In order to understand the distribution of tsunami hazards, both in simple geographical terms and in terms of correlations with other, associated, hazards we need to consider how tsunami are first generated and then propagated outwards from their sources.

		Aerial view of Chilean Coast after 1960 Tsunami generated by an offshore earthquake

Tsunami are caused by rapid perturbations of the seabed or of the water column above it, which either lift the sea surface up above its normal level (the usual case) or depress it. The perturbation may also accelerate a mass of water laterally; the resistance to the motion of this moving mass provided by the surrounding stationary water will also cause the water surface to rise. Gravity then acts to restore equilibrium in this source area, but inertia causes the moving water to overshoot the stable level in an oscillation that produces a series of waves, or wave train, which then propagates outwards from the source area until it either dissipates or collides with a coastline. The physics of this propagation process are considered later. Here we look at how these rapid perturbations are caused.

The ranges of potential sizes, frequency, size distributions and patterns of occurrence of these different types of events differ widely. For example, as discussed below, earthquakes cause most tsunami but have a distinct and well-defined upper size limit. At the other end of the spectrum, while no historical tsunami have been definitively linked to impact events these have a well publicised potential to cause the largest "megatsunami".

Four main processes by which tsunami are generated can be distinguished, although it should be noted that some of them can occur together in the same event and also that it is not always possible to determine which has produced a particular tsunami.