A tsunami (Japanese for "harbour wave") is series of huge ocean waves caused by a rapid, large-scale disturbance of the sea water. Tsunamis can be caused by submarine volcanic eruptions, submarine landslides, meteor impact, and major earthquakes occuring beneath the seabed causing large vertical movements. In deep water, tsunami waves are less than a metre high, but they can travel at speeds exceeding 800 kilometres per hour and can easily cross an entire ocean basin. When they reach shallow water or narrow inlets the waves slow down and the height can build into a wall of water which causes devastation on the shore.
On January 26, 1700 one of the world's largest earthquakes occurred along the west coast of North America and created a tsunami which completely destroyed the winter village of the Pachena Bay people with no survivors. These events are recorded in the oral traditions of the First Nations people on Vancouver Island. There is evidence of repeated tsunamis inundating our west coast following giant megathrust earthquakes in the Cascadia seismic zone.
In the twentieth century, 27 people were killed on November 18, 1929 in the tsunami that struck the Burin Peninsula of Newfoundland. This tsunami was caused by a large submarine slump along the continental slope triggered by the magnitude 7.2 earthquake that struck approximately 250 km to the south along the edge of the Laurentian Slope.
QUICK TIPS - If you live or find yourself by the ocean
West Coast and Alaska Tsunami Warning Centre (WCATWC) Message Definitions
Data from selected NRCan seismometers are forwarded to the National (United States) Oceanic and Atmospheric Administration’s (NOAA) West Coast and Alaska Tsunami Warning Centre (WCATWC) in Palmer, Alaska. This information is integrated with other seismic, tide gauge, and deep ocean buoy system data to produce tsunami information statements, alerts, watches, or warnings for all North American coastlines (including the Atlantic and Arctic). WCATWC distributes these messages to Emergency Measures Organizations (EMO) and other clients 5 to 15 minutes after a potentially tsunamigenic earthquake has occurred and provide updates at regular intervals.
WCATWC product definitions changed to the definitions provided below on February 12, 2008. The products issued by the center are warning, watch, advisory, and information statements. Each has a distinct meaning relating to local emergency response. In summary:
|Warning||->||Inundating wave possible||->||Full evacuation suggested|
|Watch||->||Danger level not yet known||->||Stay alert for more info|
|Advisory||->||Strong currents likely||->||Stay away from the shore|
|Information||->||Minor waves at most||->||No action suggested|
Tsunami Warning - a tsunami warning is issued when a potential tsunami with significant widespread inundation is imminent or expected. Warnings alert the public that widespread, dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after arrival of the initial wave. Warnings also alert emergency management officials to take action for the entire tsunami hazard zone. Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas, and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled. To provide the earliest possible alert, initial warnings are normally based only on seismic information.
Tsunami Watch - a tsunami watch is issued to alert emergency management officials and the public of an event which may later impact the watch area. The watch area may be upgraded to a warning or advisory - or canceled - based on updated information and analysis. Therefore, emergency management officials and the public should prepare to take action. Watches are normally issued based on seismic information without confirmation that a destructive tsunami is underway.
Tsunami Advisory - a tsunami advisory is issued due to the threat of a potential tsunami which may produce strong currents or waves dangerous to those in or near the water. Coastal regions historically prone to damage due to strong currents induced by tsunamis are at the greatest risk. The threat may continue for several hours after the arrival of the initial wave, but significant widespread inundation is not expected for areas under an advisory. Appropriate actions to be taken by local officials may include closing beaches, evacuating harbors and marinas, and the repositioning of ships to deep waters when there is time to safely do so. Advisories are normally updated to continue the advisory, expand/contract affected areas, upgrade to a warning, or cancel the advisory.
Tsunami Information Statement - a tsunami information statement is issued to inform emergency management officials and the public that an earthquake has occurred, or that a tsunami warning, watch or advisory has been issued for another section of the ocean. In most cases, information statements are issued to indicate there is no threat of a destructive tsunami and to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. An information statement may, in appropriate situations, caution about the possibility of destructive local tsunamis. Information statements may be re-issued with additional information, though normally these messages are not updated. However, a watch, advisory or warning may be issued for the area, if necessary, after analysis and/or updated information becomes available.