Sequential inversion of local earthquake traveltimes and gravity anomaly - The example of the western Alps

Abstract

We present a joint analysis of gravity anomaly and seismic arrival time data recorded in the western Alps. Seismological data were collected by a network of 126 permanent and temporary stations implemented in 1996. A set of Ο550 local events has been recorded. Gravity data result from the addition of two new gravity surveys to an existing data base. A published velocity model obtained by local earthquake tomography (LET), was used to construct an initial 3-D gravity model, using a linear velocity-density relationship (Birch's law). While the synthetic Bouguer anomaly field calculated for this model has the same shape and wavelength as the observed anomaly, its amplitude is strongly underestimated. To derive a crustal velocity-density model that accounts for both types of observations, we performed a sequential inversion of seismological and gravity data. The variance reduction of the arrival time data for the final sequential model was comparable to the variance reduction obtained by simple LET. Moreover, the sequential model explained Ο90 per cent of the observed gravity anomaly. The main features of our model compared with the LET model are: (1) an important broadening of the high-velocity anomaly associated with the high-velocity high-density Ivrea Body, (2) a 10 km thick low-velocity zone beneath the nappes of Digne and Castellane and (3) a high-velocity zone at more than 25 km depth under the internal zone of the range.