Compressional velocity structure and anisotropy in the uppermost mantle beneath Italy and surrounding regions

Travel times of about 39,000 Pn arrivals recorded from regional earthquakes by the Italian Telemetered Seismic Network and by stations of nearby countries are inverted to image lateral variations of seismic velocity and anisotropy at subcrustal depth in Italy and surrounding regions. This method allows simultaneous imaging of variations of Pn velocity and anisotropy, as well as crustal thickness variations. The Po plain, the Adriatic Sea, and the Ionian Sea have normal to high Pn velocities. In contrast, lower velocities (7.9-8.0 km/s) are imaged in Italy beneath the western Alps, the northern Apennines, and eastern Sicily and nearby Calabria, as well as in northern Albania and beneath the Pannonian basin. Low Pn velocities beneath the northern Apennines correlate with present-day extension and may have resulted from thermal anomalies; in the uppermost mantle possibly due to delamination processes. Low velocities are consistent with the high-attenuation zone inferred in the uppermost mantle beneath the internal Apennine units and the Tyrrhenian margin of the peninsula by Mele et al. [1996, 1997]. On the contrary, low velocities beneath the western Alps may be an apparent effect due to the abrupt thickening of the crustal roots. Pn velocity is anisotropic in the study area with a maximum amplitude of +/- 0.2 km/s. The largest anisotropic velocity anomalies are observed along the major are structures of Italy, i.e., the northern Apennines and the Calabrian Are, indicating that these features are controlled by uppermost mantle processes. The anisotropy anomaly along the Calabrian Are extends as far as Albania but ends abruptly north of this area, suggesting that a lithospheric discontinuity is present along the northern Albanian border.