Often a foot high when created, their hundred kilometer wavelengths produce
Often a foot high when created, their hundred kilometer wavelengths produce interesting properties.  Since their frequencies are constant, their wavelengths sharply decrease and their amplitudes increase as their speed slows.  This occurs in shallow water, especially near coastlines where they can become 30 meters tall. For ten points, what oceanic phenomenon is usually caused by submarine earthquakes.
Often a foot high when created, their hundred kilometer wavelengths produce interesting properties.  
tsunami or tidal wave