Puget Sound faults

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.

[10] 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).

This pocket is catching a stream of terranes (crustal blocks about 20 to 30 km thick[18]) 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.

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).

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.

[39] 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).

[49] 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.

[51] 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.

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.

[60] 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.

[62] These ridges (part of a broader regional pattern that reflects the roots of the former Calkins Range[63]) are formed of sediments that collected in the Everett basin during the Eocene, and were subsequently folded by northeast-directed compression against the older Cretaceous and Jurassic rock to the east that bound the Puget Lowland.

At the edge of this older rock is the Rogers Belt, a geologically interesting zone running from the area of Sultan (due east of Everett) to Mount Vernon (just north of the bend in the Devils Mountain Fault).

[75] 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.

[85] The Tokul Creek Fault (TCF) strikes NNE from Snoqualmie, aligned with a possible offset of the Western Melange Belt[86] and with a valley that cuts through to the Skykomish River; it is now believed to be of regional significance.

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.

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.

[129] 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.

[131] 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.

This is the Dewatto lineament, believed to result from an east-dipping low-angle thrust fault where the western flank of the Seattle Uplift has been pushed into the northwestern corner of the Tacoma Basin.

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.

Kinematic analysis suggests that if shortening (compression) in the Puget Lowland is directed to the northeast (i.e., parallel to Hood Canal and the Saddle Mountain deformation zone) and thus oblique to the Dewatto lineament, it should be subject to both strike-slip and dip-slip forces, implying a fault.

Based on this and geophysical anomalies it was inferred that there is a major, active strike-slip fault zone running from the south end of Hood Canal, up Dabob Bay, and continuing north on land.

The Olympia structure – also known as the Legislature fault[168] – 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).

Aeromagnetic mapping in 1999 showed a very prominent anomaly[172] (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".

[176] A marine seismic reflection study[177] 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.

The WRZ and SHZ are associated with the southern Washington Cascades conductor (SWCC), a formation of enhanced electrical conductivity[194] lying roughly between Riffe Lake and Mounts St. Helens, Adams, and Rainier, with a lobe extending north (outlined in yellow, right).

[195] 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.

[199] 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.

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]";[203] it has been attributed to the geometry of the subducting Juan de Fuca plate.

Leech River Fault Devils Mountain Fault Zone Strawberry Point Fault Utsalady Point Fault Southern Whidbey Island Fault Woods Creek Fault Lake Chaplain Fault Mount Vernon Fault Cherry Creek Fault Zone Tokul Creek Fault Zone Rattlesnake Mountain Fault Zone Little River Fault Sequim Fault Dabob Bay Fault Zone Seattle Fault Zone Hood Canal Fault (questioned) Saddle Mountain Faults Canyon River Fault Frigid Creek Fault Dewatto Lineament/fault Tacoma Fault Zone East Passage Zone Olympic–Wallowa Lineament White River Fault Olympia Structure (suspected fault) Doty Fault Saint Helens Zone Western Rainier Zone Victoria, British Columbia San Juan Islands Lake Shannon Olympic Mountains Black Hills Mount Rainier Riffe Lake Lofall Fault
The principal Puget Sound faults (approximate location of known extents) and other selected peripheral and minor faults. Southern tip of Vancouver Island and San Juan Islands at top left (faults not shown), Olympic Mountains at center left, Mount Rainier at lower right (near WRZ). Faults north to south: Devils Mountain , Utsalady Point , Strawberry Point , Mount Vernon Fault/Granite Falls FZ/Woods Creek , Monroe Fault , Little River , Sequim , Southern Whidbey Island Fault , Cherry Creek , Tokul Creek , Rattlesnake Mountain Fault Zone , Lofall , Canyon River , Frigid Creek , Saddle Mountain faults , Hood Canal , Dabob Bay , Seattle Fault Zone , Dewatto Lineament , Tacoma Fault Zone , East Passage , White River (extends east), Olympia Structure , Scammon Creek , Doty (extends west), Western Rainier Zone , Saint Helens Zone (extends south). Also shown: Victoria (V), part of the Leech River Fault (unlabeled), and part of the Olympic–Wallowa Lineament .
Concentration of mid-crustal (10–20 km deep) seismicity in the Puget Lowland. ( Fig. 48 from USGS OFR 99-311 )
Simplified view of tectonic forces affecting Washington. The "accretionary complex" (in grey) is sediments and basalts that have accumulated in the trough where the Juan de Fuca plate bends downward. The lobe extending past Vancouver Island is where bending of the subduction zone to the south raised a wrinkle (now the Olympic Mountains) in the subducting plate. A stream of terranes that have been flowing northward in the trough above the subduction zone are being caught between this wrinkle and the basement rock ("fixed block") of the North Cascades, the latter consisting of other terranes which have accreted to the North American craton . As a result, Washington is crumpling in a series of folds (dotted lines show synclines and anticlines ) and faults, and Oregon is rotating in a manner similar to a jack-knifing trailer. Folding has exposed patches of Crescent Formation basalt ("mafic crust", black). (USGS [ 15 ] )
Bouguer gravity anomaly map of the Puget Sound region showing basins and uplifts, and principal faults and folds, over outline of Puget Sound, Hood Canal, and east end of Strait of Juan de Fuca. Blue and green generally indicate basins (with lower density sedimentary rock), red is generally uplifted basalt of the Crescent Formation. Unlabeled lines northwest of Everet Basin = Strawberry Point & Utsalady Point faults; E-F = Seattle Fault zone; C-D = Tacoma Fault zone; A = Olympia Fault; Doty Fault is east–west dashed line just north of Chehalis Basin; curved dashed line = Hood Canal Fault; Dewatto Lineament (western flank of Seattle uplift) runs south from "D", Saddle Mountain Faults are due west. (Adapted from Pratt et al. 1997 , plate 1.)
Puget Lowland and other areas divided from the "North Cascade Crystalline Core" by the Straight Creek Fault. The green colored area on the left has been pushed north, the purple area ("HH Melange") on the Darrington—Devils Mountain Fault originally being at or southwest of the Olympic Wallowa Lineament. (Fig. 1 from USGS I-2538 , modified.)
Location and known extent (prior to 2004) of Southern Whidbey Island Fault (SWIF). Also shown: Devils Mountain, Strawberry Point, and Utsalady Point faults (crossing northern Whidbey Island), Seattle Fault zone, southern part of Rattlesnake Mountain Fault Zone, Tokul Creek Fault (striking NNE from RMFZ). Not shown: southeastward extension of SWIF and various faults running northward from the RMFZ and east of Everett. This map is approximately one-quarter the scale of the map below. (USGS [ 41 ] )
Simplified geologic map of the Snoqualmie Valley (east of Seattle) from North Bend to Duvall, showing various strands of the Rattlesnake Mountain Fault (RMF), and the Snoqualmie Valley (SVF), Griffin Creek (GCF), and Tokul Creek (TCF) faults. The stream NNE of Carnation lies in the Cherry Creek Fault Zone. Southeastern limit of Southern Whidbey Island Fault at Duvall ( 3 ), other faults south of I-90 not shown. Tiger Mountain is the uplifted "Evc" formations southeast of Issaquah, between I-90 and Hwy. 18. (Figure 2 from DGER Geological Map GM-73 )
Approximate location of the Seattle Fault, showing eastern junction with SWIF and RMFZ. Western extension uncertain past Blue Hills uplift (marked "OP"). (Excerpt from DGER Geological Map GM-52 .)
Cross-section of one model of the Seattle uplift. Models differ on the nature of the ramp and details of the faults. (From Johnson et al. 2004a , figure 17D.)
Tacoma fault zone, with multiple southeast-striking strands, and part of the Olympia fault.(USGS [ 128 ] )
Excerpt from Bouguer gravity anomaly map ( above ), showing approximate locations of the Seattle Fault zone (line E), Seattle Uplift (red, and adjoining yellow border), parts of the Tacoma Fault Zone (green; the pair of green lines mark the Rosedale Monocline), Tacoma Basin (light blue), Dewatto Basin (northwestern portion of Tacoma Basin at C), Dewatto lineament/fault (vertical black bar at D), city of Belfair at white triangle.
Excerpt from main map showing the putative Hood Canal Fault (dashed line) running up Hood Canal and Dabob Bay, the Dabob Bay Fault Zone (DBFZ), Saddle Mountain faults (SM), and western end of the Seattle Fault zone (approximate).
In red: Saddle Mountain faults (west and east) extension to the southwest inferred from aeromagnetic and LIDAR evidence, Dow Mountain fault (offset by SM east), and Frigid Creek fault.
Excerpt from Geologic Map GM-34 , showing faults in the Centralia—Chehalis Coal District, Lewis County, Washington. Doty—Salzer Creek Fault runs east–west between Centralia and Chehalis (black squares). Map available on-line . Click on image for enlargement.
Mid-crustal (10–20 km deep) seismicity in western Washington. (Fig. 48 from USGS OFR 99-311 )
The Southern Washington Cascades Conductor (SWCC, yellow) located at depth approximately between Mount St. Helens (MSH), Mount Adams (MA), Goat Rocks (GR), Mount Rainier (MR), and Riffe Lake, with a lobe extending towards Tiger Mountain (TM). Also shown: Entiat Fault, Straight Creek Fault (inactive, southern continuation unknown), Southern Whidbey Island Fault, Rattlesnake Mountain Fault Zone, Olympic–Wallowa Lineament , White River/Naches River fault, Rimrock Lake Inlier (outlined in green), surface outcrops of the Crescent Formation (outlined in brown), Olympia Structure, Portland Hills fault zone.