Shear wave attenuation in the lithosphere beneath Italy and surrounding regions: Tectonic implications

Abstract

More than 700 waveforms produced by 51 shallow earthquakes and recorded at regional distances (250-1400 km) by the Italian seismic network have been analyzed to characterize the amplitude and frequency contents of the crustal and uppermost mantle shear waves Lg and Sn, respectively. The crustal phase Lg efficiently propagates through the relatively stable Adriatic continental crust, while it is not observed along propagation paths crossing major physiographic features, like the Apennine chain and the basinal domain of the Tyrrhenian and Ionian Seas. Similar to Lg, efficient Sn propagation is observed in the uppermost mantle beneath the Po plain and the Adriatic Sea. Efficient Sn transmission is also observed across the northern Ionian Sea and Sicily and in the area between Sardinia and the northern coasts of Africa. Sn are efficiently transmitted across the Sicily Channel, and rather efficient Sn propagate beneath the Ligurian Sea. On the contrary, inefficient Sn transmission characterizes the uppermost mantle beneath the Apennines, the western margin of the Italian peninsula, and the southern Tyrrhenian Sea. Shear wave attenuation suggests the presence of asthenospheric material in the uppermost mantle, probably related to the present-day extension along the Apennine chain and in the Tyrrhenian basin. This interpretation is consistent with the presence of extensive Neogene and Quaternary volcanic activity in these areas and related high heat flow. Proposed lithospheric delamination processes beneath the Apennines and subduction beneath the Tyrrhenian Sea can reasonably explain the observed high-attenuation zones in the uppermost mantle. In contrast, a high-strength mantle lid is inferred to underlay the Po plain, the Adriatic Sea, and the northern Ionian Sea. The available waveforms also indicate that a continuous mantle lid is present beneath Sicily and the extensional domain of the Sicily Channel, as well as in the marine area south of Sardinia.