  A measure of the correlation between two earthquakes is used to link events
to their aftershocks, generating a growing network structure. In this framework
one can quantify whether an aftershock is close or far, from main shocks of all
magnitudes. We find that simple network motifs involving links to far
aftershocks appear frequently before the three biggest earthquakes of the last
16 years in Southern California. Hence, networks could be useful to detect
symptoms typically preceding major events.
