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Tsunami (soo-NAH-mee)comes from the Japanese word that means harbor wave; a sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major sub-marine slides, or exploding volcanic islands.
Typically generated by seismic or volcanic activity or by underwater landslides, a tsunami consists of a series of high-energy waves that radiate outward like pond ripples from the area in which the generating event occurred.
In the past century, several damaging tsunamis have struck the Pacific Northwest coast ( Northern California , Oregon , and Washington ), according to the U.S. Geological Survey. All of these tsunamis were distant tsunamis generated from earthquakes located far across the Pacific basin and are distinguished from tsunamis generated by earthquakes near the coast—termed local tsunamis.
While tsunamis are rare in Puget Sound , it is important to be aware of their very real and dangerous potential, an awful reality illustrated by the December 2004 earthquake and tsunami in Southeast Asia . According to the King County Office of Emergency Management, these are the early warning signs of the arrival of tsunami waves:
- A sudden and unexpected recession of water
- The first wave will be followed by additional waves a few minutes or even a few hours later
- Wave size typically increases over time
- Coastal flooding often may precede the largest waves
Not all earthquakes produce tsunamis. To generate a tsunami, an earthquake must:
- Occur underneath or near the ocean
- Be very large (approximately Richter magnitude 7 or greater), and
- Create vertical movement of the sea floor.
Recent studies regarding the potential for a great Cascadia Subduction zone earthquake off the Washington , Oregon , and Northern California coastlines indicate local tsunami waves may reach nearby coastal communities within minutes of the earthquake, thereby giving little or no time to issue warnings .
The City of Seattle , in cooperation with several other RPIN agencies, recently installed a new outdoor hazard-warning system for all major hazards, including tsunamis, in three waterfront locations. In an emergency the new siren will sound with "Westminster chimes." To familiarize your family with this warning sound, click here.
Source: King County Office of Emergency Management
The following links provide useful information on tsunamis:
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