Cascadia Earthquake

The Cascadia Earthquake was a magnitude 8.7 – 9.2 megathrust earthquake 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 the Pacific Northwest of the United States.

The fault slipped along about 1000 kilometers — around 600 miles.

It took place at about 9:00 in the evening of January 26, 1700. Although there was no European settlement and no written records in the region at the time, the earthquake's precise date is nevertheless known from Japanese records of a tsunami that could not be tied to any other earthquake. The most important initial clue came from studies of tree rings ( dendrochronology) which showed a large number of fallen trees whose last year of growth was 1699– 1700; with this information, the Japanese tsunami records were then used to determine the exact date. Oral traditions also exist among the region's original inhabitants, although these do not specify the date.

Its occurrence was confirmed in 2003 in an article by geologist Brian Atwater.

The Cascadia Earthquake may be linked to the Bonneville slide.

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.

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. [1]

Similar megathrust earthquakes

Other megathrust earthquakes are the slightly more powerful 1964 American Good Friday Earthquake measured at magnitude 9.2, the 1960 Great Chilean Earthquake measured at 9.5, and the 2004 Indian Ocean earthquake at a minimum of 9.0.

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