Waiting for the 'Big One'
Giant earthquakes have occurred along the Cascadia Subduction Zone throughout the past several million years. The last one, on January 26, 1700, caused a tsunami that produced large waves on Vancouver Island and waves which crossed the Pacific Ocean bringing damage to coastal Japan. Geological evidence of this earthquake and earlier large earthquakes is well preserved along the coasts of British Columbia and northwestern United States where buried salt-marsh peat deposits attest to sudden sinking of coastal areas during the earthquakes.
This mathematical simulation shows the tsunami created by the Cascadia Subduction Zone earthquake on January 26, 1700, as it reaches Hawaii on its way across the Pacific Ocean.
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Changes in the elevations and distances between positions on the Earth’s surface can be determined from precise measurements obtained from satellites of the global positioning system (GPS).
With the use of GPS satellite technology and other geodetic methods it is possible to make extremely accurate measurements of distances between points on the Earth’s surface and to detect changes in their elevations. Employing these and other techniques, scientists have concluded that the Juan de Fuca Plate is currently locked to the North America Plate along the Cascadia Subduction Zone fault beneath the edge of the continent. Where the fault is locked, the temperature is estimated to be less than 350°C. Whereas at temperatures less than 350°C, rocks behave in a brittle fashion and are thus susceptible to earthquakes, those at higher temperatures tend to flow while under stress, with no earthquakes occurring. Due to locking, the western part of Vancouver Island is being flexed upward (shown here as greatly exaggerated) and compressed horizontally as the North America and Juan de Fuca plates converge upon one another. Eventually the locked zone must release, causing a giant earthquake (magnitude 8 to 9) to occur at any time within the next few hundred years. At that time, areas along Vancouver Island’s west coast will suddenly drop by as much as 1 m or more.
Additional information can be obtained on the Geodynamics Program Web site.