This plaque on the southern Oregon coast provides information regarding the Cascadia Subduction Zone and Tsunamis.
Copied from the sign:
Tsunamis
Devastating waves called "tsunamis can strike Oregon's coast at any time. These giant waves are cased by great undersea earthquakes. Such earthquakes can occur along the Cascadia Subduction Zone, one of the largest active faults in North America. This fault zone lies 32 to 70 miles offshore and roughly parallels the coast.
Tsunamis are dangerous and destructive. They have struck the Oregon coast repeatedly and will again in the future. Tsunamis can follow within minutes of an earthquake. They move rapidly but quickly run out of water as they sweep inland and uphill. Flooding can occur several miles inland along rivers and streams. Remember, most tsumanis are not solitary giant waves, instead, many large waves may strike the shore over the course of several hours.
The earth's surface consists of a series of "plates". These plates are constantly shifting and sliding over, under or past each other. When a sudden movement occurs between two plates, we experience an earthquake.
The Juan de Fuca Plate is moving away from the Juan de Fuca Ridge and is being forced under the overriding North American Plate. This geologic process is called subduction.
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A great deal more information is available on this website: (
visit link) and a portion of which is copied here:
"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 generate earthquakes. The "locked" zone is storing up energy for an earthquake, and the "transition" zone, although somewhat plastic, could probably rupture. HOW BIG ARE CASCADIA SUBDUCTION ZONE QUAKES?
Great Subduction Zone earthquakes are the largest earthquakes in the world, and can exceed magnitude 9.0. Earthquake size is porportional to fault area, and 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. Because of the very large fault area, the Cascadia Subduction Zone could produce a very large earthqauke, magnitude 9.0 or greater, if rupture occurred over its whole area.
HOW OFTEN ARE CASCADIA SUBDUCTION ZONE QUAKES?
The last known great earthquake in the northwest was in January, 1700, just over 300 years ago. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return time of 400 to 600 years.