Lateral variations of Pn propagation in Italy: Evidence for a high attenuation zone beneath the Apennines

Abstract

Pn phases recorded by 40 stations of the Italian seismic network are analyzed using the spectral ratio technique to estimate the Q structure of the uppermost mantle beneath the Italian peninsula and nearby Adriatic Sea. A total of 344 digital waveforms are analyzed from 22 earthquakes that occurred within distances of 300 to 1600 km. The calculated apparent Q values are grouped into two categories: Q > 800 characterizes the Adriatic side of the Italian peninsula, indicating that the Adriatic lithosphere is very efficient in propagating Pn phases through the uppermost mantle; Q < 600 characterizes the uppermost mantle beneath the Apennines and western Italy, indicating less efficient wave propagation. The presence of asthenospheric mantle material at shallow depths beneath the Apennines can explain the observed Q.