Tsunamis are actually a series of waves, but are catagorized by the magnitude of the largest wave of the set. There are two types of tsunamis: Local and Teletsunamis.
Local tsunamis are waves affecting coastlines close to the generating area (whether by tectonic or landslide activity). They may be quite severe but only affect a limited area. An example is a fjord wall that collapsed in 1958 in Alaska and sent a surge of water up the other side clearing trees to an elevation of 1,725 feet (526 meters). A wave, over 100 feet in height, also swept out of the bay but was barely recorded by Hawaiian wave sensors.
Teletsunamis, however, are
waves generated more then 1000 kilometers from the affected shore.
Usually produced by major earthquakes they affect entire ocean basins,
though as they travel their energy becomes dispersed as they spread out.
They travel at speeds of 800 - 900 kilometers per hour with periods (length)
of about an hour and very small amplitudes (height), which means they pass
quite harmlessly beneath ships at sea. As they approach a coast the
front of the wave is slowed by the decreasing depth allowing the rest of
the wave to catch up and the wave builds before breaking or flooding inland.
In 1960 an earthquake of a magnitude of 8.6 occured off the coast of Chile
and produced a teletsunami that reached the Alaskan coast in 20 hours creating
a 2.3 meter wave. Usually teletsunamis are barely noticable when
they reach distant shores, however records exist of 50 foot tsunamis reaching
Alaskan shores from earthquakes off the coast of Russia.
Source: National Oceanic and Atmospheric Administration;
Tsunamis
Affecting Alaska, 1737 - 1996, NGDC Key to Geophysical Research, Documentation
No. 31; September 1996.