In one sense, all seismology is the study of historical earthquakes. Earthquakes are short-lived phenomena; over within a couple of minutes at most, well before the seismologist can arrive on the scene. Every earthquake is history, albeit recent history, by the time it can be studied. This inability to observe an earthquake in real time has coloured the development of seismology as a science. In lieu of direct observation, scientists have been obliged to rely on secondary phenomena, or to put it another way, on traces left by the earthquake. These can be grouped into three: permanent marks left on the landscape, written descriptions of the effects of an earthquake, and the recordings made by instruments specifically designed to register the movement of the ground during an earthquake. These three different types of data are the three pillars on which seismological knowledge rests.