Shallow earthquake features in the southern Tyrrhenian region: geostructural and tectonic implications

The locations and mechanisms of the Aeolian Islands seismicity during a period of background activity (March 1988 -March 1990) were analysed using data from the local seismic networks operating on these islands and in northeastern Sicily and Calabria. The results are discussed in a more general framework which also includes the information available on the most intensely active phases of recent decades (1978 and 1980). Both in the low- and high-activity periods, seismicity affected mostly the southern and western sectors of the area investigated, where NW -SE lithospheric fault systems define the southern margin of the Tyrrhenian basin and allow SE lateral dislocation of the southern Tyrrhenian crustal structures with respect to the Sicilian ones. The notable lack of shallow seismicity observed in the northern and eastern sectors is interpreted as due to a lesser stress accumulation in the inner part of the SE migrating crustal mass and at some distance from its highly stretched margins. Even if a rigorous identification of the faults which have generated the earthquakes is often difficult because of the location uncertainties both of fault systems and hypocenters, it can be said that a relevant percentage of the Aeolian Islands seismic events occur in correspondence to the main faults (Sisifo and Vulcano), in the depth range 0-25 km and, especially, 8-16 km. In particular, the two strongest events of recent decades (Gulf of Patti, April1978, M=5.5 and Alicudi, May 1980, M=5.8) were located near the bottom of the active layer, approximately at the level of the crust-mantle transition. However, while the fault-plane solutions of these two events are coherent with the presumed dynamics of the Tyrrhenian crust, the P-polarity distributions of the smaller and shallower earthquakes occurring in the same fault systems from periods of background activity show some mechanism heterogeneity. This is explained in terms of stress regime complexity and structural heterogeneity at shallow depth for these faults, which is also coherent with the fairly high values of some of the earthquake statistical parameters such as the temporal clustering of shocks and the slope of the frequency-magnitude relationship. The above data led us to interpret the background activity as being closely related to the tectonic process generating the strongest shocks (the southern Tyrrhenian crust SE migration). Furthermore, bearing in mind a model of seismic energy accumulation-release recently proposed for this area, and considering both the present low seismic rate and the depth distribution of the different magnitude earthquakes, we hypothesize that energy could be presently accumulating in the Aeolian region, possibly at the level of the crust-mantle transition, while a comparatively low release is occurring in the shallower and more fractured structures under a quite complex stress regime due to the high rock fracture and stretch.