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 the permanent monitoring networks, obtaining an average interstation distance of ∼10 km. Travel time data from 446 local earthquakes and 104 quarry blasts were inverted simultaneously for hypocenter parameters and three‐dimensional velocity structure. The P wave velocity model displays strong lateral contrasts both at shallow and deeper levels. A low‐velocity anomaly stands out at shallow depths beneath the Digne and Castellane nappes in the southwestern part of the investigated area. Farther east, the Monviso ophiolitic massif appears to have a much larger extension at depth than previously assumed. The largest and strongest anomaly is located under the Dora Maira massif and the westernmost Po plain. It correlates with the well‐known Ivrea body, which is classically interpreted as a wedge of Adriatic upper mantle. At the best resolved depths (10 and 15 km) it appears as a rather 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 km north of the surface trace of the Frontal Penninic Thrust. Special care was taken with regard to the quantitative estimation of the resolution for the main anomalies using the inversion of synthetic travel time data.