The Cascadia subduction zone is a convergent

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

that stretches from northern

Vancouver Island Vancouver Island is an island in the northeastern Pacific Ocean and part of the Canadian Provinces and territories of Canada, province of British Columbia. The island is in length, in width at its widest point, and in total area, while are o ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

Northern California Northern California (colloquially known as NorCal) is a geographic and cultural region that generally comprises the northern portion of the U.S. state of California. Spanning the state's northernmost 48 counties, its main population centers incl ...

in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. It is a very long, sloping subduction zone where the

Explorer Exploration refers to the historical practice of discovering remote lands. It is studied by geographers and historians. Two major eras of exploration occurred in human history: one of convergence, and one of divergence. The first, covering most ...

Juan de Fuca Juan de Fuca (10 June 1536, Cefalonia 23 July 1602, Cefalonia)Greek Consulate of Vancouver,Greek Pioneers: Juan de Fuca. was a Greeks, Greek maritime pilot, pilot who served Philip II of Spain, PhilipII of Spanish Empire, Spain. He is best know ...

, and Gorda plates move to the east and slide below the much larger mostly continental

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

. The zone varies in width and lies offshore beginning near

Cape Mendocino Cape Mendocino (Spanish: ''Cabo Mendocino'', meaning "Cape of Mendoza"), which is located approximately north of San Francisco, is located on the Lost Coast entirely within Humboldt County, California, United States. At 124° 24' 34" W longitude ...

, Northern California, passing through

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

and

Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...

, and terminating at about

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...

. The Explorer, Juan de Fuca, and Gorda plates are some of the remnants of the vast ancient

Farallon Plate The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west c ...

which is now mostly subducted under the North American Plate. The North American Plate itself is moving slowly in a generally southwest direction, sliding over the smaller plates as well as the huge oceanic

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

(which is moving in a northwest direction) in other locations such as the

San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...

in central and southern California. Tectonic processes active in the Cascadia subduction zone region include accretion,

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

, deep

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

s, and active

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...

of the Cascades. This volcanism has included such notable eruptions as

Mount Mazama Mount Mazama (''Giiwas'' in the Native American language Klamath language, Klamath) is a complex volcano in the state of Oregon, United States, in a segment of the Cascade Volcanoes, Cascade Volcanic Arc and Cascade Range. Most of the mountai ...

(

Crater Lake Crater Lake (Klamath language, Klamath: ''Giiwas'') is a volcanic crater lake in south-central Oregon in the western United States. It is the main feature of Crater Lake National Park and is famous for its deep blue color and water clarity. The ...

) about 7,500 years ago, the

Mount Meager massif The Mount Meager massif is a group of volcanic peaks in the of the Coast Mountains in southwestern British Columbia, Canada. Part of the Cascade Volcanic Arc of western North America, it is located north of Vancouver at the northern end of the ...

(

Bridge River Vent The Bridge River Vent is a volcanic crater in the Pacific Ranges of the Coast Mountains in southwestern British Columbia, Canada. It is located west of Bralorne on the northeastern flank of the Mount Meager massif. With an elevation of , it lies ...

) about 2,350 years ago, and

Mount St. Helens Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...

in 1980. USGS Major cities affected by a disturbance in this subduction zone include

Vancouver Vancouver ( ) is a major city in western Canada, located in the Lower Mainland region of British Columbia. As the List of cities in British Columbia, most populous city in the province, the 2021 Canadian census recorded 662,248 people in the ...

and

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

, British Columbia;

Seattle Seattle ( ) is a seaport city on the West Coast of the United States. It is the seat of King County, Washington. With a 2020 population of 737,015, it is the largest city in both the state of Washington and the Pacific Northwest regio ...

, Washington; and

Portland Portland most commonly refers to: * Portland, Oregon, the largest city in the state of Oregon, in the Pacific Northwest region of the United States * Portland, Maine, the largest city in the state of Maine, in the New England region of the northeas ...

, Oregon.

History

Tradition

There are no contemporaneous written records of the

1700 Cascadia earthquake The 1700 Cascadia earthquake occurred along the Cascadia subduction zone on January 26, 1700, with an estimated moment magnitude of 8.7–9.2. The megathrust earthquake involved the Juan de Fuca Plate from mid-Vancouver Island, south along the P ...

. Orally transmitted legends from the

Olympic Peninsula The Olympic Peninsula is a large arm of land in western Washington that lies across Puget Sound from Seattle, and contains Olympic National Park. It is bounded on the west by the Pacific Ocean, the north by the Strait of Juan de Fuca, and the ...

area tell of an epic battle between a thunderbird and a whale. In 2005, seismologist Ruth Ludwin set out to collect and analyze anecdotes from various

First Nations First Nations or first peoples may refer to: * Indigenous peoples, for ethnic groups who are the earliest known inhabitants of an area. Indigenous groups *First Nations is commonly used to describe some Indigenous groups including: **First Natio ...

groups. Reports from the Huu-ay-aht,

Makah The Makah (; Klallam: ''màq̓áʔa'')Renker, Ann M., and Gunther, Erna (1990). "Makah". In "Northwest Coast", ed. Wayne Suttles. Vol. 7 of ''Handbook of North American Indians'', ed. William C. Sturtevant. Washington, D.C.: Smithsonian Institut ...

Hoh Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "univer ...

Quileute The Quileute , are a Native American people in western Washington state in the United States, currently numbering approximately 2,000. They are a federally recognized tribe: the ''Quileute Tribe of the Quileute Reservation''. The Quileute peo ...

Yurok The Yurok (Karuk language: Yurúkvaarar / Yuru Kyara - "downriver Indian; i.e. Yurok Indian") are an Indigenous people from along the Klamath River and Pacific coast, whose homelands are located in present-day California stretching from Trinidad ...

, and

Duwamish Duwamish may refer to: * Duwamish tribe, a Native American tribe in Washington state * Duwamish River, in Washington state * ''Duwamish'' (fireboat) See also * Elliott Bay Elliott Bay is a part of the Central Basin region of Puget Sound. It is ...

peoples referred to earthquakes and saltwater floods. This collection of data allowed the researchers to come up with an estimated date range for the event; the midpoint was in the year 1701.

Ghost forests

During low tide one day in March 1986, paleogeologist Brian Atwater dug along

Neah Bay Neah Bay is a census-designated place (CDP) on the Makah Reservation in Clallam County, Washington, United States. The population was 865 at the 2010 census. It is across the Canada–US border from British Columbia. Originally called "Scarboro ...

with a ''nejiri gama'', a small hand hoe. Under a top layer of sand, he uncovered a distinct plant— arrowgrass—that had grown in a layer of marsh soil. This finding was evidence that the ground had suddenly sunk under sea level, causing saltwater to kill the vegetation. The event had happened so quickly that the top layer of sand sealed away the air, thus preserving centuries-old plants. In 1987, Atwater mounted another expedition paddling up the

Copalis River The Copalis River is a stream on the Olympic Peninsula in the U.S. state of Washington. It originates in the foothills on the southwest side of the Olympic Mountains and empties into the Pacific Ocean.USGS topographic maps accessed via https://mapp ...

with Dr. David Yamaguchi, who was then studying the eruptions of

. The pair happened upon a section of " ghost forest", so-called due to the dead, gray stumps left standing after a sudden inundation of salt water had killed them hundreds of years ago. Originally thought to have died slowly due to a gradual rise in sea level, closer inspection yielded a different story: the land plummeted up to two meters during an earthquake. Having initially tested

spruce A spruce is a tree of the genus ''Picea'' (), a genus of about 35 species of coniferous evergreen trees in the family Pinaceae, found in the northern temperate and boreal (taiga) regions of the Earth. ''Picea'' is the sole genus in the subfami ...

using tree-ring dating, they found that the stumps were too rotted to count all the outer rings. However, upon having examined those of the

western red cedar ''Thuja plicata'' is an evergreen coniferous tree in the cypress family Cupressaceae, native to western North America. Its common name is western redcedar (western red cedar in the UK), and it is also called Pacific redcedar, giant arborvitae ...

and comparing them to the living specimens meters away from the banks, they were able to approximate their year of death. There were rings up until the year 1699, indicating that the incident had occurred shortly thereafter. Root samples confirmed their conclusion, narrowing the time frame to the winter of 1699 to 1700. As with the arrowgrass site, the banks of the Copalis River are lined with a layer of marsh followed by a layer of sand. Jody Bourgeois and her team went on to demonstrate that the sand cover had originated with a tsunami surge rather than a storm surge. In 1995, an international team led by Alan Nelson of the

USGS The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, a ...

further corroborated these findings with 85 new samples from the rest of the

Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though ...

. All along British Columbia, Washington State, and Oregon, the coast had fallen due to a violent earthquake and been covered by sand from the subsequent tsunami. A further ghost forest was identified by Gordon Jacoby, a

dendrochronologist Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...

from Columbia University, underwater in Lake Washington. Unlike the other trees, these suffered from a landslide rather than a dip in the fault during a separate event around 900 CE.

Activity

In the 1960s, underground fractures were uncovered by oil companies in

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

. These were believed to be inactive through the 1990s. In the 1980s, geophysicists

Tom Heaton Thomas David Heaton (born 15 April 1986) is an English professional footballer who plays as a goalkeeper for club Manchester United. Heaton began his career in the Manchester United academy but was unable to break into the first team and sp ...

and

Hiroo Kanamori is a Japanese seismologist who has made fundamental contributions to understanding the physics of earthquakes and the tectonic processes that cause them. Career Kanamori and American seismologist Thomas C. Hanks developed the moment magnitud ...

Caltech The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...

compared the generally quiet Cascadia to more active subduction zones elsewhere in the

Ring of Fire The Ring of Fire (also known as the Pacific Ring of Fire, the Rim of Fire, the Girdle of Fire or the Circum-Pacific belt) is a region around much of the rim of the Pacific Ocean where many volcanic eruptions and earthquakes occur. The Ring o ...

. They found similarities to faults in Chile, Alaska, and Japan's

Nankai Trough The is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately offshore. The underlying fault, the ''Nankai megathrust,'' is the source of the devastating Nankai megathrust earthquakes, w ...

, locations known for

megathrust earthquake Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthqua ...

s, a conclusion that was met with skepticism from other geophysicists at the time.

Orphan tsunami

A 1996 study published by seismologist

Kenji Satake Kenji Satake is a Japanese seismologist who has made significant contributions to subduction and tsunami research. Along with Brian Atwater and David Yamaguchi, Satake assembled disparate pieces of information regarding a Japanese tsunami that h ...

supplemented the research by Atwater et al. with tsunami evidence across the Pacific. Japanese annals, which have recorded natural disasters since approximately 600 CE, had reports of a sixteen-foot tsunami that struck the coast of

Honshu Island , historically called , is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island sep ...

during the

Genroku was a after Jōkyō and before Hōei. The Genroku period spanned the years from the ninth month of 1688 to the third month of 1704. The reigning emperor was .Titsingh, Isaac. (1834). ''Annales des empereurs du japon'', p. 415. The period was ...

. Since no earthquake had been observed to produce it, scholars dubbed it an "orphan tsunami." Translating the

Japanese calendar Japanese calendar types have included a range of official and unofficial systems. At present, Japan uses the Gregorian calendar together with year designations stating the year of the reign of the current Emperor. The written form starts with t ...

, Satake found the incident had taken place around midnight of 27–28 January 1700, ten hours after the earthquake occurred. The original magnitude 9.0 earthquake in the Pacific Northwest had thus occurred around 9pm local time on 26 January 1700.

Geophysics

The Cascadia Subduction Zone (CSZ) is a long dipping fault that stretches from Northern Vancouver Island to

in northern California. It separates the Juan de Fuca and North America plates. New Juan de Fuca plate is created offshore along the

Juan de Fuca Ridge The Juan de Fuca Ridge is a mid-ocean spreading center and divergent plate boundary located off the coast of the Pacific Northwest region of North America. The ridge separates the Pacific Plate to the west and the Juan de Fuca Plate to the east ...

. USGS The Juan de Fuca plate moves toward, and eventually is shoved beneath, the continent (North American plate). The zone separates the Juan de Fuca Plate,

Explorer Plate The Explorer Plate is an oceanic tectonic plate beneath the Pacific Ocean off the west coast of Vancouver Island, Canada, which is partially subducted under the North American Plate. Along with the Juan de Fuca Plate and Gorda Plate, the Explor ...

Gorda Plate The Gorda Plate, located beneath the Pacific Ocean off the coast of northern California, is one of the northern remnants of the Farallon Plate. It is sometimes referred to (by, for example, publications from the USGS Earthquake Hazards Program) a ...

, and

. Here, the

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

of the Pacific Ocean has been sinking beneath the continent for about 200 million years, and currently does so at a rate of approximately 40 mm/ yr. At depths shallower than or so, the Cascadia zone is locked by friction while stress slowly builds up as the subduction forces act, until the fault's frictional strength is exceeded and the rocks slip past each other along the fault in a

. Below the plate interface exhibits

episodic tremor and slip Episodic tremor and slip (ETS) is a seismological phenomenon observed in some subduction zones that is characterized by non-earthquake seismic rumbling, or tremor, and slow slip along the plate interface. Slow slip events are distinguished from ...

. The width of the Cascadia subduction zone varies along its length, depending on the angle of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As the edge of the plate sinks and becomes hotter and more molten, the subducting rock eventually loses the ability to store mechanical stress;

s may result. On the Hyndman and Wang diagram (not shown, click on reference link below) the "locked" zone is storing up energy for an earthquake, and the "transition" zone, although somewhat plastic, could probably rupture. The Cascadia subduction zone runs from

triple junction A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can ...

s at its north and south ends. To the north, just below

Haida Gwaii Haida Gwaii (; hai, X̱aaydag̱a Gwaay.yaay / , literally "Islands of the Haida people") is an archipelago located between off the northern Pacific coast of Canada. The islands are separated from the mainland to the east by the shallow Hecat ...

, it intersects the

Queen Charlotte Fault The Queen Charlotte Fault is an active transform fault that marks the boundary of the North American plate and the Pacific plate. It is Canada's right-lateral strike-slip equivalent to the San Andreas Fault to the south in California. The Queen Ch ...

and the

Explorer Ridge The Explorer Ridge is a mid-ocean ridge, a divergent tectonic plate boundary located about west of Vancouver Island, British Columbia, Canada. It lies at the northern extremity of the Pacific spreading axis. To its east is the Explorer Plate, ...

. To the south, just off Cape Mendocino in California, it intersects the

and the

Mendocino Fracture Zone The Mendocino Fracture Zone is a fracture zone and transform boundary over 4000 km (2500 miles) long, starting off the coast of Cape Mendocino in far northern California. It runs westward from a triple junction with the San Andreas Fault a ...

at the

Mendocino Triple Junction The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in th ...

Recent seismicity

Subduction zones experience various types of earthquakes (or seismicity); including

slow earthquake A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. First detected using long term strain measurements, most ...

interplate earthquake An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates. Earthquakes of this type account for more than 90 percent of the total seismic energy released around the world. If one plate is trying to move past ...

s, and

intraplate earthquake The term intraplate earthquake refers to a variety of earthquake that occurs ''within the interior'' of a tectonic plate; this stands in contrast to an interplate earthquake, which occurs ''at the boundary'' of a tectonic plate. Intraplate earth ...

s. Unlike other subduction zones on Earth, Cascadia presently experiences low levels of seismicity and has not generated a

since January 26, 1700. Despite low levels of seismicity compared to other subduction zones, Cascadia hosts various types of earthquakes that are recorded by seismic and

geodetic Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...

instruments, such as

seismometer A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...

s and GNSS receivers. Tremor, a type of slow fault slip, occurs along almost the entire length of Cascadia at regular intervals of 13–16 months. Tremor occurs deeper on the subduction interface than the locked area where megathrust earthquakes occur. The depth of tremor along the subduction interface in Cascadia ranges from 28 km to 45 km, and the motion is so slow that it is not felt at the surface by people or animals, but it can be measured geodetically. The highest density of tremor activity in Cascadia occurs from northern Washington into southern Vancouver Island, and in northern California. Tremor in Cascadia is monitored by the Pacific Northwest Seismic Network's semi-automatic tremor detection system. The majority of

s, or earthquakes that occur near the boundaries of tectonic plates, near the Cascadia subduction zone occur in the

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

of the overriding

in Washington, west of the

Cascade Volcanic Arc The Cascade Volcanoes (also known as the Cascade Volcanic Arc or the Cascade Arc) are a number of volcanoes in a volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to Northern Calif ...

and east of where tremor occurs. These earthquakes are sometimes referred to as crustal earthquakes, and they are capable of causing significant damage due to their relatively shallow depths. A damaging magnitude 7 interplate earthquake occurred on the

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

around 900–930 CE that generated 3 meters of uplift and a 4-5 meter tsunami. A substantial number of

s also occur in northern California. Much less interplate seismicity occurs in Oregon compared to Washington and northern California, although Oregon hosts more

volcanic activity Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

than its neighboring states. Intraslab earthquakes, frequently associated with stresses within the subducting plate in convergent margins, occur most frequently in northern Cascadia along the west coast of Vancouver Island and in Puget Sound, and in southern Cascadia within the subducting

, near the

offshore of northern California. The

1949 Olympia earthquake The 1949 Olympia earthquake occurred on April 13 at with a moment magnitude of 6.7 and a maximum Mercalli Intensity of VIII (''Severe''). The shock was located in the area between Olympia and Tacoma, and was felt throughout the state, as well ...

was a damaging magnitude 6.7 intraslab earthquake that occurred at 52 km depth and caused 8 deaths. Another notable intraslab earthquake in the

region was the magnitude 6.8

2001 Nisqually earthquake The 2001 Nisqually earthquake occurred at on February 28, 2001 and lasted nearly a minute. The intraslab earthquake had a moment magnitude of 6.8 and a maximum Mercalli intensity of VIII (''Severe''). The epicenter was in the southern Puget So ...

. Intraslab earthquakes in Cascadia occur in areas where the subducting plate has high

curvature In mathematics, curvature is any of several strongly related concepts in geometry. Intuitively, the curvature is the amount by which a curve deviates from being a straight line, or a surface deviates from being a plane. For curves, the canonic ...

. Much of the seismicity that occurs off the coast of northern California is due to

intraplate deformation Intraplate deformation is the folding, breaking, or flow of the Earth's crust within plates instead of at their margins. This process usually occurs in areas with especially weak crust and upper mantle, such as the Tibetan Plateau (Figure 1). Int ...

within the

. Similar to the distribution of interplate earthquakes in Cascadia, intraslab earthquakes are infrequent in Oregon, with its strongest earthquake since statehood being the 5.6 magnitude 1993 Scotts Mills earthquake, an oblique-slip quake.

Megathrust earthquakes

Earthquake effects

Megathrust earthquake Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthqua ...

s are the most powerful earthquakes known to occur, and can exceed

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

9.0, which releases 1,000 times more energy than magnitude 7.0 and 1 million times more energy than a magnitude 5.0. They occur when enough energy (stress) has accumulated in the "locked" zone of the fault to cause a rupture. The magnitude of a megathrust earthquake is proportional to length of the rupture along the fault. The Cascadia Subduction Zone, which forms the boundary between the Juan de Fuca and North American plates, is a very long sloping fault that stretches from mid-Vancouver Island to Northern California. Because of the great length of the fault, the Cascadia Subduction Zone is capable of producing very large earthquakes if rupture occurs along its entire length. Thermal and deformation studies indicate that the region 60 kilometers (about 40 miles) downdip (east) of the deformation front (where plate deformation begins) is fully locked (the plates do not move past each other). Further downdip, there is a transition from fully locked to aseismic sliding. In 1999, a group of Continuous

Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...

sites registered a brief reversal of motion of approximately 2 centimeters (0.8 inches) over a 50 kilometer by 300 kilometer (about 30 mile by 200 mile) area. The movement was the equivalent of a 6.7 magnitude earthquake. The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures. In 2004, a study conducted by the Geological Society of America analyzed the potential for land subsidence along the Cascadia subduction zone. It postulated that several towns and cities on the west coast of Vancouver Island, such as

Tofino Tofino ( ) is a town of approximately 2,516 residents on the west coast of Vancouver Island in the Canadian province of British Columbia. The District of Tofino is located at the western terminus of Highway 4 on the tip of the Esowista Peninsu ...

and

Ucluelet Ucluelet (, also Ukee) is a district municipality (population 1,717) on the Ucluelet Peninsula on the west coast of Vancouver Island in British Columbia, Canada. Ucluelet means "people of the safe harbour" in the indigenous Nuu-chah-nulth (No ...

, are at risk for a sudden, earthquake initiated, 1–2 m subsidence.

San Andreas Fault connection

Studies of past earthquake traces on both the northern

and the southern Cascadia subduction zone indicate a correlation in time which may be evidence that quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas during at least the past 3,000 years or so. The evidence also shows the rupture direction going from north to south in each of these time-correlated events. The

1906 San Francisco earthquake At 05:12 Pacific Standard Time on Wednesday, April 18, 1906, the coast of Northern California was struck by a major earthquake with an estimated moment magnitude of 7.9 and a maximum Mercalli intensity of XI (''Extreme''). High-intensity sha ...

seems to have been a major exception to this correlation, however, as it was not preceded by a major Cascadia quake.

Earthquake timing

The last known great earthquake in the northwest was the

, years ago.

Geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...

evidence indicates that great earthquakes (> magnitude 8.0) may have occurred sporadically at least seven times in the last 3,500 years, suggesting a return time of about 500 years. Seafloor core evidence indicates that there have been forty-one subduction zone earthquakes on the Cascadia subduction zone in the past 10,000 years, suggesting a general average earthquake recurrence interval of only 243 years. Of these 41, nineteen have produced a "full margin rupture", wherein the entire fault opens up. By comparison, similar subduction zones in the world usually have such earthquakes every 100 to 200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip. There is also evidence of accompanying

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...

s with every earthquake. One strong line of evidence for these earthquakes is convergent timings for fossil damage from tsunamis in the

and historical Japanese records of tsunamis. USGS Professional Paper 1707 The next rupture of the Cascadia Subduction Zone is anticipated to be capable of causing widespread destruction throughout the

Forecasts of the next major earthquake

Prior to the 1980s, scientists thought that the subduction zone did not generate earthquakes like other subduction zones around the world, but research by Brian Atwater and

tied together evidence of a large tsunami on the Washington coast with documentation of an orphan tsunami in Japan (a tsunami without an associated earthquake). The two pieces of the puzzle were linked, and they then realized that the subduction zone was more hazardous than previously suggested. In 2009, some geologists predicted a 10% to 14% probability that the Cascadia Subduction Zone will produce an event of magnitude 9.0 or higher in the next 50 years. In 2010, studies suggested that the risk could be as high as 37% for earthquakes of magnitude 8.0 or higher. Geologists and civil engineers have broadly determined that the Pacific Northwest region is not well prepared for such a colossal earthquake. The earthquake is expected to be similar to the

2011 Tōhoku earthquake and tsunami The occurred at 14:46 JST (05:46 UTC) on 11 March. The magnitude 9.0–9.1 (M) undersea megathrust earthquake had an epicenter in the Pacific Ocean, east of the Oshika Peninsula of the Tōhoku region, and lasted approximately six minutes ...

, because the rupture is expected to be as long as the

2004 Indian Ocean earthquake and tsunami An earthquake and a tsunami, known as the Boxing Day Tsunami and, by the scientific community, the Sumatra–Andaman earthquake, occurred at 07:58:53 local time (UTC+7) on 26 December 2004, with an epicentre off the west coast of northern Suma ...

. The resulting

might reach heights of approximately 30 meters (100 ft).

FEMA The Federal Emergency Management Agency (FEMA) is an agency of the United States Department of Homeland Security (DHS), initially created under President Jimmy Carter by Presidential Reorganization Plan No. 3 of 1978 and implemented by two Ex ...

estimates some 13,000 fatalities from such an event, with another 27,000 injured, which would make it the deadliest natural disaster in American, and North American, history. FEMA further predicts that a million people will be displaced, with yet another 2.5 million requiring food and water. An estimated 1/3 of public safety workers will not respond to the disaster due to a collapse in infrastructure and a desire to ensure the safety of themselves and their loved ones. Other analyses predict that even a magnitude 6.7 earthquake in Seattle would result in 7,700 dead and injured, $33 billion in damage, 39,000 buildings severely damaged or destroyed, and 130 simultaneous fires.

Cascade Volcanic Arc

The Cascade Volcanic Arc is a continental volcanic arc that extends from northern

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

to the coastal peninsula of

Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...

. The arc consists of a series of Quaternary age stratovolcanoes that grew on top of pre-existing geologic materials that ranged from

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

volcanics to

glacial ice A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...

. The Cascade Volcanic arc is located approximately 100 km inland from the coast, and forms a north-to-south chain of peaks that average over 3,000 m (10,000 ft) in elevation. The major peaks from south to north include: *

Lassen Peak Lassen Peak ( ), commonly referred to as Mount Lassen, is a lava dome volcano and the southernmost active volcano in the Cascade Range of the Western United States. Located in the Shasta Cascade region of Northern California, it is part of the ...

and

Mount Shasta Mount Shasta ( Shasta: ''Waka-nunee-Tuki-wuki''; Karuk: ''Úytaahkoo'') is a potentially active volcano at the southern end of the Cascade Range in Siskiyou County, California. At an elevation of , it is the second-highest peak in the Cascades ...

(California) *

(Mazama), Three Sisters, Mount Jefferson,

Mount Hood Mount Hood is a potentially active stratovolcano in the Cascade Volcanic Arc. It was formed by a subduction zone on the Pacific coast and rests in the Pacific Northwest region of the United States. It is located about east-southeast of Portlan ...

(Oregon) * Mount Adams,

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

Glacier Peak Glacier Peak or Dakobed (known in the Sauk-Suiattle dialect of the Lushootseed language as "Tda-ko-buh-ba" or "Takobia") is the most isolated of the five major stratovolcanoes (composite volcanoes) of the Cascade Volcanic Arc in the U.S state of ...

Mount Baker Mount Baker (Lummi: '; nok, Kw’eq Smaenit or '), also known as Koma Kulshan or simply Kulshan, is a active glacier-covered andesitic stratovolcano in the Cascade Volcanic Arc and the North Cascades of Washington in the United States. Mount ...

(Washington) *

Mount Garibaldi Mount Garibaldi (known as Nch'kaý to the indigenous Squamish people) is a dormant stratovolcano in the Garibaldi Ranges of the Pacific Ranges in southwestern British Columbia, Canada. It has a maximum elevation of and rises above the surroundi ...

and

(British Columbia) The most active volcanoes in the chain include Mount St. Helens, Mount Baker, Lassen Peak, Mount Shasta, and Mount Hood. Mount St. Helens captured worldwide attention when it erupted catastrophically in 1980. St. Helens continues to rumble, albeit more quietly, emitting occasional steam plumes and experiencing small earthquakes, both signs of continuing magmatic activity. Most of the volcanoes have a main, central vent from which the most recent eruptions have occurred. The peaks are composed of layers of solidified

andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

dacitic Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyol ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

, and the more siliceous (and explosive)

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

Volcanoes above the subduction zone

The volcanoes above the subduction zone include: *

Silverthrone Caldera The Silverthrone Caldera is a potentially active caldera complex in southwestern British Columbia, Canada, located over northwest of the city of Vancouver and about west of Mount Waddington in the Pacific Ranges of the Coast Mountains. The cald ...

Mount Cayley Mount Cayley is an eroded but potentially active stratovolcano in the Pacific Ranges of southwestern British Columbia, Canada. Located north of Squamish and west of Whistler, the volcano resides on the edge of the Powder Mountain Icefield. ...

* Mount Adams *

* Mount Jefferson * Three Sisters *

Newberry Volcano Newberry Volcano is a large active shield-shaped stratovolcano located about south of Bend, Oregon, United States, east of the major crest of the Cascade Range, within the Newberry National Volcanic Monument. Its highest point is Paulina Peak ...

(Crater Lake) *

Mount McLoughlin Mount McLoughlin is a dormant steep-sided stratovolcano, or composite volcano, in the Cascade Range of southern Oregon and within the United States Sky Lakes Wilderness. It is one of the volcanic peaks in the Cascade Volcanic Arc, within the Hi ...

Medicine Lake Volcano Medicine Lake Volcano is a large shield volcano in northeastern California about northeast of Mount Shasta. The volcano is located in a zone of east-west crustal extension east of the main axis of the Cascade Volcanic Arc and the Cascade R ...

* Black Butte

References

External links

* *
"Cascadia Peril '09"
at dailywireless.org
9.0 Shakemap Scenario

The Really Big One
–

University of California Television University of California Television (known simply as UCTV) is a 24-hour television channel presenting educational and enrichment programming from the campuses, national laboratories, and affiliated institutions of the University of California sy ...

Great Earthquakes of the Pacific Northwest
–

Central Washington University Central Washington University (CWU) is a public university in Ellensburg, Washington. Founded in 1891, the university consists of four divisions: the President's Division, Business and Financial Affairs, Operations, and Academic and Student Lif ...

Toast, tsunamis and the really big one , Chris Goldfinger , TEDxMtHood
– TEDx {{Authority control Subduction zones Geology of Oregon Geology of Washington (state) Geologic provinces of California Geology of British Columbia Natural hazards in British Columbia Seismic zones of British Columbia Cascade Volcanoes