Most waves are generated by strong winds blowing across vast stretches of the ocean. As the waves approach shore, the underwater part of the wave slows down as it feels the sand beneath the shallower water, whereas the top of the wave continues its speed. With the top of the wave moving faster than the bottom, it breaks or spills over, often creating a surfing wave.
In contrast, a tsunami is generally caused by an earthquake (magnitude 7.0 or greater) adjacent to or under the ocean. If the earthquake involves a large segment of land that displaces a large volume of water, the water will travel outwards in a series of waves (like concentric ripples from a pebble dropped in a pond). In this case, the displaced water is not just at the surface like it is with wind-generated waves, but extends from the ocean surface to the sea floor where the earthquake originated. This makes a tsunami wave far more powerful and dangerous than surfing waves.
Tsunami waves are only a foot or so high at sea, but they are100 miles long and travel at 500 miles per hour. When they approach shore, they too begin to feel bottom and slow down, but not into a surf-shaped wave. Instead the water is forced to stack up in height and rush far inland at tremendous speed. The water then recedes, and more waves follow at intervals of 10-20 minutes.
Hawaiʻi experiences tsunamis from two sources. One is earthquakes from the Pacific rim, such as Alaska and Chile. For these earthquakes, there is a warning time of a few hours. The second source is locally-generated earthquakes, such as the 1975 Halapē M 7.2 earthquake. For these events, the warning time is only seconds to minutes, so beachgoers must move to higher ground and far inland immediately.
In Hawaiʻi, tsunamis have killed more people in the last hundred years than earthquakes, volcanoes, hurricanes, brushfires, and floods combined. Make sure you stay far away from the coastline anytime a tsunami threatens.