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Science-Pacific Region-Ocean Sciences-Research Activities |
Tsunamis and Tsunami Research |
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Tsunamis
in Coastal British Columbia
The tsunamis in coastal British Columbia The tsunamis along coastal British Columbia could be generated by remote or local earthquakes. For example, in 1964, a large earthquake in Alaska triggered a tsunami that caused damage all the way to California, including several million dollars damage in Port Alberni here in British Columbia. The local tsunami source is the Cascadia subduction zone earthquake. What is the Cascadia subduction zone? 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 diagram on the following shows subduction of the oceanic Juan de Fuca plate beneath the
continental crust of North America.
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 earthquakes? 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 earthquakes? There is abundant geological evidence in tidal marshes along the Pacific coast from Vancouver Island to northern California for repeated, historically unprecedented great earthquakes in the recent past. 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.
The last known great earthquake in the northwest was in January, 1700, just over 300 years ago.
The computer-generated image (courtesy of Kenji Satake) shows the tsunami produced by the great (magnitude 9)
Cascadia earthquake of January 26, 1700, six hours after initiation. The tsunami moved across the
Pacific Ocean and produced destructive waves up to several meters high along a 1000-km length of the
coast of Honshu in Japan. Much larger waves struck the west coast of North American less than 30 minutes
after the shaking stops. Deposits from this tsunami are preserved in tidal marshes and low-elevation
coastal lakes on the Pacific coast. Their distribution provides information on the wave run-up
that can be expected from future Cascadia tsunamis. Where can you get Tsunami warnings in British Columbia? Warning of tsunamis in the Pacific Basin is the subject of an international protocol established by the United Nations Intergovernmental Oceanographic Commission (IOC) in 1946. The tsunami warning system, managed by the United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), is designed to detect tsunamis and provide prompt notification to all nations bordering the Pacific Ocean. In British Columbia, the Provincial Emergency Program (PEP) receives alerts and warnings from the West Coast/Alaska Tsunami Warning Center and relay those alerts and warnings (with refinements for Canadian coast and tidal conditions) to all sectors of the population at risk in the province. |
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