fault
- Saddle Mountain Faults
- Olympia structure (suspected fault)
- Doty Fault
- Saint Helens Zone and Western Rainier Zone
- 1 General background
- 2 Devils Mountain Fault
- 3 Strawberry Point and Utsalady Point faults
- 4 Rogers Belt
- 5 Southern Whidbey Island Fault
- 6 Rattlesnake Mountain Fault Zone
- 7 Seattle Fault
- 8 Tacoma Fault Zone
- 9 Hood Canal Fault
- 10 Saddle Mountain Faults
- 11 Olympia Structure
- 12 Doty Fault
- 13 Saint Helens Zone, Western Rainier Zone
- 14 Deeper structure
- 15 Other faults
- 16 See also
- 17 Notes
- 18 References
- 19 External links
Earthquake sources and hazard
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 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, caused by slippage of the entire Cascadia subduction zone, from approximately Cape Mendocino in northern California to Vancouver Island in British Columbia.
- Intraslab (Benioff zone) earthquakes, such as the M 6.7 2001 Nisqually earthquake, caused by slippage or fracturing on a small part of the subducting plate at a depth of around 50 km (31 miles).
- Relatively shallow crustal earthquakes, generally less than 25 km (16 miles) 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.
While the great subduction events are large and 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 in being closer to the surface will have 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 of 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 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.
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 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 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 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 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 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 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 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 the Siletz River Volcanics). 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 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:
- The basement rock 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 of the upper-crustal formations (such as the Western and Eastern Melange Belts, see map) have been pushed onto the older (pre-Tertiary) 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 (vertical movement) thrust or reverse faults.
- Some strike-slip (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 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 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 bowed fault seen here). 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 and crustal uplifts. 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 (line E)
- Seattle Uplift
- Tacoma Fault Zone (line D)
- Tacoma Basin
- Olympia fault (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.
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 from the Eocene epoch (about 50 million years ago), with an origin variously attributed to a seamount chain, or continental margin rifting. 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 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.
The previous model studied seismicity, surface geology, and geophysical data 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 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 the Seattle Fault is attributed to that area riding on single block (terrane), bounded approximately by the Seattle Fault, the Olympia structure (fault?), Hood Canal (Saddle Mountain Faults?), and vaguely east of Seattle/Tacoma. 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 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, 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, and on towards Victoria, British Columbia, where the DMF is believed to join the Leech River fault system at 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 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 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 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 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,
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.
Strawberry Point and Utsalady Point faults 
Strands of 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 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 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 1700 Cascadia earthquake. 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 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).
Rogers Belt 
In the northeast corner of the Puget Lowland, running from approximately Monroe to a point on the Devils Mountain Fault (DMF) just short of Mount Vernon, is a belt about 15 miles wide of strikingly parallel drainages, indicating a strong albeit subdued pattern of topographical folding. Observing this, some parallel gravity gradients, and a "very active zone of minor seismicity", William Rogers inferred a "fault or other major structural feature".
Cheney (1987) subsequently argued for a Mount Vernon Fault (MVF; originally the Bellingham Bay—Chaplain fault zone) from Lake Chaplain (northeast of Monroe) along a distinct lineament running past Lake McMurray (near where the DMF has a slight bend), Mount Vernon, and eventually Lummi Island. This interpretation was not accepted, likely because it was also argued that the MVF had offset the DMF 47 km. to the north (past Lummi Island), contrary to the prevailing consensus that the DMF is not offset.
Mapping in 2006 revealed the right-lateral McMurray Fault Zone (MFZ) near Lake McMurray, aligned with the Lake Chaplain—Mount Vernon lineament, and suspected of being a major bounding fault that may correlate with the MVF. The implications of this for the rest of the Rogers belt have yet to be assessed. The interaction of the MFZ and adjacent Mount Washington Fault Zone with the DMF (part of the Darrington—Devils Mountain Fault Zone, a major regional structural boundary) is complex, with hints of northward extensions; the geological structure to the north has yet to be adequately mapped. The southern end of the Rogers belt runs into an area of complex faulting and active seismicity between Monroe and North Bend which is associated with the Southern Whidbey Island Fault and Rattlesnake Mountain Fault Zone; this will be discussed below.
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 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 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 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 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 — 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 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 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 and Snohomish, with several scarps in the vicinity of King County's controversial Brightwater regional sewage treatment plant showing a 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 multipe zones of disturbed ground.
Rattlesnake Mountain Fault Zone 
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 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 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 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 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 the RMFZ (e.g., the Snoqualmie Valley and Johnson's Swamp fault zones) extend the general trend of NNW faulting as far as Monroe
East of the RMFZ faulting is complex and not yet understood. Two faults stand out. 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.
The Cherry Creek Fault Zone, a few miles north of the TCF and parallel, is the location of a notable concentration of low-level seismic activity, and the 1965 Duvall earthquake. The Cherry Creek Fault Zone follows a NNE trending topographical feature that extends to Rockport on Highway 20; it may coincide with a tectonic contact of formations of the Western Melange Belt.
(Rattlesnake Mountain Fault Zone not included in QFFDB.)
Coast Range Boundary Fault 
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 is various pre-Tertiary (older than 65 Ma) metamorphic rock. The contact between these geological provinces is expected somewhere between Puget Sound and the Cascades foothills. 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, which has been named the Coast Range Boundary Fault (CRBF).
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 studes 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 on-shore parts of the SWIF 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 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 Fig. 48), along the edge of a formation known as the Southern Washington Cascades Conductor.
However, gravity data constrains the eastern edge of the Crescent Formation to be near Seattle. This has led to a suggestion that the CRBF lies below Seattle, and runs south-southeast 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 Stanley, Villaseñor & Benz (1999) (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 thrust and reverse faults – between lines E and F on the 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. It is the most studied fault in the region; it will be treated in somewhat greater detail.
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 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. This seems geologically reasonable, as both the SWIF and RMFZ appear to be the contact between Tertiary 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.
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). In 1994 it was stated (without support) 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 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.
An extension of some 10 km west of Hood Canal into the Olympic Mountains has been suggested on evidence that the Saddle Mountain Fault (discussed below), to the west of Hood Canal, might extend as far north as the Seattle Fault, and might be even be kinematically linked. Other studies show the Seattle Fault wrapping around Green Mountain to strike southwest towards Hood Canal, and likely joining the Saddle Mountain Fault.
Another complication is an unusually strong north-south striking geophysical anomaly (appearing in gravitational, aeromagnetic, and seismic tomography data) that bounds the west side of the Seattle uplift (vertical black line below "D" on the map), separating it from the Dewatto Basin to the west. This appears to be a deep fault, possibly with significant vertical and horizontal offsets, which has recently (2009) been named the Tahuya Fault. To the south it appears to terminate at the Tacoma Fault; the nature of its junction with the Seattle Fault is undetermined. It is suggestively aligned with Dabob Bay, where the Hood Canal Fault is believed to strike north, but any possible connection is as yet pure speculation.
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 Pratt et al. (1997) 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 Brocher et al. (2001), 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.
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 above), considerably more energy would be released, on the order of ~M 8.
Tacoma Fault Zone 
The Tacoma Fault (at right, and also between lines C and D on the 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 Seattle Fault 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 diagram above) in a way that north-south compression tends to force the Seattle Uplift up, resulting in dip-slip movement on both fault zones.
The Tacoma Fault was first identified by Gower, Yount & Crosson (1985) 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. Not until 2001 was it identified as a fault zone, and only in 2004 did trenching reveal Holocene activity.
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 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, under the Muckleshoot Basin (see 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.
The western end of the Tacoma Fault is curious, as near the small town of Allyn (near the extreme tip of Hood Canal) it appears to make a sharp turn to the north, and follows a strong geophysical lineament which has recently (2009) been named the Tahuya Fault. This appears to be the western termination of the Seattle Uplift. The gap between this and Hood Canal is the Dewatto Basin, which thus forms an appendage to the northwestern corner of the Tacoma Basin. One hypothesis as to how this formed is based on the regional compressive force coming not from the south, but southwest (normal to the Rosedale monocline); this raises the prospect of left-lateral strike-slip motion on the east-west oriented parts of the Tacoma and Seattle faults.
Hood Canal Fault 
Hood Canal marks an abrupt change in physiography between the Puget Lowland and the Olympic Mountains to the west. Gravitational anomalies and seismic reflection data suggest that there is a major fault zone from the south end of Hood Canal to Dabob Bay, and continuing north on land. It is inferred that if this fault is a terrane boundary between the Olympics and the Puget Lowland (but read about the Saddle Mountain Faults, below), then it must connect with various faults in the Strait of Juan de Fuca. But whether this is via the Discovery Bay Fault, or further east (passing close to Port Townsend) remains speculative due to lack of definite scarps and paleoseismological investigation.
This fault is "largely inferred" due to a paucity of evidence. Possible Holocene movement is likewise inferred from a possible connection with the Seattle Fault, but even that connection is doubted (discussed above). Evidence is accumulating that the Saddle Mountain Faults (discussed next), which show recent activity, are the structural boundary of the Puget Lowland. It is possible that there is no fault under Hood Canal. A 2001 study questioned its existence on the basis of high-resolution seismic tomography, though a recent study interprets a different variety of tomographic data as showing a major, active fault zone. Recent mapping (2010, 2012) "found no convincing evidence for the existence of this 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), they appear to be kinematically related to the Seattle Fault: 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 Seattle Fault 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.
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 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. Gower, Yount & Crosson (1985), labelling it "structure L", mapped it from Shelton (near the Olympic foothills) southeast to Olympia (pretty nearly right under the state Legislature), directly under the town of 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 Sherrod (1998) saw sufficient similarity with the Seattle Fault to speculate that this is a thrust fault. Pratt et al. (1997), 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 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 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 Seattle Fault 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 
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 and 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 (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 (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—Salzar Creek Fault does not fully fit the regional pattern of basins and uplifts bounded by faults described 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—Salzar 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 Mossy Rock. They are also on-strike with a swarm of faults on the Columbia River, bracketing The Dalles. As all of these are thrust 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.
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 image), do not participate in the 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 strike-slip 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 (see QFFDB 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 (an uplifted block of the Puget Group of sedimentary and volcanic deposits typical of the Puget Lowland) and the adjacent Raging River anticline (see 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.
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 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, and 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 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.
Each of the faults reviewed here is more typically a zone of related faults, or strands. There are numerous other seismogenic faults (or fault zones) in the Puget Lowland, sketchily studied and largely unnamed. These are usually fairly short, and not believed to be significantly seismogenic, though the Snoqualmie Valley Fault, Griffin Creek Fault, and Tokul Creek Fault (see map) are in an area of active seismicity. The newly discovered (2010) Cherry Creek Fault Zone north of Carnation (north end of the RMFZ, map) may be the source of a 1996 M 5.3 earthquake just east of Duvall.
The San Juan Island and Leach River faults crossing the southern end of Vancouver Island are significant and undoubtably 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 the 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, see fault database map), 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), crosses Discovery Bay (and various possible extensions of the Hood Canal Fault) and bounds the Port Ludlow Uplift ("uplift of unknown origin" on the map); it appears to extend to the Southern Whidbey Island Fault.
An Everett Fault, running east-northeast along the bluffs between Mukilteo and 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 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 of the Puget Lowland, it may actually connect under the Muckleshoot Basin to the East Passage Zone and the Tacoma Fault (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 North Cascades, 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.
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 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.
Each of the principal Puget Lowland faults discussed here is notable largely for demonstrated or suspected seismic activity, usually a geologically recent (Holocene) major earthquake. However, significant movements on these faults (such as the Great Seattle Quake of ~930 AD) are rare, and 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.
- "The Puget Lowland is a north-south-trending structural basin that is flanked by Mesozoic and Tertiary rocks of the Cascade Range on the east and by Eocene rocks of the Olympic Mountains on the west." Barnett et al. 2010, p. 2, and see Figure 1. The Georgia Basin to the north is structurally related, but topographically demarcated by the Chuckanut Mountains near Bellingham.
- Ballantyne, Pierepiekarz & Chang 2002, p. 2.
- Bucknam, Hemphill-Haley & Leopold 1992, p. 1611; Fisher et al. 2005, p. 8; Karlin & Abella 1996, p. 6138.
- Ballantyne, Pierepiekarz & Chang 2002, p. 11
- Stanley, Villaseñor & Benz 1999, figures 46—50. See the maps.
- Harding & Berghoff 2000, p. 2.
- Daneš et al. 1965.
- Gower 1980; Gower, Yount & Crosson 1985.
- Barnett et al. 2010, p. 1
- Adams 1992.
- Haugerud et al. 2003; Harding & Berghoff 2000; Nelson et al. 2003, p. 1369; Sherrod et al. 2004; Johnson et al. 2004b, p. 2299.
- Brocher et al. 2001.
- Blakely & Wells Weaver1999; Blakely et al. 2002.
- Calvert & Fisher 2001.
- Parsons et al. 1999, figure 5a.
- Brandon & Calderwood 1990.
- Troost, The Origin of Puget Sound.
- Pratt et al. 1997, p. 27,471.
- Stanley, Villaseñor & Benz 1999, p. 43.
- Blakely et al. 2002
- Tabor 1994, pp. 217, 230.
- Because of the geometry of the SWIF and the Kingston arch, the "uplift of unknown origin" between them is smaller, and the fault separating the uplift from the arch (the Lofall Fault, discovered relatively recently by Brocher et al. 2001, p. 13,557) is shorter; it is not notably seismogenic.
- Strictly speaking the southern edge of the Black Hills Uplift would be the southeast striking Scammon Creek Fault that converges with the east striking Doty Fault at Chehalis. In the angle between these is located the minor Lincoln Creek uplift, the Doty Hills, and, further west, an impressive chunk of Crescent basalt. If the pattern is continued to the southwest, along cross-section A-A' in Pratt's figure 11 (and missing the mapped trace of the Doty Fault), then the next basin is at Grays Harbor (not shown here). The Doty Fault/Chehalis Basin sequence follows the cross-section X-X' shown on the map.
- Blakely et al. 2009b; Blakely et al. 2011, §5.2.1.
- See Stanley et al. 1996, figure 2.
- Pratt et al. 1997.
- Babcock et al. 1992, p. 6799.
- Pratt et al. 1997, see figure 2; Johnson et al. 2004a, see figure 17.
- Pratt et al. 1997, p. 27,486.
- Logan et al. 1998.
- Stanley, Villaseñor & Benz 1999, p. 46, and see figure 64.
- Stanley, Villaseñor & Benz 1999, pp. 45, 46.
- Tabor 1994.
- Hayward et al. 2006 possibly an ancient coastal shore.
- Geologic Map GM-61 (McMurray).
- QFFDB Fault 573.
- Johnson et al. 2004b, p.2313.
- Geologic Map GM-59 (Oak Harbor and Crescent Harbor).
- Hayward et al. 2006, p.444.
- Interstate 5 runs nearly due north from Everett to Mount Vernon, except for a stretch southeast of Conway that parallels one of these low-amplitude folds. In some places, such as along the South Fork of the Stillaguamish River between Arlington and Granite Falls, there are also contrasting geological contacts. Geologic Map GM-50.
- Rogers 1970, p. 55.
- The lineament and general context is readily seen on GM-50, though the MVF itself is not marked. Lummi Island can be seen at top edge of the map.
- Dragovich et al. 1998 (OFR 98-5, Bow and Alger) p. 44.
- GM-61, p. 10.
- Blakely & Sherrod 2006.
- QFFDB Fault 572; Johnson et al. 1996.
- Johnson et al. 1996.
- Brocher et al. 2001, §5.10.
- Clowes et al. 1987; Johnson et al. 1996, p. 336.
- Clowes et al. 1987.
- Brocher et al. 2001, §5.10. This contact is the Coast Range Boundary Fault, discussed below.
- Johnson et al. 1996, Fig. 1; Barnett et al. 2010, Map 5, on-line.
- Sherrod et al. 2008, paragraph 11.
- Johnson et al. 1996, p. 351.
- Brocher et al. 2001, §5.10.
- Sherrod et al. 2008, par. 71.
- Blakely et al. 2004 (USGS OFR 04-1204); Sherrod et al. 2005 (USGS OFR 05-1136); Sherrod et al. 2008; Liberty & Pape 2008.
- GM-67, pp. 11, 12 (Fall City); GM-73 (North Bend); Dragovich et al. 2010 (Carnation).
- Sherrod et al. 2008, paragraphs 75, 78, & 84; Geologic Map GM-67.
- Sherrod et al. 2008, §6.3, par. 78.
- Blakely et al. 2011. Their preferred interpretation is that the SWIF is right-offset along the RMFZ (par. 71). See fig. 22.
- Dragovich et al. 2012 (Lake Joy).
- The long-range mapping plan area and current status of planned mapping can be seen at Washington State DNR.
- Kelsey & Sherrod 2001, p. 2.
- Sherrod et al. 2005, pp. 15, 2.
- For the County's interpretation of the geological hazard and anticipated impacts of a major earthquake, see the Environmental Impact Statements.
- GM-73 (North Bend).
- Geologic Map GM-73, pp. 29–30.
- Geologic Maps GM-67 (Fall City) and GM-73, p. 31.
- Dragovich et al. 2010 (Carnation), p. 2; Dragovich et al. 2011 (Monroe), p. 2.
- Blakely et al. 2011, par. 71.
- Geological Map GM-52, pp. 5, 6.
- GM-73, Figures 3B and 3C, and throughout. Whether RMFZ is also the Crescent-Cascade contact, and thereby the Coast Range Boundary Fault, depends on whether the Crescent Formation reaches this far. Gravity studies (Finn 1990, p. 19,538) suggest not, or at least not near the surface. The situation at depth is not known. There is a suggestion of a décollement about 18 km down, (GM-73, p. 31), but at a similar décollement further south (under the SWCC) the underlying basement is believed to be pre-Tertiary.
- Dragovich et al. 2011. Detailed mapping north of Monroe has not yet been done.
- GM-52, Fig. 1.
- Dragovich et al. 2012 (Lake Joy), Appendix H.
- However, according to Stanley, Villaseñor & Benz (1999, p. 34) the Duvall earthquake was on a fault striking 350°. This suggests the quake was actually on the cross-cutting Cherry Valley fault, the northern most member of the RMFZ, and possibly an extension of the Griffin Creek fault. Dragovich et al. 2010, p. 2; Dragovich et al. 2012, plate 2.
- Geological Map GM-52.
- Johnson et al. 1996, p. 340: "must occur".
- Johnson et al. 1996, p. 336.
- Johnson et al. 1996, p. 336, and see fig. 1.
- Brocher et al. 2001, §5.10.
- GM-73, p. 30 and map. There is some evidence for a décollement (horizontal separation) at about 18 km depth, and it is possible that the surface patterns of faulting do not reflect faulting or structure below the décollement. See GM-73, p. 31 and preceeding figures.
- The SWCC appears to be Tertiary marine sediments, not the pre-Tertiary metamorphic rock of the Cascades province; this would seem to make it part of the Coast Range province, with the Coast Range—Cascade contact further east. However, the SWCC is relatively shallow (no more than 15 km deep), and likely is draped over pre-Tertiary bedrock. (See Stanley et al. 1996, Fig. 5.) The Crescent Formation is expected to be in contact with the pre-Tertiary rock along the SHZ at depth.
- Finn 1990, p. 19,538. This constraint might not apply at depth.
- Johnson, Potter & Armentrout 1994; Johnson et al. 1996.
- Snelson et al. 2007, Figures 6 & 7.
- As indicated in GM-50 and the location maps of Brocher et al. 2001, Van Wagoner et al. 2002, Johnson et al. 2004a, Snelson et al. 2007, and Ramachandran 2012.
- There is a preliminary report of aeromagnetic and gravity mapping placing the eastern edge of the Siletz terrane under Lake Washington. See Anderson et al. 2011 (abstract).
- Liberty 2009, p. 3.
- Daneš et al. 1965, pp. 5576–5577, and figure 5.
- See Adams 1992, and additional references at Seattle Fault.
- Geologic Map GM-67 (Fall City), p. 11; Geologic Map GM-73 (North Bend), pp. 9, 12.
- Bucknam, Hemphill-Haley & Leopold 1992, see figure 1.
- Johnson, Potter & Armentrout 1994, p. 74.
- Daneš et al. 1965, pp. 5577–5579.
- Johnson et al. 1999, figure 6; Brocher et al. 2001, figure 1; Blakely et al. 2002, figures 1, 2, and 3. Curiously, Johnson et al. (1999), having failed to find any definite indications of a fault zone in seismic-reflection profiles in Hood Canal, claimed (p. 1048) that "the Seattle fault does not extend west as far as Hood Canal" (emphasis added).
- Blakely et al. 2002, figures 2 and 3; Liberty 2009, p. 6.
- Blakely et al. 2009a, p. 15.
- Haeussler & Clark 2000 (Wildcat Lake map); Liberty 2009, p. 2, and see Figures 3 and 10; Contreras et al. 2012, p. 2.
- Brocher et al. 2001, §5.4, p. 13,553, and various plates. Also observed by Daneš et al. 1965, Pratt et al. 1997 (from which the map above is taken), and others.
- Lamb et al. 2009. Although it has been suggested (Polenz et al. 2010b, p. 17) that there is no fault.
- And further amplified by Brocher, Blakely & Wells 2004 and Johnson et al. 2004a.
- ten Brink, Song & Bucknam 2006, p. 588.
- Stanley, Villaseñor & Benz 1999, p. 46.
- Nelson et al. 2008 (SIM 3060)
- Brocher et al. 2001, §6.1.
- Sherrod et al. 2004, p. 11.
- Johnson et al. 2004a, §5 and figure 17.
- Brocher et al. 2001.
- Sherrod et al. 2004. See also Brocher et al. 2001, §6.1 (p. 13,558).
- The QFFDB, citing lack of consensus, ignores the eastern part.
- Brocher et al. 2001; Johnson et al. 2004a, see figure 4, and compare the differences in cross-sections A-A' (west) and B-B' (east) in figure 17.
- Brocher et al. 2001, §6.3.
- Blakely et al. 2009b (abstract); Carley, Liberty & Pratt 2007 (abstract); Liberty 2007, figure 3; Blakely et al. 2011, §5.2.1, and see Fig. 22. Alternately, the Tacoma Fault may be only a splay, with the main part of the WRF fault continuing WNW past Kent and Bremerton (Washington Narrows).
- Brocher et al. 2001, §5.4.
- Lamb et al. 2009.
- Johnson et al. 2004a, figure 18.
- Daneš et al. 1965, p. 5579.
- QFFDB Fault 552.
- Van Wagoner et al. 2002, §4.1.9.
- Ramachandran 2012, 3.5.
- Contreras et al. 2010 (OFR 2010-4, Lilliwaup), p. 4; Contreras et al. 2012 (OFR 2011-3, Hoodsport) p. 3.
- See QFFDB Fault 562a and Fault 562b
- Carson 1973; Carson & Wilson 1974.
- Blakely et al. 2009a, p. 1.
- Blakely et al. 2009a, pp. 13–15, and figure 4.
- Walsh & Logan 2007 (OFR 2007-1).
- Witter & Givler 2005, p. 16; Blakely et al. 2009a, pp. 1, 15.
- Figure 64 of Stanley, Villaseñor & Benz 1999 (USGS OFR 99-0311) shows additional dates of various co-seismic events. See also Logan et al. 1998.
- See Cushman Dam No. 1 and Cushman Dam No. 2.
- Witter & Givler 2005, p. 1, and see Figure 2.
- Walsh & Logan 2007.
- Sherrod 1998, pp. 99, 131, and figure 4-19.
- Stanley, Villaseñor & Benz 1999 (OFR 99-311), figures 46—50. See seismic maps.
- Daneš et al. 1965, figures 3 and 4.
- Pratt et al. 1997, p.27,472.
- Blakely, Wells & Weaver 1999 (OFR 99-514). Download the map and see the aeromagnetic anomaly. Additional aeromagnetic and gravitational imagery of the Olympia and other structures available on the Summit Lake geological map.
- Sherrod 2001, p. 1308.
- Magsino et al. 2003.
- E.g., Logan & Walsh 2004 (Summit Lake map). More recently it has been suspected that a natural berm across the delta of the Skokomish River may be due to faulting, which could implicate the OS as an active fault. But the researchers are not yet ready to assert that. Polenz et al. 2010a; Polenz et al. 2010b.
- Geologic Map GM-56 (East Olympia).
- Clement 2004; Clement et al. 2010.
- Weaver & Smith 1983, pp. 10,376, 10,380.
- Blakely & Jachens 1990, plate 2.
- See QFFDB 580, "Faults near The Dalles".
- Sherrod 1998; Sherrod 2001, p. 1308 and generally.
- Logan & Walsh 2004 (Summit Lake map).
- Snavely, Brown & Rau 1958.
- Finn & Stanley 1997, p. 4; Finn 1999, p. 330.
- Wong et al. 2000, Table 1, p. 7.
- See report from the Washington State Department of Transportation for the economic costs when flooding closed the freeway for just several days.
- Wong et al. 2000, Table 1, p. 7. 40 miles would include the combined Doty—Salzer Creek fault plus a 15 mile extension west to South Bend, on Willapa Bay. Finn (1990), without identifying it, associated the Doty Fault with notable gravity and aeromagnetic anomalies (Plates 1 and 2) that extend towards Willapa Bay.
- Pratt et al. 1997, Plate 1.
- Gower, Yount & Crosson 1985 (Map I-1613).
- Geologic Map GM-34 (Southwest Quadrant).
- Weaver, Grant & Shemata 1987.
- Stanley, Villaseñor & Benz 1999, figures 46—49; Stanley et al. 1996, p. 5.
- Weaver & Smith 1983; Stanley, Finn & Plesha 1987, p. 10,179; Stanley et al. 1996, pp. 6—7.
- Several possible explanations of the enhanced conductivity have been considered; Eocene marine sediments containing brine are most likely (Stanley, Finn & Plesha 1987, pp. 10,183—10,186). Egbert & Booker (1993) discuss evidence that the conductivity anomaly may be broader than shown here, and suggest it is a remnant of an "early Cenozoic subduction zone which is analogous to the present-day Olympic Peninsula." (p. 15,967)
- Stanley, Finn & Plesha 1987; Stanley et al. 1996, pp. 6—7.
- Stanley et al. 1996, p. 6.
- Stanley et al. 1996, p. 4
- Vine 1962, pp. 7–8; Stanley & Johnson 1993, p. 3; Stanley et al. 1996, figure 13, pp. 15—16. If the Carbon River and Raging River anticlines are related, and the SWCC and Raging River formations correlative, the RMFZ would be the eastern edge of the SWCC. That the fault strands of the RMFZ turn easterly, and seismicity jumps from a fault contact to an anticline, suggests there is more to learn about the OWL/WR-NR zone.
- Geologic Map GM-53, (Washington State).
- Weaver & Smith 1983, pp. 10,383, and 10,371. See also p. 10,376, and figure 8.
- Weaver, Grant & Shemata 1987, pp. 10,170, 10,176; Stanley et al. 1996, p. 16.
- Weaver, Grant & Shemata 1987, p. 10,175.
- Evarts, Ashley & Smith 1987, p. 10,166.
- Tabor & Crowder (1969, p. 60, and see figure 60) (possibly relying on an earliar writer) reported a "zone of basaltic dikes and cinder cones that trends north-northeast" (NNE), including Mount Rainier and Mount St. Helens "to the southwest". Evarts, Ashley & Smith (1987, p. 10,166) state that "Mount Rainier and Glacier Peak are aligned along the projection of this trend," described as NNE, or "roughly N25E". While MR does bear nearly N25E from MSH, calculation from latitude and longitude shows the MSH—GP bearing to be more accurately N21E; lining up all three volcanoes would require a slight bowing of the lineament. However, features near MSH (such as Yale Lake and Spirit Lake) bear N20E, not aligned with MR. It is more likely that MR, in rising to the surface, has "drifted" off of the underlying lineament. This NNE striking lineament should not be confused with other lineaments striking N50°E. See Evarts, Ashley & Smith 1987, p. 10,166, Weaver, Grant & Shemata 1987, p. 10,175, and Hughes, Stoiber & Carr 1980, figure 1.
- Evarts, Ashley & Smith 1987, p.10,166.
- Hughes, Stoiber & Carr 1980, p. 16; Guffanti & Weaver 1988, p. 6523.
- Stanley et al. 1996, p. 5; Stanley, Villaseñor & Benz 1999, "Seismicity Patterns", and figures 46—49. The location and orientation of line "B" as shown here is approximate.
- The apparent gap north of Riffe Lake is possibly due to obscuration by volcanic deposits of the Northcraft Formation. Stanley et al. 1996, p. 4 and figure 3.
- Along part of the Sultan River and the west end of Blue Mountain, the east sides of Mount Pilchuck, Three Fingers, and Whitehorse Mountain, and (north of Darrington and the DDMF) the west side of North Mountain and part of the North Fork of the Stillaguamish River. North of Hwy 20 it is paralleled by Lake Shannon.
- However, according to Stanley, Villaseñor & Benz (1999, p. 34) the Duvall earthquake was on a fault striking 350°. This suggests the quake was actually on the cross-cutting Cherry Valley fault, the northern most member of the RMFZ, and possibly an extension of the Griffin Creek fault. Dragovich et al. 2010, p. 2.
- Geological map GM-52 (Tectonic elements).
- Evarts, Ashley & Smith 1987, p. 10,166.
- Olbinski 1983, pp. 149—151.
- Hughes, Stoiber & Carr 1980, p. 15.
- Geologic Map GM-75 (Snoqualmie).
- Dragovich et al. 2010.
- Brocher et al. 2001, p. 13,557.
- Molinari & Burk 2003 (abstract).
- Brocher et al. 2001, p. 13,557.
- Blakely et al. 2009b (abstract); Carley, Liberty & Pratt 2007 (abstract); Blakely et al. 2011, §5.2.1, and see Fig. 22. Or it might continue WNW in association with a topographical lineament extending from Lake Meridian (Kent) past Southworth, the Washington Narrows (entrance to Dyes Inlet), the western end of the Seattle Fault, and the southern tip of the Toandos Peninsula.
- Vance & Miller 1994.
- Johnson 1984; Johnson et al. 1999.
- Johnson et al. 1996, pp. 336, 341, 348; Geologic Map GM-67 (Fall City).
- Snelson et al. 2007, p. 1442.
- Finnegan et al. 2008.
- Rogers (2002, p. 145): "... there is little evidence of fault planes aligning with spatial trends of epicentres. Instead, most crustal seismicity seems to be occurring on random faults, all responding to the same regional stress."
- Yelin 1982. See also Map view of quakes in western Washington at PNSN.
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- 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.
- Earthquake locations.
- Geologic map of northwestern Washington (GM-50).
- Geologic map of southwestern Washington (GM-34).
- Various maps from Washington DGER.
- Aeromagnetic anomaly maps (USGS OFR 99-514).