Spatio-temporal correlations in global seismicity.

It is known that earthquakes are not distributed uniformly both in space and in time. It has often been noted that their occurrence presents temporal clustering but this is apparently not visible when analyzing the whole world seismicity. Indeed, in catalogs that refer to very long periods of time or to very large areas, the occurrence of earthquakes can be assimilated to a random process. Instead, there are precise limits of space and time within which seismicity is characterized by a fractional dimension. This result is made possible by an innovative analysis that simultaneously involves inter-times and distances between events. The results also show the existence of a slow increase over time of the area subject to aftershocks.