City and Borough of Juneau
155 S. Seward Street
Juneau, Alaska 99801
tel. 907-586-5240
fax 907-586-5385

Emergency Management

Tsunamis And Seiches

Tsunamis are a series of traveling ocean waves of extremely long length generated by disturbances associated primarily with earthquakes.  In the deep ocean, their length from wave crest to wavedcrest may be a hundred miles or more but with a wave height of only a few feet or less.  They cannot be felt aboard ships nor can they be seen from the air in the open ocean.  In deep water, the waves may reach speeds exceeding 500 miles per hour.

A seiche is a similar wave that is confined in a partially or totally enclosed body of water. Seiches can also be caused by earthquakes and landslides, and can be particularly devastating because the wave may strike repeatedly as it rebounds back and forth across the body of water.

Tsunamis may seem unlikely in Juneau because Juneau is not exposed to the open ocean. However, Southeast Alaska has been the site of ten of Alaska's historical tsunamis, including the tsunami with the highest wave height ever recorded. This famous tsunami, was induced by an earthquake-triggered landslide in Lituya Bay and reached a wave height of over 1700 feet. 

Landslides and Tsunamis

Landslide-generated tsunamis occur not only in ocean bays, but in lakes as well. Tsunamis generated by landslides occur more in Alaska than in any other part of the United States. This kind of tsunami is possible in Juneau because of the presence of unstable soils and landslide-prone areas.

Earthquakes and Tsunamis

Not all earthquakes generate tsunamis. For a tsunami to be triggered, an earthquake must occur underneath or near the ocean and create movement of the sea floor.  All oceanic regions of the world can experience tsunamis, but large, destructive tsunamis more frequently occur in the Pacific Ocean because of the many large earthquakes along the margins of the Pacific Ocean.

Tsunamis can also be caused by landslides or ice falls. When a large mass of earth slides into a body of water, it displaces a large amount of water and causes a tsunami. If the event occurs in a lake or channel, the water may slosh back and forth until the wave loses momentum - a phenomenon known as a seiche. Seiches were observed in Skagway during the Denali Fault earthquake of 2002.

Southeast Alaska has been the site of ten of Alaska's historical tsunamis, including the tsunami with the highest wave height ever recorded. This famous tsunami was induced by a major landslide in Lituya Bay an dreached a wave height of over 1700 feet.  Juneau's history of landslides and unstable ground make landslide-induced tsunamis a particular concern.

The 1964 Good Friday Earthquake in Southcentral Alaska resulted in a tsunami that struck Southeast Alaska.  Fortunately the series of waves traveled through the Panhandle during low tide and resulted in minimal damage.  In Juneau, tsunami waves crested only four feet over the expected tide level.  Juneau luckily escaped damage, but the event shows that distant-source tsunamis are indeed possible throughout the inner channels of Southeast Alaska.

What you can do

  • A strong earthquake felt in a low-lying coastal area is a natural worning of possible immediate danger.  Keep calm and quickly move to higher ground away from the earthquake.
  • All large earthquakes do not cause tsunamis, but many do. If the quake is located near or directly under the ocean, the probablity of a tsunami increases. When you hear that an earthquake has occurred in the ocean or coastline regions, prepare for a tsunami emergency.
  • Tsunamis can occur at any time, day or night.  They can travel up rivers and streams that lead to the ocean.
  • A tsunami is not a single wave, but a series of waves. Stay out of danger until an "ALL CLEAR" is issued by a competent authority.
  • Approaching tsunamis are sometimes heralded by a noticeable rise or fall of coastal waters.  This is nature's tsunami warning and should be heeded.
  • A small tsunami at one beach can be a giant a few miles away.  Do not let the modest size of one make you lose respect for all.
  • Sooner or later, tsunamis visit every coastline in the Pacific. All tsunamis-like hurricanes-are potentially dangerous even though they may not damage every coastline they strike.
  • Never go down to the beach to watch for a tsunami!  WHEN YOU CAN SEE THE WAVE YOU ARE TOO CLOSE TO ESCAPE.   Tsunamis can move much faster than any person can run!
  • During a tsunami emergency, your local emergency management office, police, fire and other emergency organizations will try to save your life. Give them your full cooperation.
  • Homes and other buildings located in low lying coastal areas are not safe. Do not stay in such buildings if there is a tsunami warning.
  • The upper floors of high, multi-story, reinforced concrete hotels can provide refuge if ther eis not time to quickly move inland or to higher ground.
  • If you are on a boat or ship and there is time, move your vessel to deeper water (over 100 fathoms).  If there is concurrent severe weather, it may be safer to leave the boat at the pier and move overland to higher ground.
  • Damaging wave activity and unpredictable currents can affect harbor conditions for a period of time after the tsunami's initial impact.  Be sure conditions are safe before you return your boat or ship to the harbor.
  • Stay tuned to your local radio, marine radio, NOAA Weather Radio, or television stations during a tsunami emergency - bulletins issud through your local emergency management office and National Weather Service offices can save your life.

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