Nicole Béthoux

Attenuation of crustal waves across the Alpine Range

We present observational evidence of an anomalous propagation of Lg waves across the south-western part of the Alpine range, and we use numerical simulations to model these observations. We consider a set of 48 earthquakes which occurred in Switzerland, northern Italy, and southeastern France and were recorded by the French (Laboratoire de Detection et de Geophysique) and northwest Italian (Istituto Geofisico di Genova) seismic networks. While the amplitude of the Pn phase is stable throughout the region studied, Lg wave amplitude undergoes strong variations. We map this anomaly in Lg wave propagation by dividing the region into a grid and attributing to each cell a value equal to the mean value of the Lg/Pn amplitude ratios computed for all the paths which cross this cell. The image obtained shows that the extinction of Lg waves occurs in a limited region of the western Alps which corresponds to the zone of highest positive Bouguer anomaly. This zone located to the east of the high peaks of the Massifs Cristallins Externes does not correspond either to the region of the highest topographies or to the one of the deepest Moho. At a frequency of 2 Hz, the crustal waves that cross this anomalous region have amplitudes divided by more than 10 with respect to waves that propagate along other paths. In order to investigate the cause of the anomaly we perform numerical simulations of SH wave propagation through a model of the western Alps which includes the main characteristics of the geological structure. Our results indicate that the geometrical effect of the lateral variations of the medium does not account entirely for the actual vanishing of crustal waves. The simulation predicts a decay of the amplitude by only a factor between 2 and 3. The introduction of corrugation in the interface shapes further adds to the decay. However, other sources of attenuation such as anelasticity or severe heterogeneity must be invoked to explain fully the observations.

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.

New insights on the interseismic active deformation along the North Ecuadorian–South Colombian (NESC) margin

[1] The North Ecuadorian–South Colombian subduction zone was the site of the 1906 Mw 8.8 megathrust earthquake. This main shock was followed by three large events in 1942, 1958, and 1979 whose rupture zones were located within the 500 km long 1906 rupture area. Acombined onshore andoffshore temporary seismic network covering from thetrench to the Andes was deployed during 3 months in the area of large earthquakes, in order to obtain a detailed knowledge of the seismic background activity. Resulting earthquakes location and mechanisms bring new insights on interseismic active deformation distribution in the three main tectonic units of the margin, namely, the Interplate Seismogenic Zone, the fore‐arc region which is part of the North Andean Block and the downgoing oceanic Nazca plate. The interplate seismic activity presents along strike variations, suggesting that the seismicity and the associated stress buildup along the plate interface depend on the time elapsed since the last large earthquakes. According to our results, the updip and downdip limits of the seismogenic zone appear to be located at 12 and 30 km depth, respectively. Shallow to intermediate depth seismicity indicates a slab dip angle of ≈25°. North of the Carnegie Ridge, the Wadati‐Benioff plane is defined beneath the fore arc down to ≈100 km depth. Facing the ridge, the Wadati‐Benioff plane extends beneath the Andes, down to ≈140kmdepth.Thisobservationconflictswiththehypothesisofthepresenceofaflatslabat a depth of 100 km facing the ridge. In the overlying fore‐arc region, the crustal seismicity occurs down to 40 km depth and is mainly concentrated in a roughly NW‐SE 100 km wide stripe stretching from the coast, at about 1°N, to the Andes. The location of this active deformation stripe coincides with observed tectonic segmentation of the coastal domain as evidenced by the presence of an uplifting segment to the south and a subsiding segment to the north of the stripe. It also corresponds to a ≈30° change in the trend of the Andes, suggesting that the curvature of the volcanic arc might play an important role in the deformation of the fore‐arc region.

Volcano-tectonic interactions revealed by inversion of focal mechanisms: stress field insight around and beneath the Vatnajökull ice cap in Iceland

Volcano-tectonic processes in the central part of Iceland, covered by the Vatnajokull glacier, are investigated by inversion of focal mechanisms. Working on a large catalogue of focal mechanisms determined by the Icelandic Meteorological Office, we used a damped regional-scale stress inversion method to obtain an insight of kilometric variations of the stress field. To evaluate the resolution and the stability of this stress field solution, we computed checkerboard tests, stress field models and error propagation tests. Stress field models showed a continuous stress regime between normal and strike-slip faulting, associated with a high stress shape ratio (i.e.; σ1 ≈ σ2). Two main directions of σhmin were evidenced: the first one was in agreement with the regional spreading direction of Iceland and the second one was deviated, being almost perpendicular to the first one. The deviated stress direction is sustained through the 20 year time-span of the study around the Barðarbunga and Grimsvotn central volcanoes while the spreading direction remains predominant around the Hamarinn volcano. This result supports the hypothesis that this volcano lacks collapse caldera and shares a fissure swarm with the larger Barðarbunga volcano. On a smaller temporal scale, during the 1996 volcanic crisis, a bimodal distribution of σhmin showed two opposite strike-slip regimes where the deviated direction dominated. Because these two states of stress T1 and T2 show stress regimes away from the Andersonian positions, P, B and T axes, the rapid flip between these two regimes may be associated with the progressive melt intrusion of a dyke.