Tsunami Preparedness

The West Coast and Alaska Tsunami Warning Center uses seismic data as its starting point, but then takes into account oceanographic data when calculating possible threats. Tide gauges in the area of the earthquake are checked to establish if a tsunami has formed. The center then forecasts the future of the tsunami, issuing warnings to at-risk areas all around the Pacific basin if needed.


Tsunami Warning?

The highest level of tsunami alert. Warnings are issued due to the imminent threat of a tsunami from a large undersea earthquake or following confirmation that a potentially destructive tsunami is underway. They may initially be based only on seismic information as a means of providing the earliest possible alert. Warnings advise that appropriate actions be taken in response to the tsunami threat. Such actions could include the evacuation of low-lying coastal areas and the movement of boats and ships out of harbors to deep water. Warnings are updated at least hourly or as conditions warrant to continue, expand, restrict, or end the warning.
Tsunami Watch?

The second highest level of tsunami alert. Watches are issued by the Tsunami Warning Centers based on seismic information without confirmation that a destructive tsunami is underway. It is issued as a means of providing an advance alert to areas that could be impacted by destructive tsunami waves. Watches are updated at least hourly to continue them, expand their coverage, upgrade them to a Warning, or end the alert. A Watch for a particular area may be included in the text of the message that disseminates a Warning for another area.
Tsunami Advisory?

The third highest level of tsunami alert. Advisories are issued to coastal populations within areas not currently in either warning or watch status when a tsunami warning has been issued for another region of the same ocean. An Advisory indicates that an area is either outside the current warning and watch regions or that the tsunami poses no danger to that area. The Center(s) issuing the Advisory will continue to monitor the event, issuing updates at least hourly. As conditions warrant, the Advisory will either be continued, upgraded to a watch or warning, or ended. 

  • A tsunami is a series of waves or surges most commonly caused by an earthquake beneath the sea floor.
  • An unusual lowering of ocean water, exposing the sea floor, is a warning of a tsunami or other large wave. This “draw back” means the water will surge back strongly.
    Beaches, lagoons, bays, estuaries, tidal flats and river mouths are the most dangerous places to be. It is rare for a tsunami to penetrate more than a mile inland. Tsunami waves are unlike normal coastal waves.
  • Tsunamis are more like a river in flood or a sloping mountain of water and filled with debris.
  • Tsunamis cannot be surfed. They have no face for a surfboard to dig into and are usually filled with debris.
  • Large tsunamis may reach heights of twenty to fifty feet along the coast and even higher in a few locales. The first tsunami surge is not the highest and the largest surge may occur hours after the first wave. It is not possible to predict how many surges or how much time will elapse between waves be for a particular tsunami.
  • The entire California Coast is vulnerable to tsunamis. The Crescent City Harbor on California’s North Coast suffered significant tsunami damage as recently as 2006. A dozen people were killed in California following the 1964 Alaska earthquake.

  1. If indoors, DROP under a sturdy table or object, COVER your head and neck and HOLD ON.
  2. If outdoors, move to a clear area if you can safely do so - away from trees, beach cliffs, signs and other hazards - and drop to the ground.
  3. If you have mobility impairments that prevent you from getting up on your own, do not drop to the ground but do cover your head and neck and hold on.
  1. As soon as it is safe to move, go to higher ground. DO NOT WAIT for an official tsunami warning.
  2. Avoid downed power lines and weakened overpasses.
  3. If you are outside of a tsunami hazard zone, stay where you are.
  1. Remain on high ground. Waves from a tsunami may arrive for eight hours or longer.
  2. Return to the coast only when officials have announced that it is safe to do so.
Additional Tsunami Resources
(Cal OES) – “How to Survive a Tsunami” (pdf download)
(Cal OES) - "My Hazards"  (See potential hazards near your home or business)
California Geological Survey (CGS) -Tsunami Web Site 
California Geological Survey (CGS) - Tsunamis Note 55 (pdf download)
National Oceanic and Atmoshepric Administration (NOAA) - "Designing for Tsunamis" (pdf download)
Humboldt State University, Department of Geology - "Living on Shaky Ground" 
National Weather Service - "TsunamiReady" 
California Geological Survey - "Tsunami Information"
Tsunami-Related Maps
CA Department of Conservation (California Geological Survey) - "Inundation Maps" 
CA Department of Conservation (California Geological Survey) - Inundation Maps by County"
City of Half Moon Bay - "Tsunami Evacuation Map"
Humboldt County - "Tsunami Hazard Maps" "Tsunami Hazard Maps"