1700 Cascadia earthquake

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A map of the Cascadia Earthquake

The 1700 Cascadia Earthquake was a magnitude 8.7 – 9.2 megathrust earthquake that occurred in the Cascadia subduction zone in 1700. The earthquake involved the Juan de Fuca Plate in the Pacific ocean, from mid-Vancouver Island of southwest Canada off British Columbia to northern California, off what is now known as the Pacific Northwest. The length of the fault slip was about 1000 kilometers (600 miles).

The Cascadia Earthquake is linked to a tsunami that struck the coast of Japan, and may also be linked to the Bonneville slide.

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[edit] 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 in the evening of 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 [1].

[edit] Future threats

The geological record strongly indicates 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 for at least 13 events at intervals of from 300 to 900 years, with an average of 590 years. Previous earthquakes are estimated to have occurred in 1310, 810 and 170 BC.

As the subduction zone ruptured, it would have unleashed a magnitude 9.0 earthquake and a tsunami headed for the coast. The shaking would have lasted for about 4 or more minutes devastating natural spaces and local habitations. Then the tsunami would have hit land, destroying many remaining structures on the coast. To this date, this was probably the strongest earthquake to hit the territory that is now the Contiguous United States in recorded history.

As displayed in the 2004 Indian Ocean Earthquake, great damage can come from subduction zone tsunamis. In the case of a future Cascadia Earthquake, however, most of the damage would probably be from the earthquake itself due to the immense numbers of urban areas including three major cities and large quantity of vulnerable structures like brick buildings and highrises in the Cascadia region. The Cascadia coast is also armed with various tsunami alerts and escape routes.

Other subduction zones usually have such earthquakes every 100–200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.

The rate of convergence between the Juan de Fuca Plate and the North American Plate is 40 mm/yr. [2]

[edit] 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 a minimum of 9.3.

See also New Madrid Earthquake, which is estimated to have measured 8.0 or higher on the modern Richter Scale.

[edit] See also

[edit] External links and references

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