Cascadia subduction zone
45°N 124°W / 45°N 124°W The Cascadia subduction zone is a 960 km (600 mi) fault at a
The Explorer, Juan de Fuca, and Gorda plates are some of the remnants of the vast ancient Farallon Plate 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 (which is moving in a northwest direction) in other locations such as the San Andreas Fault in central and southern California.
Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism of the Cascades. This volcanism has included such notable eruptions as Mount Mazama (Crater Lake) about 7,500 years ago, the Mount Meager massif (Bridge River Vent) about 2,350 years ago, and Mount St. Helens in 1980.[3] Major cities affected by a disturbance in this subduction zone include Vancouver and Victoria, British Columbia; Seattle, Washington; and Portland, Oregon.
History
Tradition
There are no contemporaneous written records of the
Ghost forests
During low tide one day in March 1986, paleogeologist
In 1987, Atwater mounted another expedition paddling up the Copalis River with Dr. David Yamaguchi, who was then studying the eruptions of Mount St. Helens.[7] 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.[5] Originally thought to have died slowly due to a gradual rise in sea level,[2] closer inspection yielded a different story: the land plummeted up to two meters during an earthquake.[5] Having initially tested spruce 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 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.[4][7]
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.[5]
In 1995, an international team led by Alan Nelson of the
A further ghost forest was identified by Gordon Jacoby, a dendrochronologist from Columbia University, 60 feet (18 m) 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.[6]
Activity
In the 1960s, underground fractures were uncovered by oil companies in Puget Sound. These were believed to be inactive through the 1990s.[6]
In the 1980s, geophysicists Tom Heaton and Hiroo Kanamori of Caltech compared the generally quiet Cascadia to more active subduction zones elsewhere in the Ring of Fire. They found similarities to faults in Chile, Alaska, and Japan's Nankai Trough, locations known for megathrust earthquakes, a conclusion that was met with skepticism from other geophysicists at the time.[7]
Orphan tsunami
A 1996 study published by seismologist
Geophysics
The Cascadia Subduction Zone (CSZ) is a 1,000 km (620 mi) long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino 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.[8][9]
The Juan de Fuca plate moves toward, and eventually is pushed under 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.[8][9]
At depths shallower than 30 km (19 mi) 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 megathrust earthquake. Below 30 km (19 mi) the plate interface exhibits episodic tremor and slip.
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; earthquakes 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.[10]
The Cascadia subduction zone runs from triple junctions at its north and south ends. To the north, just below Haida Gwaii, it intersects the Queen Charlotte Fault and the Explorer Ridge. To the south, just off Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino Fracture Zone at the Mendocino Triple Junction.
Recent seismicity
Subduction zones experience various types of earthquakes (or seismicity); including slow earthquakes, megathrust earthquakes, interplate earthquakes, and intraplate earthquakes. Unlike other subduction zones on Earth, Cascadia presently experiences low levels of seismicity and has not generated a megathrust earthquake 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 instruments, such as seismometers and GNSS receivers.
Tremor, a type of slow fault slip, occurs along almost the entire length of Cascadia[11] at regular intervals of 13–16 months.[12] 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,[13] 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.[13] Tremor in Cascadia is monitored by the Pacific Northwest Seismic Network's semi-automatic tremor detection system.[13]
The majority of
Intraslab earthquakes, frequently associated with stresses within the subducting plate in
Megathrust earthquakes
Earthquake effects
Megathrust earthquakes are the most powerful earthquakes known to occur, and can exceed magnitude 9.0, which releases 1,000 times more energy than magnitude 7.0 and 1 million times more energy than a magnitude 5.0.[18][19][20] 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.[18]
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.[18]
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.[21] The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures.[22]
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 and Ucluelet, are at risk for a sudden, earthquake initiated, 1–2 m subsidence.[23]
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.[24]
Earthquake timing
estimated year | interval | |
---|---|---|
2005 source[25] | 2003 source[26] | (years) |
NS) | about 9 pm, January 26, 1700 (780 | |
CE | 780–1190880–960 CE | 210 |
690–730 CE | 550–750 CE | 330 |
350–420 CE | 250–320 CE | 910 |
BCE | 660-440610–450 BCE | 400 |
980–890 BCE | 910–780 BCE | 250 |
1440–1340 BCE | 1150–1220 BCE | unknown |
The last known great earthquake in the northwest was the
There is also evidence of accompanying tsunamis with every earthquake. One strong line of evidence for these earthquakes is convergent timings for fossil damage from tsunamis in the Pacific Northwest and historical Japanese records of tsunamis.[28]
The next rupture of the Cascadia Subduction Zone is anticipated to be capable of causing widespread destruction throughout the Pacific Northwest.[29]
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 Kenji Satake 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.[30] In 2010, studies suggested that the risk could be as high as 37% for earthquakes of magnitude 8.0 or higher.[31][32]
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
Cascade Volcanic Arc
The Cascade Volcanic Arc is a continental volcanic arc that extends from northern California to the coastal peninsula of Alaska.[35] 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.[3] 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.[3] The major peaks from south to north include:
- Lassen Peak and Mount Shasta (California)
- Crater Lake (Mazama), Three Sisters, Mount Jefferson, Mount Hood (Oregon)
- Mount Adams, Mount St. Helens, Mount Rainier, Glacier Peak, Mount Baker (Washington)
- Mount Garibaldi and Mount Meager massif (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.[3] St. Helens continues to rumble, albeit more quietly, emitting occasional steam plumes and experiencing small earthquakes, both signs of continuing magmatic activity.[3]
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.
Volcanoes above the subduction zone
The volcanoes above the subduction zone include:
|
See also
- Astoria Fan
- Cascade Range
- Cascadia (region)
- Cascadia Channel
- Cascadia Region Earthquake Science Center (CRESCENT)
- Geology of the Pacific Northwest
- Neskowin Ghost Forest
- North Cascades National Park
- Plate tectonics
References
- ^ "Oregon Department of Emergency Management : Cascadia Subduction Zone : Hazards and Preparedness : State of Oregon".
- ^ a b c d e f g h Schulz, Kathryn (July 20, 2015). "The Really Big One: An earthquake will destroy a sizable portion of the coastal Northwest. The question is when". The New Yorker. Retrieved July 14, 2015.
- ^ a b c d e "Cascadia Subduction Zone Volcanism in British Columbia". Archived from the original on 2010-06-02. Retrieved 2008-12-18. USGS
- ^ a b c d e f g Stefan Lovgren (8 December 2003). "Did North American Quake Cause 1700 Japanese Tsunami?". National Geographic. Archived from the original on December 11, 2003. Retrieved 15 July 2015.
- ^ a b c d e f "Ghosts of Tsunamis Past". American Museum of Natural History. Archived from the original on 28 August 2018. Retrieved 15 July 2015.
- ^ a b c d e f Kevin Krajick (March 2005). "Future Shocks: Modern science, ancient catastrophes and the endless quest to predict earthquakes". Smithsonian Magazine. Retrieved 15 July 2015.
- ^ a b c d e f g h Jerry Thompson (13 March 2012). "The Giant, Underestimated Earthquake Threat to North America". Discover Magazine. Retrieved 15 July 2015.
- ^ a b "Juan de Fuca Volcanics". Retrieved 2008-05-06. USGS
- ^ ISBN 978-0-87842-063-6.
- ^ "The rupture zone of Cascadia great earthquakes from current deformation and thermal regime". ResearchGate.
- S2CID 6682060.
- S2CID 2672381.
- ^ S2CID 134015941.
- ISSN 0895-0695.
- ISSN 0037-1106.
- ISSN 2156-2202.
- ^ Strauss, Mark (20 October 2014). "Oregon Earthquake History". Oregon Insurance Advisor. Archived from the original on 2015-04-01. Retrieved 2021-06-21.
- ^ S2CID 4383885.
- ^ Haas, Ryan (2015-03-09). "What Is A 9.0 Earthquake?". Oregon Public Broadcasting. Archived from the original on 2015-01-26. Retrieved 2021-06-21.
- ^ ""How Much Bigger…?" Calculator". USGS Earthquake Hazards Program. Archived from the original on 2019-09-28. Retrieved 2021-06-21.
- S2CID 10928887.
- S2CID 2672381.
- S2CID 62833386.
- ^ "Earthquakes Along The Cascadia And San Andreas Faults May Be Linked, Affecting Risk To San Francisco Bay Region". ScienceDaily.
- ^
Brian F Atwater; Musumi-Rokkaku Satoko; Satake Kenji; Tsuji Yoshinobu; Ueda Kazue; David K Yamaguchi (2005). The Orphan Tsunami of 1700 — Japanese Clues to a Parent Earthquake in North America (U.S. Geological Survey Professional Paper 1707 ed.). Seattle and London: University of Washington Press. p. 100 (timeline diagram). ISBN 978-0-295-98535-0.
- ^
Brian F Atwater; Martitia P Tuttle; Eugene S Schweig; Charles M Rubin; David K Yamaguchi; Eileen Hemphill-Haley (2003). Earthquake Recurrence Inferred from Paleoseismology (PDF). Developments in Quaternary Sciences. Vol. 1. Elsevier BV. Figures 10 and 11 (pp 341, 342); article pp 331–350. ISSN 1571-0866. Archived from the original (PDF) on 2012-03-19. Retrieved 2011-03-15.)
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:|journal=
ignored (help - ^ "Cascadia Subduction Zone". Pacific Northwest Seismic Network.
- ^ "The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America" (PDF). Retrieved 2008-05-06. USGS Professional Paper 1707
- ^ "Cascade Range Earthquake Workgroup – Magnitude 9 scenario" (PDF). Archived from the original (PDF) on 2012-10-24. Retrieved 2012-03-27.
- ^ a b Tobias, Lori (April 19, 2009). "Big earthquake coming sooner than we thought, Oregon geologist says". The Oregonian.
- . Retrieved 2010-06-08.
- Science Daily.
- ^ Roos, Dave (2020-09-18). "The Deadliest Natural Disasters in U.S. History". HISTORY. Archived from the original on 2018-12-03. Retrieved 2021-06-21.
- ^ Powell, Kimberly (2018-07-19). "10 Deadliest Natural Disasters in U.S. History". ThoughtCo. Archived from the original on 2017-10-14. Retrieved 2021-06-21.
- ^ King, Hobart M. "Plate Tectonics Map - Plate Boundary Map". geology.com. Retrieved 2020-11-18.
External links
- Chris Goldfinger; C. Hans Nelson; Ann E. Morey; Joel E. Johnson; Jason R. Patton; Eugene Karabanov; Julia Gutiérrez-Pastor; Andrew T. Eriksson; Eulàlia Gràcia; Gita Dunhill; Randolph J. Enkin; Audrey Dallimore; Tracy Vallier (2012). Robert Kayen (ed.). "Turbidite event history—Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone". U.S. Geological Survey. Professional Paper 1661–F.
- Atwater, BF (1987). "Evidence for great Holocene earthquakes along the outer coast of Washington State". S2CID 39790838.
- "Cascadia Peril '09" at dailywireless.org
- 9.0 Shakemap Scenario
- The Really Big One – University of California Television
- Great Earthquakes of the Pacific Northwest – Central Washington University
- Toast, tsunamis and the really big one | Chris Goldfinger | TEDxMtHood – TEDx