This is often a question many people never ask themselves, but at times they can be affected by the answer when they go to the beach. Ocean surface waves are generated basically two things; earthquakes that cause tsunami and ocean surface winds that cause “wind waves” on the surface.
It turns out that if waves have enough energy, once formed they will continue moving until they reach land! Yes we have tracked very large waves caused by strong winds in the South Indian Ocean that end up breaking on the coasts of Alaska more than 8,000 miles away. This is one reason why most hurricanes in the Atlantic cause waves that reach some or all portions of the US East Coast. The hurricane and its winds can remain thousands of miles away. This is why you can be at the shore in Long Island on a later summer afternoon, the wind is calm, the hurricane is a thousand miles away but the surf is 15-20 feet high!
The closer the wind generating area is to YOU the higher the waves will be for any given wind speed. In the case of strong storms the “decaying” waves that leave the wind area are called swell. Where I grew up you see swells all the time on Southern California beaches in summer and fall. These large swells have come either from a tropical storm or hurricane off the coast of Mexico/Baja typically 800 to 1,00 miles away or from a large winter storm in the Southern Hemisphere 2,500-5,000 miles away (it is winter down there in our summer).
In the cause of tsunami, when it forms from an earthquake at the coast or under water, its wavelength is very long and hence it can move at speeds of more than 500 miles an hour…the water is not moving with the tsunami wave (except locally up then down)…the wave is propagating through the water. Water moves once the tsunami comes into shallow water zones at the coast and the tsunami breaks, then water moves and rushes onshore.
Dr. Steve Lyons 