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 earthq uakes by the Italian Telemetered Seismic Network and by stations of ne arby countries are inverted to image lateral variations of seismic vel ocity and anisotropy at subcrustal depth in Italy and surrounding regi ons. This method allows simultaneous imaging of variations of Pn veloc ity and anisotropy, as well as crustal thickness variations. The Po pl ain, the Adriatic Sea, and the Ionian Sea have normal to high Pn veloc ities. In contrast, lower velocities (7.9-8.0 km/s) are imaged in Ital y beneath the western Alps, the northern Apennines, and eastern Sicily and nearby Calabria, as well as in northern Albania and beneath the P annonian basin. Low Pn velocities beneath the northern Apennines corre late with present-day extension and may have resulted from thermal ano malies; in the uppermost mantle possibly due to delamination processes . Low velocities are consistent with the high-attenuation zone inferre d 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 k m/s. The largest anisotropic velocity anomalies are observed along the major are structures of Italy, i.e., the northern Apennines and the C alabrian Are, indicating that these features are controlled by uppermo st mantle processes. The anisotropy anomaly along the Calabrian Are ex tends as far as Albania but ends abruptly north of this area, suggesti ng that a lithospheric discontinuity is present along the northern Alb anian border.