A tsunami requires a massive and sudden displacement of water in the ocean in order to form. The most likely trigger for this to occur is a powerful undersea earthquake along tectonic plate boundaries. The tsunami is created when one tectonic plate violently slips under another, lifting or dropping a massive column of ocean water above the fault line. Gravity immediately pushes the displaced water back down, causing energy to ripple outward in all directions, (18) _______.
As mentioned, tsunamis generally, but not always, develop in conjunction with seismic activity. Earthquakes that result in tsunamis are typically shallow and possess a high magnitude. (19) _______, especially when the geological shift is horizontal rather than vertical. (20) _______, these deep-ocean waves travel at jet-liner speeds without losing much energy, which creates the massive wall of water that coastal residents associate with danger and destruction of property.
(21) ______. The first stage occurs deep beneath the ocean surface, where tectonic stress builds up until the rock fractures. As the energy is released into the water, it begins an invisible horizontal movement across the open sea, where waves are barely noticeable. Once the waves approach shallow coastal waters, compressed energy causes the wave height to rise dramatically, or to shift from a long, low wave to a towering wall of water. The shoreline then recedes dramatically just before the wave strikes, exposing the seabed. Waves that have accumulated immense power in the shallow bay (22) ______. Shortly after the completion of the previous step, the tsunami crashes ashore.