The 1700 Cascadia earthquake was a magnitude 8.7 to 9.2 megathrust earthquake that occurred in the Cascadia subduction zone on January 26, 1700.[1] The earthquake involved the Juan de Fuca Plate underlying the Pacific Ocean, from mid-Vancouver Island in British Columbia, Canada, south along the Pacific Northwest coast as far as northern California, USA. The length of the fault rupture was about 1,000 kilometers (620 miles) with an average slip of 20 meters (22 yards).
The 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 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 p.m. 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. Local Native American oral traditions describing a large quake also exist, although these do not specify the date.[4] There are many areas in the Pacific Northwest with drowned groves of trees that also show evidence of the earthquake.[5]
| est. year | interval (years) |
|---|---|
| 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.[6]
As seen in the 1700 quake, the 2004 Indian Ocean earthquake, and the 2011 Tōhoku 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. One expert asserts that buildings in Seattle are vastly inadequate even to withstand an earthquake of the size of the 1906 San Francisco earthquake, much less the much greater one that may well occur.[7]
Recent findings conclude that the Cascadia Subduction zone is more complex and volatile than previously believed. Geologists predict a 37 percent chance of a M8.2+ event in the next 50 years, and a 10 to 15 percent chance that the entire Cascadia Subduction will rupture with a M9+ event within the same time frame.[8] 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).[9]
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.[10]
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 earthquakes of 1737 (est. mag. 8.3) and 1952 (measured at 9.0); the 2004 Indian Ocean earthquake at 9.1; the 2010 Chile earthquake at 8.8; and the 2011 Tōhoku earthquake at 9.0.
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