Megathrust earthquakes in subduction zones can produce powerful tsunamis. The rock above the plate boundary is being compressed and bent as it stores the elastic energy during time intervals between the earthquakes. When it can no longer hold the stress, the continental plate slides up abruptly along the plate boundary causing the leading edge to push sea water upward and generating a tsunami.
Tsunami energy extends from the surface of the ocean all the way to the bottom, regardless of the ocean depth. This makes tsunami very different from wind waves that disturb only the ocean surface. This property of tsunamis makes it possible to detect them in the open ocean by bottom pressure sensors.
Sudden failure of unstable submarine slopes can produce a movement of large amount of material, which generates displacement of water causing a tsunami. Such failures can be triggered by ground motions of a strong earthquake, extreme low tides and construction activities in ports and harbors.
The tsunami floods a coastal town as a fast-moving tide destroying everything on their path. The animation demonstrates one of the important warning signs of a tsunami – the initial retreat of water from the shore and exposure of rocks and fish on the sea bottom. However, ocean doesn’t always recede first; tsunami often comes as a wall of water and quickly inundates the coast.
The 28 March 19643 Great Alaska Earthquake generated a powerful tsunami that killed 122 people in the United States. This animation shows the first 6 hours of the tsunami travel. Colors represent tsunami heights, showing that the tsunami energy was mostly directed toward the west coast of the US.
This sequence of three aerial photos of Seward, Alaska show the town just before the 1964 tsunami, a month after the disaster, and in 2003, with the new business development in the 1964 tsunami inundation zone.
This animation shows Seward, Alaska and bathymetry at the head of Resurrection Bay. The landslide material moves down the slope driven by gravity. The slide resulted in damaging local tsunamis in Seward on March 28, 1964.
The waves generated by the underwater slide hit the Seward waterfront in less than 2 minutes after the onset of the earthquake, and killed 8 people. The animation shows an initial depression of the water surface close to the shoreline, and then the wave crest forming in the same area and inundating the Seward waterfront.
The 1964 earthquake triggered multiple landslides in Resurrection Bay. At the beginning of the animation, the dark brown patches indicate initial locations of unstable material that started moving down the slopes as a result of strong ground shaking. Eventually, the slide material reached the deep flat depression in the middle of the bay and filled it completely.
Multiple underwater slides generated a complex wave pattern in Resurrection Bay. In the close-up view of the upper bay at about 1 minute 40 seconds, it is visible that the 3 wave crests coming from different directions add together in the vicinity of Seward waterfront.