An engineer explained to me that regular waves are short, while a tsunami wave can be a mile or more long. Tsunami warning buoys monitor the lengh, the wavelength of the wave, and should a very long wave come by, the buoy signals a satellite, which then relays the warning message. Hawaii, where I am now, did get hit by a tsunami in Hilo in 1946, and now has warning buoys.
The length of the wave also accounts for its’ destructiveness. A normal wave hits, then quickly recedes. A mile long tsunami wave sends that wall of water for a very long time before it finally is spent.