The strength (magnitude) and location (epicentre and depth) of a strong earthquake can be determined accurately and quickly with the measurement systems and methods of seismology. Seismology also allows an estimation, but until now, not an accurate determination of possible movements (deformations) of the Earth’s surface in the earthquake’s area. To give the important answer to the question of whether a strong earthquake has generated a tsunami or not, knowledge of these co-seismic deformations is of very high relevance for tsunami early warning. Help comes from GPS instrumentation (GPS stations) and a special analysis system, which detects and precisely determines ground motions on the Earth’s surface. GPS-based displacement measurements have their strength close to earthquake epicentres, where displacement values are high. Seismological sensors close to an epicentre may have problems (over steer effects) while measurements from seismological sensors that are further away, are delayed according to seismic signal travel times. In combination with other techniques (determination of the mechanism and area that generates a Tsunami), measurement values that are determined using GPS technology may increase the reliability of tsunami early warnings significantly (Figure 1).