Abstract Temporal clustering of seismic shocks is investigated using the Generalized Poisson Process and assuming that grouped events are distributed according to Riemann statistics. The E-parameter (inversely proportional to the clustering level) has been calculated for both volcanic and tectonic earthquakes listed in the catalogues of the Aeolian Island local network (Southern Italy). E-values from other investigations have also been used in order to extend the analysis. Clustering is generally higher for volcanic earthquakes than for tectonic earthquakes. In volcanic regions, the tendency of shocks to group in time increases during eruptive phases with respect to quiescent periods. A similar relationship between earthquake clustering and the activity state of seismogenetic structures may be preliminarily hypothesized in tectonic environments. These findings reveal an interesting analogy between seismic clustering (1/ E) and the coefficient b of the Gutenberg-Richter relationship which “controls” the earthquake distribution in the magnitude domain. A possible explanation of the behaviour of the E and b parameters is proposed, advancing some hypotheses on the role both of rock fracturing level and stress intensity, and on the combined effect of these factors on the statistical properties of earthquake sequences.