Seismic swarms are defined as a group of earthquakes occurring very close in time and space but without any larger event triggering their occurrence. Up to now no simple law has been found to describe the swarm occurrence rate. Here we find an expression able to fit the average occurrence rate on some volcanic areas. Such an expression exhibits some differences in respect of the usual Omori law. Namely the $c$ parameter of the Omori law is equal to zero and the power law decay of the average occurrence rate of the earthquakes is followed by an exponential decaying regime. Both the results can be interpreted in term of fluid injection and/or movements. Indeed this is a more impulsive phenomenon, in respect to the occurrence of a large earthquake, with a duration compatible with a $c=0$. The exponential decay following the power law one could explained by a viscoelastic relaxation of the stress induced by the injection and/or movements of fluids in the earth crust.