On the geological time scale, earthquakes are short-lived phenomena, resulting from an unstable sliding of two compartments of the upper Earth’s layer in a fracture area. (picture 2) The affected area can vary, from several m2 for the weakest earthquakes, to lots of thousands km2 for the most important quakes. In mainland areas the seismic zone, which means the zone where earthquakes arise, is close to the Earth's surface, but it can stretch out to a depth of 10 to 25 km according to the region. Earthquakes are defined as "large" when they influence the whole thickness of the seismic zone. In that case, ruptures can arise at the surface of the ground. In comparison to the seismic activity at the borders of lithosphere plates (picture 3), the occurrence of such earthquakes in the middle of the plates is rather rare. Tectonically speaking these are stable areas. With the growing vulnerability of large urban zones, earthquakes in high-populated areas cause considerable damages with far-reaching consequences. (picture 4)

The duration of a seismic cycle corresponds to the interval of time passed between two major earthquakes. The numerous studies in active zones on the borders of the (picture 5) show that the return of major quakes takes about 100 to 1000 years. In the so-called stable zones the activity is sporadic, with two or three large shocks, in a relatively short period (10.000 to 20.000 years), followed by longer periods of inactivity of several hundred thousand years. A realistic vision on the seismic activity in our region can therefore not be obtained without gathering information about the quakes and tectonic changes during the last ten thousands of years.