The Earth's crust is made up of several pieces called tectonic plates. They fit together a bit like a jigsaw puzzle; but they are separate and they do move. Their movements are dependant on the forces exerted from below, by the viscosity of the mantle of the Earth. These same floating movements of the plates are, can we say, irregular and sometimes come into conflict, one plate moving against another. When this happens, there is a building up of tremendous pressure, until one plate gives way by sliding beneath the opposing one (called subduction) forcing it in an upward direction...if the plates in question are below land...we suffer an earthquake...if the happen to be below the ocean, the sudden release of upward pressure causes a massive tidal wave we call a Tsunami.