The Cascadia subduction zone (also referred to as the Cascadia fault) is a subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to northern California. It is a very long sloping fault that separates the Juan de Fuca and North America plates. New ocean floor is being created offshore of Washington and Oregon. As more material wells up along the ocean ridge, the ocean floor is pushed toward and beneath the continent. The Cascadia Subduction Zone is where the two plates meet. Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism that has included such notable eruptions as Mazama (Crater Lake) several thousand years ago and Mount St. Helens in 1980.[1] Major cities affected by a disturbance in this subduction zone would include Vancouver, Seattle, Portland, and Victoria.
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The zone separates the Juan de Fuca Plate, Explorer Plate, Gorda Plate, and North American Plate. Here, the oceanic crust of the Pacific Ocean sinks beneath the continent at a rate of 40 mm/yr.[2]
The width of the Cascadia subduction zone varies along its length, depending on the temperature of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As it becomes hotter and more molten, it eventually loses the ability to store mechanical stress and generates earthquakes.On the Hyndman and Wang diagram the "locked" zone is storing up energy for an earthquake, and the "transition" zone, although somewhat plastic, could probably rupture.[3]
The Cascadia subduction zone presents a challenge to current tectonic theory which generally holds that subduction occurs as a plate becomes older, denser and thicker with distance from the ridge which contributes new material to it. In the case of Cascadia, its associated ridge is just a few hundred miles (and in places less) distant (or away) from the subduction zone. This puzzle is a matter of ongoing research and discussion. The current hypothesis which attempts to explain it is that once subduction begins, it continues by the process of slab-pull, i.e. the weight of the previously subducted segment exerts a force behind it on the remaining slab above the subduction zone, regardless of its density, pulling it downward.
The Cascadia subduction zone runs from triple junctions at its north and south ends. On the north just south of Queen Charlotte Island, it intersects the Queen Charlotte Fault and the Explorer Ridge. On the south, just off of Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino fault zone at the Mendocino Triple Junction.
The Cascadia subduction zone can produce very large earthquakes ("megathrust earthquakes"), magnitude 9.0 or greater, if rupture occurs over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Great Subduction Zone earthquakes are the largest earthquakes in the world, and can exceed magnitude 9.0. Earthquake size is proportional to fault area, and the Cascadia Subduction Zone is a very long sloping fault that stretches from mid-Vancouver Island to Northern California. It separates the Juan de Fuca and North America plates. Because of the very large fault area, the Cascadia Subduction Zone could produce a very large earthquake. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers (about 40 miles) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.[4]
In 1999, a group of Continuous Global Positioning System 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.[5] The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures.[6]
The last known great earthquake in the northwest was in January of 1700, the Cascadia Earthquake. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return time of 300 to 600 years. There is also evidence of accompanying tsunamis with every earthquake, and one line of evidence for these earthquakes is tsunami damage, and through Japanese records of tsunamis.[7]
A future rupture of the Cascadia Subduction Zone would cause widespread destruction throughout the Pacific Northwest.
Other similar subduction zones in the world usually have such earthquakes every 100–200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.
Studies of past earthquake traces on both the northern San Andreas Fault 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 seems to have been a major exception to this correlation, however, as it was not preceded by a major Cascadia quake. [8]
Recent findings concluded the Cascadia subduction zone was more hazardous than previously suggested. The feared next major earthquake has some geologists predicting a 10 to 14% probability that the Cascadia Subduction will rupture in the next 50 years, producing an event of magnitude 9 or higher.[9] Geologists have also determined the Pacific Northwest is not prepared for such a "Colossal" earthquake. The tsunami produced may reach heights of approximately 30 meters (100 ft).[10]
The Cascade Volcanic Arc is a continental island arc that extends from northern California to the coastal mountains of British Columbia.[11] The arc consists of a series of Quaternary age stratovolcanoes that grew on top of pre-existing geologic materials that ranged from Miocene volcanics to glacial ice.[12] The Cascade Volcanic arc lie approximately 100 km inland from the coast, and form a north-to-south oriented chain of peaks that average over 3,000 m (10,000 ft) in elevation.[13] The major peaks from south to north include:
The most active volcanoes in the chain include Mt. St. Helens, Mt. Baker, Lassen Peak, and Mt. Hood. St. Helens captured worldwide attention when it erupted catastrophically in 1980.[14] St. Helens continues to rumble albeit more quietly, emitting occasional steam plumes and experiencing small earthquakes, both signs of continuing magmatic activity.[15] 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 to dacitic magma, and the more siliceous (and explosive) rhyolite.
The volcanoes within the subduction zone include:
Coordinates: 45°N 124°W / 45°N 124°W
The Cascadia subduction zone (also referred to as the Cascadia fault) is a subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to northern California. It is a very long sloping fault that separates the Juan de Fuca and North America plates. New ocean floor is being created offshore of Washington and Oregon. As more material wells up along the ocean ridge, the ocean floor moves toward and beneath the continent. The North American Plate also moves, in a general southwest direction, overriding the oceanic plate. The Cascadia Subduction Zone is where the two plates meet. Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism that has included such notable eruptions as Mazama (Crater Lake) several thousand years ago and Mount St. Helens in 1980.[1] Major cities affected by a disturbance in this subduction zone would include Vancouver, Seattle, Portland, and Victoria.
Contents |
The zone separates the Juan de Fuca Plate, Explorer Plate, Gorda Plate, and North American Plate. Here, the oceanic crust of the Pacific Ocean sinks beneath the continent at a rate of 40 mm/yr.[2]
The width of the Cascadia subduction zone varies along its length, depending on the temperature of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As it becomes hotter and more molten, it eventually loses the ability to store mechanical stress and generates earthquakes.On the Hyndman and Wang diagram the "locked" zone is storing up energy for an earthquake, and the "transition" zone, although somewhat plastic, could probably rupture.[3]
At the Cascadia subduction zone, the oceanic plate subducts under the continental plate, which is what normally happens at an oceanic-continental convergent boundary.
The Cascadia subduction zone runs from triple junctions at its north and south ends. On the north just south of Queen Charlotte Island, it intersects the Queen Charlotte Fault and the Explorer Ridge. On the south, just off of Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino fault zone at the Mendocino Triple Junction.
The Cascadia subduction zone can produce very large earthquakes ("megathrust earthquakes"), magnitude 9.0 or greater, if rupture occurs over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Great Subduction Zone earthquakes are the largest earthquakes in the world, and can exceed magnitude 9.0. Earthquake size is proportional to fault area, and the Cascadia Subduction Zone is a very long sloping fault that stretches from mid-Vancouver Island to Northern California. It separates the Juan de Fuca and North America plates. Because of the very large fault area, the Cascadia Subduction Zone could produce a very large earthquake. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers (about 40 miles) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.[4]
In 1999, a group of Continuous Global Positioning System 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.[5] The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures.[6]
The last known great earthquake in the northwest was in January of 1700, the Cascadia Earthquake. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return time of 300 to 600 years. There is also evidence of accompanying tsunamis with every earthquake, and one line of evidence for these earthquakes is tsunami damage, and through Japanese records of tsunamis.[7]
A future rupture of the Cascadia Subduction Zone would cause widespread destruction throughout the Pacific Northwest.
Other similar subduction zones in the world usually have such earthquakes every 100–200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.
Studies of past earthquake traces on both the northern San Andreas Fault 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 seems to have been a major exception to this correlation, however, as it was not preceded by a major Cascadia quake. [8]
Recent findings concluded the Cascadia subduction zone was more hazardous than previously suggested. The feared next major earthquake has some geologists predicting a 10 to 14% probability that the Cascadia Subduction Zone will produce an event of magnitude 9 or higher in the next 50 years[9], although the most recent studies suggest that this risk could be as high as 37%.[10][11] Geologists have also determined the Pacific Northwest is not prepared for such a "Colossal" earthquake. The tsunami produced may reach heights of approximately 30 meters (100 ft).[12]
The Cascade Volcanic Arc is a continental volcanic arc that extends from northern California to the coastal mountains of British Columbia.[13] The arc consists of a series of Quaternary age stratovolcanoes that grew on top of pre-existing geologic materials that ranged from Miocene volcanics to glacial ice.[14] The Cascade Volcanic arc lie approximately 100 km inland from the coast, and form a north-to-south oriented chain of peaks that average over 3,000 m (10,000 ft) in elevation.[15] The major peaks from south to north include:
The most active volcanoes in the chain include Mt. St. Helens, Mt. Baker, Lassen Peak, and Mt. Hood. St. Helens captured worldwide attention when it erupted catastrophically in 1980.[16] St. Helens continues to rumble albeit more quietly, emitting occasional steam plumes and experiencing small earthquakes, both signs of continuing magmatic activity.[17] 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 to dacitic magma, and the more siliceous (and explosive) rhyolite.
The volcanoes within the subduction zone include:
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