Cascadia subduction zone
Coordinates: The Cascadia subduction zone (also referred to as the Cascadia fault) is a convergent plate boundary that stretches from northern Vancouver Island to northern California. It is a very long sloping subduction zone fault that separates the Juan de Fuca and North America plates. The thinner oceanic plate is subducting beneath the thicker continental plate offshore of British Columbia, Washington and Oregon. The North American Plate 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 Mount Mazama (Crater Lake) about 7,500 years ago, Mount Meager about 2,350 years ago, and Mount St. Helens in 1980. Major cities affected by a disturbance in this subduction zone would include Vancouver and Victoria, British Columbia; Seattle, Washington; Portland, Oregon; and Sacramento, California.
The Cascadia Subduction Zone (CSZ) "megathrust" fault is a 1,000 km (620 mi) long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino 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 plate moves toward, and eventually is shoved beneath, the continent (North American plate). The zone separates the Juan de Fuca Plate, Explorer Plate, Gorda Plate, and North American Plate. Here, the oceanic crust 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 30 km (19 mi) or so, the CSZ is locked by friction while strain 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 "megathrust" earthquake.
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 (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 junctions at its north and south ends. To the north, just below Queen Charlotte Island, it intersects the Queen Charlotte Fault and the Explorer Ridge. To 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 magnitude is proportional to area of fault rupture, 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 American 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.
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. The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures.
|2005 source||2003 source||(years)|
|NS)about 9 pm, January 26, 1700 (||780|
|690-730 CE||550-750 CE||330|
|350-420 CE||250-320 CE||910|
|980-890 BCE||910-780 BCE||250|
|1440-1340 BCE||1150-1220 BCE||unknown|
The last known great earthquake in the northwest was the 1700 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.
Other 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.
San Andreas Fault connection
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.
Forecasts of the next major earthquake
Prior to the 1980s, scientists thought that the subduction zone just did not generate earthquakes like the other subduction zones around the world, but research by Brian Atwater and Kenji Satake tied together evidence of 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. 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; however, the most recent studies suggest that this risk could be as high as 37% for earthquakes of magnitude 8 or higher.
Geologists and civil engineers have broadly determined that the Pacific Northwest region is not well prepared for such a colossal earthquake. The tsunami produced may reach heights of approximately 30 meters (100 ft). The earthquake is expected to be similar to the 2011 Tōhoku earthquake and tsunami, as the rupture is expected to be as long as the 2004 Indian Ocean earthquake and tsunami.
Cascade Volcanic Arc
The Cascade Volcanic Arc is a continental volcanic arc that extends from northern California to the coastal mountains of British Columbia. 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. 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 and Mt. Shasta (California)
- Crater Lake (Mazama), Three Sisters, Mt. Jefferson, Mt. Hood (Oregon)
- Mt. Adams, Mt. St. Helens, Mt. Rainier, Glacier Peak, Mt. Baker (Washington)
- Mt. Garibaldi and Mt. Meager (British Columbia)
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. 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 to dacitic magma, and the more siliceous (and explosive) rhyolite.
The volcanoes above the subduction zone include:
- Silverthrone Caldera
- Mount Meager
- Mount Cayley
- Mount Garibaldi
- Mount Baker
- Glacier Peak
- Mount Rainier
- Mount St. Helens
- Mount Adams
- Mount Hood
- Mount Jefferson
- Three Sisters
- Newberry Volcano
- Mount Mazama
- Mount McLoughlin
- Medicine Lake Volcano
- Mount Shasta
- Lassen Peak
- Black Butte
- Cascadia (disambiguation)
- Cascade Range
- Cascade Volcanoes
- Geology of the Pacific Northwest
- North Cascades National Park
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- 9.0 Shakemap Scenario