A three-dimensional crustal velocity model of the southwestern Alps from local earthquake tomography

A temporary network of 65 short-period seismological stations was installed in the southwestern Alps during the second half of 1996. It complemented t he permanent monitoring networks, obtaining an average interstation distanc e of similar to 10 kin. Travel time data from 446 local earthquakes and 104 quarry blasts were inverted simultaneously for hypocenter parameters and t hree-dimensional velocity structure. The P wave velocity model displays str ong lateral contrasts both at shallow and deeper levels. A low-velocity ano maly stands out at shallow depths beneath the Digne and Castellane nappes i n the southwestern part of the investigated area. Farther east, the Monviso ophiolitic massif appears to have a, much larger extension at depth than p reviously assumed. The largest and strongest anomaly is located under the D ora Maim massif and the westernmost Po plain. It correlates with the well-k nown Ivrea body, which is classically interpreted as a wedge of Adriatic up per mantle. At the best resolved depths (10 and 15 kin) it appears as a rat her thin (10 to 15 km), north-south elongated, high-velocity (7.4 to 7.7 km s(-1)) anomaly with very sharp edges, extending to the south as far as 10 kin north of the surface trace of the Frontal Penninic Thrust. Special care was taken with regard to the quantitative estimation or the resolution for the main anomalies using the inversion of synthetic travel time data.