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Cascadia subduction zone

cascadia subduction zone, cascadia subduction zone prediction
Coordinates: 45°N 124°W / 45°N 124°W / 45; -124 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 that separates the Explorer, Juan de Fuca, and Gorda plates, on the one hand, and the North American Plate, on the other

The denser oceanic plate is subducting beneath the less dense continental plate offshore of British Columbia, Washington, Oregon, and northern California The North American Plate is moving in a 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 of the Cascades This volcanism 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 19801 Major cities affected by a disturbance in this subduction zone include Vancouver and Victoria, British Columbia; Seattle, Washington; and Portland, Oregon


  • 1 History
    • 11 Oral history
    • 12 Ghost forests
    • 13 Orphan tsunami
  • 2 Geology
  • 3 Earthquakes
    • 31 Earthquake effects
    • 32 San Andreas Fault connection
    • 33 Earthquake timing
    • 34 Forecasts of the next major earthquake
  • 4 Cascade Volcanic Arc
  • 5 See also
  • 6 References
  • 7 External links


In the 1960s, underground fractures were uncovered by oil companies in Puget Sound These were believed to be inactive through the 1990s2

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 time3

Oral historyedit

At the time of the 1700 earthquake, there were no written records of the event in Cascadia Orally-transmitted legends from the Olympic Peninsula area talk of an epic battle between a thunderbird and a whale Therefore, in a 2005 study, seismologist Ruth Ludwin set out to collect and analyze anecdotes from various First Nations groups Reports from the Huu-ay-aht,4 Makah,4 Hoh,5 Quileute,25 Yurok,2 and Duwamish2 peoples referred to earthquakes and saltwater floods This collection of data allowed her team to come up with an estimated date range for the event, whose midpoint fell in the year 17014

Ghost forestsedit

During low tide one day in March 1986, paleogeologist Brian Atwater dug along Neah Bay using a nejiri gama, a small hand hoe Underneath the top layer of sand, he uncovered a distinct plant—arrowgrass—that had grown in a layer of marsh soil This was proof that the ground had suddenly sunk under sea level, causing saltwater to kill the vegetation The events had happened so quickly as to cause the top layer of sand to seal away any air, thus preserving the centuries-old plants3

In 1987, Atwater mounted another expedition paddling up the Copalis River with Dr David Yamaguchi, who was then studying the eruptions of Mount St Helens3 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 ago5 Originally thought to have died slowly due to a gradual rise in sea level,6 closer inspection yielded a different story: the land plummeted up to two meters during an earthquake5 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 170034

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 surge5

In 1995, an international team led by Alan Nelson of the USGS further corroborated these findings with 85 new samples from the rest of the Pacific Northwest 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 tsunami3

Yet another 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 CE2

Orphan tsunamiedit

A 1996 study published by seismologist Kenji Satake supplemented the research by Atwater et al with tsunami evidence across the Pacific4 Japanese annals, which have recorded natural disasters since approximately 600 CE,6 had reports of a sixteen-foot tsunami that struck the coast of Honshu Island during the Genroku34 Since no earthquake had been observed to produce it, scholars dubbed it an "orphan tsunami"6 Translating the Japanese calendar, Satake found the incident had taken place around midnight of 27–28 January 1700, ten hours after the earthquake occurred


Structure of the Cascadia subduction zone

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 Ridge78

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/yr78

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 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 it becomes hotter and more molten, it eventually loses the ability to store mechanical stress and generate 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 rupture9

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 of Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino Fracture Zone at the Mendocino Triple Junction


Cascadia earthquake sources

Earthquake effectsedit

Megathrust earthquakes are the most powerful earthquakes known to occur, and can exceed magnitude 90 They occur when enough energy stress has accumulated in the "locked" zone of the fault to cause a rupture known as a megathrust earthquake 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 California10

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 sliding10

In 1999, a group of Continuous Global Positioning System sites registered a brief reversal of motion of approximately 2 centimeters 08 inches over a 50 kilometer by 300 kilometer about 30 mile by 200 mile area The movement was the equivalent of a 67 magnitude earthquake11 The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures12

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 subsidence13

San Andreas Fault connectionedit

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 quake14

Earthquake timingedit

Great earthquakes
estimated year interval
2005 source15 2003 source16 years
Y about 9 pm, January 26, 1700 NS 780
W 780–1190 CE 880–960 CE 210
U 690–730 CE 550–750 CE 330
S 350–420 CE 250–320 CE 910
N 660-440 BCE 610–450 BCE 400
L 980–890 BCE 910–780 BCE 250
J 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 > magnitude 80 may have occurred sporadically at least seven times in the last 3,500 years, suggesting a return time of about 500 years345 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 years6 Of these 41, nineteen have produced a "full margin rupture," wherein the entire fault opens up3 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 slip17

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 tsunamis18

The next rupture of the Cascadia Subduction Zone is anticipated to be capable of causing widespread destruction throughout the Pacific Northwest19

Forecasts of the next major earthquakeedit

See also: 1700 Cascadia earthquake § Future threats

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 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 90 or higher in the next 50 years20 In 2010, studies suggested that the risk could be as high as 37% for earthquakes of magnitude 80 or higher2122

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, because the rupture is expected to be as long as the 2004 Indian Ocean earthquake and tsunami The resulting tsunami might reach heights of approximately 30 meters 100 ft20 FEMA estimates some 13,000 fatalities from such an event, with another 27,000 injured It predicts that a million people will be displaced, with yet another 25 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 ones6 Other analyses predict that even a magnitude 67 earthquake in Seattle would result in 7,700 dead and injured, $33 billion in damages, 39,000 buildings largely or totally destroyed, and 130 simultaneous fires2

Cascade Volcanic Arcedit

Juan de Fuca Triple Junctions and the Cascade Volcanic Arc Main article: Cascade Volcanoes

The Cascade Volcanic Arc is a continental volcanic arc that extends from northern California to the coastal mountains of British Columbia1 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 ice1 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 elevation1 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, Mount St Helens, Mt Rainier, Glacier Peak, Mt Baker Washington
  • Mt Garibaldi and Mt Meager British Columbia

The most active volcanoes in the chain include Mount St Helens, Mt Baker, Lassen Peak, and Mt Hood St Helens captured worldwide attention when it erupted catastrophically in 19801 St Helens continues to rumble, albeit more quietly, emitting occasional steam plumes and experiencing small earthquakes, both signs of continuing magmatic activity1

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:

See alsoedit

  • Cascade Range
  • Cascadia disambiguation
  • Geology of the Pacific Northwest
  • Neskowin Ghost Forest
  • North Cascades National Park
  • Plate tectonics


  1. ^ a b c d e f "Cascadia Subduction Zone Volcanism in British Columbia" Archived from the original on 2010-06-02 Retrieved 2008-12-18  USGS
  2. ^ 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 
  3. ^ 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 
  4. ^ a b c d e f g Stefan Lovgren 8 December 2003 "Did North American Quake Cause 1700 Japanese Tsunami" National Geographic Retrieved 15 July 2015 
  5. ^ a b c d e f "Ghosts of Tsunamis Past" American Museum of Natural History Retrieved 15 July 2015 
  6. ^ a b c d e 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 
  7. ^ a b "Juan de Fuca Volcanics" Retrieved 2008-05-06  USGS
  8. ^ a b Alt, David D; Hyndman, Donald W 1978 Roadside Geology of Oregon 19th ed Missoula, Montana: Mountain Press p 3 ISBN 0-87842-063-0 
  9. ^ "Hyndman and Wang" Archived from the original on 2010-05-30 Retrieved 2009-12-17  USGS dead link See fig 5 here for the diagram
  10. ^ a b Nedimović, Mladen R; Hyndman, Roy D; Ramachandran, Kumar; Spence, George D 24 July 2003 "Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface" Nature 424 6947: 416–420 Bibcode:2003Natur424416N PMID 12879067 doi:101038/nature01840 
  11. ^ Dragert, Herb; Wang, Kelin; James, Thomas S 25 May 2001 "A silent slip event on the deeper Cascadia subduction interface" Science 292 5521: 1525–1528 Bibcode:2001Sci2921525D PMID 11313500 doi:101126/science1060152 
  12. ^ Rogers, Garry; Dragert, Herb 20 June 2003 "Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip" Science 300 5627: 1942–1943 Bibcode:2003Sci3001942R PMID 12738870 doi:101126/science1084783 
  13. ^ Leonard, Lucinda J; Hyndman, Roy D; Mazzotti, Stéphane "Coseismic subsidence in the 1700 great Cascadia earthquake: Coastal estimates versus elastic dislocation models" GSA Bulletin 116 5–6: 655–670 Bibcode:2004GSAB116655L doi:101130/B253691 
  14. ^ Science Daily, April 3, 2008
  15. ^ 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 PDF US Geological Survey Professional Paper 1707 ed Seattle and London: University of Washington Press p 100 timeline diagram ISBN 0-295-98535-6 
  16. ^ 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 Science Elsevier BV 1 Figures 10 and 11 pp 341, 342; article pp 331–350 ISSN 1571-0866 doi:101016/S1571-08660301015-7 Archived from the original PDF on 2012-03-19 Retrieved 2011-03-15 
  17. ^ "Cascadia Subduction Zone" Pacific Northwest Seismic Network 
  18. ^ "The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America" PDF Retrieved 2008-05-06  USGS Professional Paper 1707
  19. ^ "Cascade Range Earthquake Workgroup – Magnitude 9 scenario" PDF 
  20. ^ a b Tobias, Lori April 19, 2009 "Big earthquake coming sooner than we thought, Oregon geologist says" The Oregonian 
  21. ^ Lovett, Richard A 31 May 2010 "Risk of giant quake off American west coast goes up" Nature doi:101038/news2010270 Retrieved 2010-06-08 
  22. ^ "Odds are about 1-in-3 that mega-earthquake will hit Pacific Northwest in next 50 years, scientists say" Press release Oregon State University May 25, 2010 – via Science Daily 

External linksedit

  • 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" US Geological Survey Professional Paper 1661–F 
  • Atwater, BF 1987 "Evidence for great Holocene earthquakes along the outer coast of Washington State" Science 236 4804: 942–44 Bibcode:1987Sci236942A PMID 17812748 doi:101126/science2364804942 
  • "Cascadia Peril '09" at dailywirelessorg
  • 90 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

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