This work investigates the spatial distribution of three seismic swarms that occurred in Western Liguria (NW Italy) in 1990 and 1993. The largest events, recorded on July and December 1993, exceeded magnitude 4.0. The routine locations of hypocenters were not able to depict the geometry of the active zone, due to the limitations of the regional seismic network. Also the adoption of different location techniques (“forward” probability estimation, relative locations) did not furnish any particular information on the real spatial distribution of these very closely spaced events.
The analysis of the recorded signals demonstrated the strong coherency among seismograms and suggested the use of doublet-multiplet techniques in order to evaluate the relative position of events. For every swarm, the relative locations obtained (with estimated errors of the order of a few meters) depict clearly the geometry of the very limited area involved in this seismic activity (of the order of 1 km2): all the analyzed events lie (within a few meters) on a plane that is in agreement with one of the planes of the focal mechanism of the mainshocks obtained by first-motion polarities. A strike-slip rupture, with a compressive component, on planes with strike around 80° and dip ranging from 56° to 74°, is in agreement with all the available information. Moreover, using some similar events belonging to the different periods of activity (bridge events), the relative position of the three groups of earthquakes was also determined, showing that the more energetic activity of 1993 reactivated a shallower area contiguous to the sector mobilized in 1990 and characterized by a slightly different dip.