The deep seismicity of the Tyrrhenian Sea

Abstract

l'he study reappraises the deep seismicity of the Tyrrhenian Sea. Careful examination of the quality of reported hypocentres shows that the earthquakes define a zone dipping NW, about 200 km along strike, 50 km thick, and reaching a depth of about 500 km. The zone is slightly concave to the NW at a depth of 300 km, but, contrary to many previous reports, is not tightly concave, nor are there significant spatial gaps in the seismicity, which is effectively continuous with depth. Seismicity is, however, concentrated in the depth interval 250-300 km, where the dip of the seismic zone changes from 70" (above 250 km) to a more gentle dip of 45" at greater depths. Seven fault-plane solutions are available for the largest earthquakes in this depth interval, all of them consistent with a P-axis down the dip of the seismic zone, and all of them requiring movement on faults out of the plane of the subducting slab.Two deep earthquakes near Naples lie well outside the main zone of activity; for one of which a fault-plane solution is available that has aP-axis not aligned with the dip of the seismic zone. The tightly concave slabgeometry favoured by other reports is supported mainly by the location of these events near Naples, which we think may represent deformation in a separate, probably shallower dipping, piece of subducted lithosphere.The lack of shallow seismicity, and particularly of thrust faulting earthquakes, at the surface projection of the Benioff zone suggests that active subduction has ceased. Estimates of the convergence rate responsible for subduction in the last 10 Myr far exceed the present convergence rate of Africa and Eurasia, suggesting that the subduction was related instead to the stretching and thinning of the crust in the Tyrrhenian Sea.

Evolution of the southern Tyrrhenian slab tear and active tectonics along the western edge of the Tyrrhenian subducted slab

“…Earthquake hypocentral distributions and seismic tomography indicate the presence of a steep, deep and narrow slab, dipping NW-wards, underneath the Calabrian Arc (Anderson & Jackson 1987;Piromallo & Morelli 2003). The seismic slab reaches a depth of about 500 km and with its 708 slope is one of the steepest observed in subduction zones (Isacks & Barazangi 1977).…”

Tectonics and seismicity of the Tindari Fault System, southern Italy: Crustal deformations at the transition between ongoing contractional and extensional domains located above the edge of a subducting slab

“…It is widely accepted, for instance, that the causes for the extensional strain regime that has led to the appearance of the Tyrrhenian oceanic basin (Figure 1b) are connected with the subduction and rollback of the Ionian plate underneath the Calabria [Malinverno and Ryan, 1986;Kastens et al, 1988]. At present, the occurrence of the subducted Ionian slab beneath the Calabria and the Tyrrhenian Sea is inferred by subcrustal earthquakes (Figure 1b), which define, to the east of the Aeolian Islands, a NW dipping, narrow, Wadati-Benioff zone [Isacks and Molnar, 1971;Anderson and Jackson, 1987;Giardini and Velonà, 1988].…”

Southern Adriatic Sea as a Potential Area for CO2Geological Storage

“…The Adriatic region is characterized by high-seismicity in particular along its collisional margins (Anderson and Jackson, 1987;Favali et al, 1993;Kastelic and Carafa, 2012). At present-day, the Southern Adriatic area is characterized by a relatively moderate tectonic activity mainly concentrated along two main E-W trending lineaments located on the Mid-Adriatic Ridge and close to the Gargano promontory (Scisciani and Calamita, 2009) (Fig.…”