Tacoma Fault
Encyclopedia
The Tacoma Fault, just north of the city of Tacoma, Washington
, is an active east–west striking north dipping reverse fault with approximately 35 miles (56 km) of identified surface rupture. It is believed capable of generating 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.
mapping; trenching and other paleoseismological studies have documented late Holocene
uplift. It extends west to the small town of Allyn (near the tip of Hood Canal), terminating at the same north-striking geophysical anomaly (tentatively named the Tahuya Fault) that terminates the Seattle Fault to the north. To the east one strand is aligned with Commencement Bay and the Puyallup River, other strands (or related faults) cross the East Passage of south-central Puget Sound. How far east these faults extend is not known, but probably as far as the Kent Valley.
The Tacoma Fault Zone marks the south end of the Seattle Uplift, of which the similar and related Seattle Fault Zone
marks the north end. This uplift is believed to be either a slab of rock about 15 km thick being pushed up a ramp, or a wedge being popped up between these two faults, by tectonic forces from the south or south-west as tectonic plates
riding on top of the Juan de Fuca Plate
are pushed against the North American craton.
The relationship of the Seattle Uplift with other neighboring blocks, and the nature of the faults between them, is not well known. If tectonic strain is from the south, and therefore perpendicular to the Seattle and Tacoma Faults, the motion on them should be entirely dip-slip (vertical). If tectonic strain is from the south-west (see adjoining diagram), perpendicular to the Rosedale monocline, and also to the Olympia Fault (south-west boundary of the Tacoma Basin) and South Whidbey Island Fault (north-east of the Seattle Basin), both of which are parallel to the Rosedale monocline (and also to the Olympic-Wallowa Lineament
, whose significance here is not known), then there should be some component of left-lateral strike-slip motion on parts of the Seattle and Tacoma faults. There has been a suggestion that the position of the Seattle and Tacoma faults may correlated with strain accumulation in the Olympic Mountains
(to the west), but this is yet to be worked out.
Calculations of ground motions for a M 7.1 earthquake on the Tacoma Fault indicates that most of Tacoma would experience moderate damage (depending on type of construction and local conditions). Heavy damage would be expected in a zone just north of the fault, especially on Maury Island
, and extending across Federal Way to the cities of Auburn
and Kent
.
Computer simulations show that the same M 7.1 earthquake would generate a tsunami. It is expected that the industrial areas on Commencement Bay, most of the low-lying areas on the Puyallup River delta (including Fife
), and parts of Interstate 5 would be inundated within about five minutes. Strong shaking may cause slope failures, including landslides; underwater landslides may cause additional tsunamis. The small beach communities common along Puget Sound, usually at the base of high bluffs, will thus be doubly endangered, by landslides and tsunamis.
Tacoma, Washington
Tacoma is a mid-sized urban port city and the county seat of Pierce County, Washington, United States. The city is on Washington's Puget Sound, southwest of Seattle, northeast of the state capital, Olympia, and northwest of Mount Rainier National Park. The population was 198,397, according to...
, is an active east–west striking north dipping reverse fault with approximately 35 miles (56 km) of identified surface rupture. It is believed capable of generating 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
Seattle Fault
The Seattle Fault is a zone of multiple shallow east-west thrust faults that cross the Puget Sound Lowland and through Seattle in the vicinity of Interstate Highway 90...
24 miles (38 km) to the north.
Geology
The Tacoma Fault – actually a zone of connected faults – was first suspected from gravitational surveying in the 1960s, subsequently confirmed by seismic reflection and other geophysical data, and traced by detailed LIDARLIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
mapping; trenching and other paleoseismological studies have documented late Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
uplift. It extends west to the small town of Allyn (near the tip of Hood Canal), terminating at the same north-striking geophysical anomaly (tentatively named the Tahuya Fault) that terminates the Seattle Fault to the north. To the east one strand is aligned with Commencement Bay and the Puyallup River, other strands (or related faults) cross the East Passage of south-central Puget Sound. How far east these faults extend is not known, but probably as far as the Kent Valley.
The Tacoma Fault Zone marks the south end of the Seattle Uplift, of which the similar and related Seattle Fault Zone
Seattle Fault
The Seattle Fault is a zone of multiple shallow east-west thrust faults that cross the Puget Sound Lowland and through Seattle in the vicinity of Interstate Highway 90...
marks the north end. This uplift is believed to be either a slab of rock about 15 km thick being pushed up a ramp, or a wedge being popped up between these two faults, by tectonic forces from the south or south-west as tectonic plates
Tectonic Plates
Tectonic Plates is a 1992 independent Canadian film directed by Peter Mettler. Mettler also wrote the screenplay based on the play by Robert Lepage. The film stars Marie Gignac, Céline Bonnier and Robert Lepage.-Plot summary:...
riding on top of the Juan de Fuca Plate
Juan de Fuca Plate
The Juan de Fuca Plate, named after the explorer of the same name, is a tectonic plate, generated from the Juan de Fuca Ridge, and subducting under the northerly portion of the western side of the North American Plate at the Cascadia subduction zone...
are pushed against the North American craton.
The relationship of the Seattle Uplift with other neighboring blocks, and the nature of the faults between them, is not well known. If tectonic strain is from the south, and therefore perpendicular to the Seattle and Tacoma Faults, the motion on them should be entirely dip-slip (vertical). If tectonic strain is from the south-west (see adjoining diagram), perpendicular to the Rosedale monocline, and also to the Olympia Fault (south-west boundary of the Tacoma Basin) and South Whidbey Island Fault (north-east of the Seattle Basin), both of which are parallel to the Rosedale monocline (and also to the Olympic-Wallowa Lineament
Olympic-Wallowa Lineament
The Olympic-Wallowa lineament – first reported by cartographer Erwin Raisz in 1945 on a relief map of the continental United States – is a physiographic feature of unknown origin in the state of Washington running approximately from the town of Port Angeles, on the Olympic Peninsula...
, whose significance here is not known), then there should be some component of left-lateral strike-slip motion on parts of the Seattle and Tacoma faults. There has been a suggestion that the position of the Seattle and Tacoma faults may correlated with strain accumulation in the Olympic Mountains
Olympic Mountains
The Olympic Mountains is a mountain range on the Olympic Peninsula of western Washington in the United States. The mountains, part of the Pacific Coast Ranges, are not especially high - Mount Olympus is the highest at - but the western slopes of the Olympics rise directly out of the Pacific...
(to the west), but this is yet to be worked out.
Hazard
Based on observed length (actual length is probably substantially greater) the Tacoma Fault is believed capable of generating an earthquake of at least magnitude 7, comparable to the earthquake on the Seattle Fault 1100 years ago (A.D. 900—930). Although this would release only one percent of the energy of a magnitude 9 subduction zone earthquake, in being closer to the surface and confined to a smaller area damage would be more severe. It has been estimated that such an earthquake on the similar Seattle Fault would damage 80 bridges in the Seattle—Tacoma highway corridor, comparable to an estimated 87 bridges damaged in all of western Washington from a M 9 subduction earthquake. For a M 6.7 quake on the Tacoma Fault it was estimated that 20 to 35 bridges would be damaged; losses to the regional economy due just to the damaged highway infrastructure would be over $3 billion.Calculations of ground motions for a M 7.1 earthquake on the Tacoma Fault indicates that most of Tacoma would experience moderate damage (depending on type of construction and local conditions). Heavy damage would be expected in a zone just north of the fault, especially on Maury Island
Maury Island
Maury Island is a small island in Puget Sound in the U.S. state of Washington. It is connected to Vashon Island by an isthmus built by the United States Army Corps of Engineers. Before construction of the isthmus, the island was connected to Vashon only during low tide. The island is rural with...
, and extending across Federal Way to the cities of Auburn
Auburn, Washington
-Parks:Auburn has an extensive system of parks, open space and urban trails comprising 29 developed parks, 5 undeveloped sites under planning, 2 skate parks, 2 water roatary parks, and over of trails , and almost of open space for passive and active recreation.-Environmental Park:The Auburn...
and Kent
Kent, Washington
Kent is a city located in King County, Washington, United States, and is the third largest city in King County and the sixth largest in the state. An outlying suburb of Seattle, Kent is also the corporate home for companies such as REI and Oberto Sausage...
.
Computer simulations show that the same M 7.1 earthquake would generate a tsunami. It is expected that the industrial areas on Commencement Bay, most of the low-lying areas on the Puyallup River delta (including Fife
Fife, Washington
Fife is a city in Pierce County, Washington, United States. The population was 9,173 at the 2010 census.-History:In 1940, it was described as "Fife...at a valley crossroads in the midst of a thickly settled berry growing and truck-gardening district is represented by a string of markets, taverns,...
), and parts of Interstate 5 would be inundated within about five minutes. Strong shaking may cause slope failures, including landslides; underwater landslides may cause additional tsunamis. The small beach communities common along Puget Sound, usually at the base of high bluffs, will thus be doubly endangered, by landslides and tsunamis.
External links
- USGS Quaternary fault and fold database, Fault number 581 Comprehensive description of and bibliography for the Tacoma Fault.
- Earthquake Hazard at Puyallup (south-east of Tacoma)