François Thouvenot

Widespread extension in the core of the western Alps revealed by earthquake analysis

The western Alps are an active collision belt whose current stress field is inhomogeneous [Muller et al., 1992]. We report new seismological data which significantly improve our knowledge of this stress field. About 1600 earthquakes which occurred in the western Alps during the last 10 years were precisely located, and 79 new focal solutions were computed. The analysis of this database shows that widespread extension affects all the internal zones of the belt. To better constrain the associated stress regime, six stress tensors were computed using the Gephart and Forsyth [1984] method. They show that the current tectonics of the western Alps are contrasted with close variation in the stress regime (transpression to the front of the belt contrasting with extension in the core of the belt). The extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone, suggesting extensional reactivation of this inherited crustal discontinuity. Such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision-related tectonics. We propose that intrabelt buoyancy forces, such as those produced by a slab retreat or slab break-off, interfere with the boundary forces driven by the ongoing Europe-Africa convergence.

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.

Changes in effective stress during the 2003―2004 Ubaye seismic swarm, France

[1] We study changes in effective stress (normal stress minus pore pressure) that occurred in the French Alps during the 2003–2004 Ubaye earthquake swarm. Two complementary data sets are used. First, a set of 974 relocated events allows us to finely characterize the shape of the seismogenic area and the spatial migration of seismicity during the crisis. Relocations are performed by a double‐difference algorithm. We compute differences in travel times at stations both from absolute picking times and from cross‐correlation delays of multiplets. The resulting catalog reveals a swarm alignment along a single planar structure striking N130°E and dipping 80°W. This relocated activity displays migration properties consistent with a triggering by a diffusive fluid overpressure front. This observation argues in favor of a deep‐seated fluid circulation responsible for a significant part of the seismic activity in Ubaye. Second, we analyze time series of earthquake detections at a single seismological station located just above the swarm. This time series forms a dense chronicle of +16,000 events. We use it to estimate the history of effective stress changes during this sequence. For this purpose we model the rate of events by a stochastic epidemic‐type aftershock sequence model with a nonstationary background seismic rate l0(t). This background rate is estimated in discrete time windows. Window lengths are determined optimally according to a new change‐point method on the basis of the interevent times distribution. We propose that background events are triggered directly by a transient fluid circulation at depth. Then, using rate‐and‐state constitutive friction laws, we estimate changes in effective stress for the observed rate of background events. We assume that changes in effective stress occurred under constant shear stressing rate conditions. We finally obtain a maximum change in effective stress close to −8 MPa, which corresponds to a maximum fluid overpressure of about 8 MPa under constant normal stress conditions. This estimate is in good agreement with values obtained from numerical modeling of fluid flow at depth, or with direct measurements reported from fluid injection experiments.

SEISMICITY ALONG THE NORTHWESTERN EDGE OF THE ADRIA MICROPLATE

This contribution is an overview of the seismic activity observed along the northwestern edge of the Adria microplate over the last 15 years. The study area stretches from Lyons to Turin, and from Geneva to Nice. The discussion focuses on several key zones where significant earthquakes occurred during this period, or where the connection between seismicity and tectonics provides clear-cut results. Our main conclusions are: E–W to NW–SE compression in the Ligurian Sea and at depth beneath the southwestern Po plain; in the core of the western Alps, widespread extension radial to the chain, with a component of right-lateral strike slip along faults longitudinal to the chain; in the European foreland, almost exclusively strike slip, either right-lateral along faults longitudinal to the chain or left-lateral in a conjugate direction. The observed right-lateral strike slip is consistent with an anticlockwise rotation of Adria about a pole in the Po plain, but the extension observed in the root zone cannot be explained with a simple rigid-plate model. This extension probably also involves gravitational collapse and/or slab roll-back or break-off.