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Cascadia subduction zone

The 1700 Cascadia earthquake was a magnitude 8.7 to 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 British Columbia, southwest Canada, 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 earthquake caused a tsunami that struck the coast of Japan,[2] and may also be linked to the Bonneville slide.[3]


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 (NS). Although there were no written records in the region at the time, the earthquake's precise time 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 describing a large quake also exist among the region's inhabitants, although these do not specify the date.

Future threats

Cascadia earthquake sources
Great Earthquake Summary
est. year interval
1700 AD -
1310 AD 390
810 AD 500
400 AD 410
170 BC 570
600 BC 430

The geological record reveals that "great earthquakes" (those with moment 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, 170 BC and 600 BC.[citation needed]

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

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.

Recent findings conclude that the Cascadia Subduction zone is more volatile than previously suggested. The feared "big one" has many geologists predicting a 10 to 14 percent chance that the Cascadia Subduction will rupture in the next 50 years and is more likely to be a magnitude 9 or higher. Geologists have also determined the Pacific Northwest is not prepared for such a colossal quake. The tsunami produced could reach heights of 80 to 100 feet (24 to 30 m).[4]

Some other subduction zones have such earthquakes every 100 to 200 years; the longer interval results from slower plate motions. The rate of convergence between the Juan de Fuca Plate and the North American Plate is 60 millimetres (2.4 in) per year.[5]

Similar megathrust earthquakes

Other megathrust earthquakes are the slightly more powerful 1964 Alaskan Good Friday Earthquake measured at moment magnitude 9.2; the 1960 Great Chilean Earthquake measured at 9.5; the Kamchatka quakes of 1737 (est. mag. 8.3) and 1952 (measured at 9.0); the 2004 Indian Ocean earthquake at 9.3; and the 2010 Chilean earthquake at 8.8.

See also


External links



Native and Japanese accounts

Current hazards


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