Tsunami is a word of Japanese origin: ‘tsu' means ‘harbour' and ‘nami' means ‘high wave'. Tsunamis can have different causes: a seaquake (= underwater earthquake), a meteoroid or asteroid impact, a volcanic eruption below water or a big landslide. All of the above result in a big mass falling into the sea. Because of this shift, an enormous amount of water is displaced and lifted. In open sea, you can hardly notice a tsunami, for instead of a high wave, a very long wave is formed (wave length: 100 up to 400 kilometres). This wave travels at a far higher speed than normal.
However, if this wave approaches the coast, it hits shallow water. This causes the wave to slow down. The water in the front falls to the back, upon the coming water, creating an ever increasing huge wave.
It is unlikely that a tsunami will unfold along the North Sea coastline. The North Sea is a shallow sea, and thus the waves cannot travel at a very high speed.
Still, the coast guard partners have taken the necessary precautions and have developed warning systems.