Space (3-D) and space-time (4-D) analysis of aftershock sequences: the Friuli (NE Italy) case

Space-time hypocentral coordinates from a fixed number of aftershocks in a time window sliding along the sequence may be used to compute a set of geometric moment matrices thought to represent 4-D rupture hyperellipsoids. Time variations of the hyperellipsoids size and shape, bound to the time variations of the shocks space-time distribution clearly mark the sequence beginning and the onset of different subsequences. The four principal axes of the hyperellipsoids, projected onto 3-D space, depict the seismogenic fault orientation, slip and rupture propagation directions, and the layout of the possible stack of the same-order faults. Discrimination of the single subsequences which constitute a seismic series and the successive directional analysis allow the construction of a reliable evolutionary fracture model. -Authors