Identification and spatiotemporal organization of aftershocks

We propose a fast method able to discriminate between Poissonian independent earthquakes and aftershocks. The method is based on the evaluation of the variability coefficient, defined as the ratio between the standard deviation and the average value of the interoccurrence time between two successive earthquakes. We apply this technique to the California and a properly constructed synthetic catalogue in order to estimate the level of background seismicity and identify seismic sequences. We then investigate the spatiotemporal organization of aftershocks focusing on the distributions of interevent times and interevent distances between two successive events. We find evidence for the existence of a characteristic spatial length scale, related to the size of the aftershock zone, whereas no typical timescale is detected.