So far, there are no early warning systems worldwide that that constantly monitor the entire ocean, which can be used to discover and track tsunamis. Model calculations help to determine the possible extent of a tsunami across the ocean. In the tsunami early warning system, for the first time, measurement data, such as seismological parameters, GPS measurements of land displacements and coastal tide gauge measurements of the sea level are being included in the model calculations. This results in a dynamic-updated model, that constantly reflects the processes in the ocean and can forecast their future progress. Such models form the sound quantitative basis for a modern tsunami early warning system such as GITEWS.

Traditionally, tsunami modelling is divided into two steps: (1) Modelling of the tsunami source (source modelling) and (2) modelling of the tsunami propagation (tsunami modelling) including coastal flooding (run-up modelling). Both steps are effectively uncoupled from one another. The physical background is that the seismic fracture that lifts on the ocean floor and thereby triggers a tsunami wave, usually does not take place locally for longer than 1 to 2 minutes and can be regarded as an instant initial condition from the point of view of wave propagation physics. In spite of the major importance of wave modelling, source monitoring must not be underestimated: this is precisely where several uncertainties accrue, which are compressed up to the coast and determine the majority of the resulting accuracy of the early warning. The calculation is also made more difficult by the fact that particularly major earthquakes have fracture lengths of around a thousand kilometres and the entire quake process can take up to ten minutes.

As a third step, GITEWS adds a particularly high-resolution simulation of flooding for densely populated coastal areas, in order to produce risk maps and gain a quick overview of the endangered population and infrastructure in an emergency.

In the event of an earthquake, only a few minutes often remain in Indonesia to decide on a warning, as the earthquake zones are close to the coasts. Therefore, a database of tsunami scenarios was produced for the Sunda Trench in the Indian Ocean, in order to conclude the expected danger from the measurement data within seconds.