City and Borough of Juneau
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