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Cascadia Earthquake

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1700 Cascadia earthquake

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Cascadia subduction zone

Cascadia earthquake sources

The 1700 Cascadia Earthquake was a magnitude 8.7 – 9.2 megathrust earthquake that occurred in the Cascadia subduction zone in 1700.^[1] The earthquake involved the Juan de Fuca Plate underlying the Pacific ocean, from mid-Vancouver Island in southwest Canada off British Columbia to northern California, along the Pacific Northwest coast. The length of the fault rupture was about 1000 kilometers (600 miles) with an average slip of 20 meters.

The Cascadia Earthquake caused a tsunami that struck the coast of Japan^[2], and may also be linked to the Bonneville slide.^[3]

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Evidence of the earthquake

Evidence supporting the occurrence of the 1700 earthquake has been gathered into the 2005 book, The Orphan Tsunami of 1700, by geologist Brian Atwater and others.

The evidence suggests that it took place at about 9:00 PM on January 26, 1700. Although there were no written records in the region at the time, the earthquake's precise date is nevertheless known from Japanese records of a tsunami that has not been tied to any other Pacific Rim earthquake. The most important clue linking the tsunami in Japan and the earthquake in the Pacific Northwest comes from studies of tree rings (dendrochronology) which show that red cedar trees killed by lowering of coastal forests into the tidal zone by the earthquake have outermost growth rings that formed in 1699, the last growing season before the tsunami. Oral traditions also exist among the region's original inhabitants, although these do not specify the date.

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Future threats

The geological record reveals that "great earthquakes" (those with magnitude 8 or higher) occur in the Cascadia subduction zone about every 500 years on average, often accompanied by tsunamis. There is evidence of at least 13 events at intervals from about 300 to 900 years with an average of 590 years. Previous earthquakes are estimated to have occurred in 1310 AD, 810 AD, 400 AD, 600 BC and 170 BC.

As the subduction zone ruptured in a magnitude 9 earthquake, it sent a strong tsunami to the coast. The shaking lasted for four minutes or more, triggering landslides. Then the tsunami would have hit land, destroying structures at the coast. This was probably the strongest earthquake to strike the Contiguous United States in recorded history.

As seen in the 1700 quake and the 2004 Indian Ocean Earthquake, subduction zone earthquakes can cause large tsunamis, and many coastal areas in the region have prepared tsunami evacuation plans in anticipation of a possible future Cascadia earthquake. However, the major nearby cities, notably Seattle, Portland, Vancouver, Victoria, and Tacoma, which are located on inland waterways rather than on the coast, would be sheltered from the full brunt of a tsunami. These cities do have many vulnerable structures, especially bridges and unreinforced brick buildings; consequently, most of the damage to the cities would probably be from the earthquake itself.

Some other subduction zones have such earthquakes every 100–200 years; the longer interval here results from slower plate motions. The rate of convergence between the Juan de Fuca Plate and the North American Plate is 40 mm/yr. [1]

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Similar megathrust earthquakes

Other megathrust earthquakes are the slightly more powerful 1964 Alaskan Good Friday Earthquake measured at magnitude 9.2, the 1960 Great Chilean Earthquake measured at 9.5, The Kamchatka quake measured at 9.0, and the 2004 Indian Ocean earthquake at 9.3.

See also New Madrid Earthquakes, which are estimated to have measured magnitude 7.5–8.0.

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See also

• Geology of the Pacific Northwest
• List of earthquakes
• 1949 Queen Charlotte earthquake

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References

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External links

This article or section is missing citations or needs footnotes. Using inline citations helps guard against copyright violations and factual inaccuracies. (January 2008)

• National Resources Canada: evaluation of Cascadia subduction zone hazards
• USGS on the Cascadia earthquake
• Repository of information on the Cascadia earthquake
• Japanese tsunami descriptions
• Science Daily on the Cascadia Earthquake & Japanese evidence
• Canadian source with a nice map
• 300th anniversary article
• Lessons from surviving a tsunami
• Seattle scenario description

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