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

Abstract

Summary 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 ∼550 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 ∼90 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.

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

“…The western boundary of the Ivrea body, corresponding to lithospheric mantle, trends approximately N-S beneath the southern part of the Western Alpine arc (Waldhauser et al, 2002;Kissling et al, 2006;Lardeaux et al, 2006), that is in strong discrepancy with the present arc shape. We propose that this N-S boundary which appears roughly rectilinear at ~15 km depth (Vernant et al, 2002;Schreiber et al, 2010) could be a relict of the western transform boundary of Adria during northward Eocene drift (WATZ, fig. 13).…”

First seismic evidence for continental subduction beneath the Western Alps

“…In the internal Alps, we built our crustal model based on the results of the 1958-1960 seismic experiments in the western Alps (Closs and Labrouste, 1963), and the local earthquake tomography (Paul et al, 2001), complemented by the sequential inversion of the local earthquake traveltimes and gravity anomaly (Vernant et al, 2002) of the GeoFrance-3D experiments. Since the 1960s (Closs and Labrouste, 1963), we have known that the shallowest part of the Ivrea surface (i.e., the top surface of the Ivrea body) is located at 10 km depth and x = 70-80 km along our profile.…”

Deciphering orogeny: a metamorphic perspective. Examples from European Alpine and Variscan belts

“…In the western Alps, the Moho geometry is documented in both the northern [Ménard and Thouvenot, 1984;Nicolas et al, 1990;Roure et al, 1996;Waldhauser et al, 1998;Schmid and Kissling, 2000] and the southern [Masson et al, 1999;Paul et al, 2001;Vernant et al, 2002;Thouvenot et al, 2007;Schreiber et al, 2010] domains. The European Moho deeps progressively eastward and underthrusts the south dipping Adriatic Moho that remains sub-horizontal at shallower depth.…”