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The Cascadia Subduction Zone is a very long sloping fault
that stretches from mid-Vancouver Island to Northern California. It separates
the Juan de Fuca and North America plates. New ocean floor is being created
offshore of Washington and Oregon. As more material wells up along the ocean
ridge, the ocean floor is pushed toward and beneath the continent. The
Cascadia Subduction Zone is where the two plates meet.
The width of the Cascadia Subduction Zone fault varies along its length, depending on the temperature of the subducted oceanic slab, which heats up as it is pushed deeper beneath the continent. As it becomes hotter and more molten it eventually loses the ability to store mechanical stress and generates earthquakes. The volcanoes along the Cascade range are also caused by this subduction. Giant earthquakes have occurred on the Cascadia continental margin at intervals of several 100 to over 1000 years according to paleoseismicity data from the coasts of British Columbia, Washington and Oregon. The average interval is about 500 years, and the last one is thought to have been on January 26, 1700 at about 9 pm. (This is based on core samples taken from the ocean floor and supported by oral traditions of west coast native populations and by Japanese accounts of the resulting tsunami.) |