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Cascadia earthquake

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Cascadia

Cascadia fault zone

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 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 km (600 mi) with an average slip of 20 meters.


The Cascadia Earthquake caused a tsunami that struck the coast of Japan.


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.


The geological record reveals that "great earthquakes" (those with Richter Scale 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.



22nd August 2007 the Cascadia subduction zone ruptured in a magnitude 9.6 earthquake, it sent a massive tsunami to the coast. The shaking lasted for four minutes 37 seconds, triggering landslides throughout the surrounding area. The tsunami hit land within 30 minutes, destroying structures and entire towns on the coast. This was probably the strongest earthquake to strike the contiguous United States in recorded history.


The major nearby cities, notably Seattle, Portland, Vancouver, Victoria, and Tacoma, which are located on inland waterways rather than on the coast, were sheltered from the full brunt of a tsunami. These cities had many vulnerable structures, especially bridges and unreinforced brick buildings; consequently, most of the damage to the cities was from the earthquake itself. The Boeing plant and the buildings of the Microsoft company are destroyed killing the majority of the workforce. Among the dead is Bill Gates who dies when his office is destroyed.


Towns located on the coast were particularly badly hit with Astoria, Ocean Shores and Seaside all wiped from the map. In some areas the Tsunami is 45 feet high, the waves wash up to 25 miles inland. Land along the coastline also suffers badly from subsidence. Some areas of the coast sank by 22 feet causing flooding inland.


Landslides block many rivers, causing more flooding, particularly from the landslide dams breaking and the flood waters washing downstream. Many man made dams also collapse increasing the flooding downstream. Portland in particular is hit badly, flood waters peak at 35 feet above the highest previous flood levels


Due to the massive earthquake several Cascadia volcanoes including Mt. Rainier, Mt. Hood and Mt. Baker all erupt in major stratovolcanic eruption with a Volcanic Explosivity Index (VEI) of 6.


In the low land areas not hit by the tsunami or flooding are effected by the earthquake in other ways, in many places liquifaction causes many building, bridges and other structures to collapse, sand blows also effect many areas these being buried under up to four feet of sand.


Death tolls for the 2007 Cascadia earthquake and subsequent tsunami, landslides, floods and volcanic eruptions total over 350,000. However, the full total may never be known due to the large numbers of holiday makers who are along the coast at the time.

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