Tsunami comes from japanese harbour wave . Wave is harmless before entering the harbour. Has nothing to do with tides but sometimes called a tidal wave. Can hit with little or no warning. The most prone areas are those associated with earthquakes and volcanoes in subduction zones. Ex: subduction zone of cascadia has potential for offshore quakes of m > 8. In shallow water, tsunami waves pile up velocities and wavelengths decrease. At the same time amplitude increases a lot. In deep water generally less than 1 meter. In shallow water can reach 40 meters or more above normal sea level. The wave will break when its height will exceed 1/7th of its wavelength. Drawdown rise or fall in sea level. Long wavelength takes along time for tsunami wave to crest. Wave may remain high for several minutes. Tsunami made up of a series of waves. Takes hours for the crests of successive waves to reach the shore.