Seismic evidence of a crustal overthrust in the Western Alps

SUMMARY In the French western Alps, east of Grenoble, we identify the Belledonne Border Fault as an active seismic fault. This identification is based on the seismic monitoring of the Grenoble area by the Sismalp seismic network over the past 12 yr (1989–2000). We located a set of earthquakes with magnitudes ranging from 0 to 3.5 along a ∼50 km long alignment which runs in a N30°E direction on the western flank of the Belledonne crystalline massif. Available focal solutions for these events are consistent with this direction (N36°E strike‐slip fault with right‐lateral displacement). These events along the Belledonne Border Fault have a mean focal depth of ∼7 km (in the crystalline basement), with a probably very low slip rate. The Belledonne Border Fault has never been mapped at the surface, where the otherwise heavily folded and faulted Mesozoic cover makes this identification difficult. Historical seismicity also shows that, over the past two and a half centuries, a few events located mainly along the southern part of the Belledonne Border Fault caused damage, with the magnitude 4.9 1963 Monteynard earthquake reaching intensity VII. The most recent damaging event in the study area is the magnitude 3.5 1999 Laffrey earthquake (intensity V–VI). Although its epicentre lies at the southern tip of the Belledonne Border Fault, there is clear evidence that aftershocks were activated by the left‐lateral slip of a N122°E‐striking fault. The length of the Belledonne Border Fault, which could easily accommodate a magnitude 6 event, as well as the proximity to the Isère valley with its unlithified Quaternary deposits up to 500 m thick known to generate marked site effects, make the identification of the Belledonne Border Fault an important step in the evaluation of seismic risk in the Grenoble area. Besides, the activity observed on the fault will now have to be taken into account in future geodynamic models of the western Alps.

Section: Fault‐plane Solutionssupporting

confidence: 79%

The Belledonne Border Fault: identification of an active seismic strike-slip fault in the western Alps

Thouvenot

Fréchet

Jenatton

et al. 2003

“…The Nafe–Drake relationship was originally established for marine sediments (Nafe & Drake 1957) so we decided to use Birch's law (1961). The velocities of the LET model are underestimated. The maximum velocity obtained at the top of the Ivrea Body by LET (∼7.4 km s −1 ) is consistent with the velocities previously obtained in the 1960s and 1970s by several refraction and wide‐angle seismic experiments (Closs & Labrouste 1963; Berckhemer 1968; Choudhury et al 1971; Perrier 1973; Giese & Prodehl 1976; Ansorge et al 1979; Thouvenot & Perrier 1981). Producing agreement between observed and synthetic residual anomalies requires an increase in the velocity anomaly of the Ivrea Body by a factor of 2.…”

Section: Failure Of the Direct Modelling Of Gravity Data From The Isupporting

confidence: 88%

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

Vernant

Masson

Bayer

et al. 2002

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.

“…Lastly, there are differences in the crustal structure near shot-point A between the present model and the two-dimensional model of Thouvenot & Perrier (1980) due to different phase correlations and interpretation. The model of Thouvenot & Perrier (1980) shows that the boundary between the upper and lower crust deepens from about 15km beneath shot-point A to about 25 km, 30 km east of shot-point A after which it remains at more than 20 km depth all the way to shot-point B and that the basement occurs at 9-10 km depth, 15-25km east of shot-point A. Depths to the Palaeozoic crystalline basement of 2-4.5 km have been found in boreholes in the vicinity of the main line near shot-point A (Menard 1979;Thouvenot & Pemer 1980).…”

Section: Discussion a N D Conclusionmentioning

confidence: 76%

A fine structural section through the crust and lower lithosphere along the axial region of the Alps

Yan¹,

Mechie²

1989

SUMMARY A unified two‐dimensional lithospheric model has been derived for the P‐wave velocity structure beneath the Alpine Longitudinal Profile (ALP'75) main line by making new phase correlations and using ray‐tracing and ray‐theoretical seismogram calculations in laterally inhomogeneous media. Due essentially to an increase in thickness of the lower crust, the total crustal thickness increases from 30–40 km in the peripheral regions to about 50 km in the central region of the Alps. The thickness change is more abrupt at the eastern end than at the western end of the line. The boundary between the upper and lower crust occurs at 18–20 km depth except in the peripheral region at the eastern end of the line where it occurs at 14–16 km depth. Along the whole line the seismic basement has velocities averaged over some tens of kilometres of 5.85‐6.15 km s‐1 with individual values having uncertainties of ±0.05‐0.1 km s‐1 at least east of shot‐point B. The lower part of the upper crust has been modelled as a low velocity layer, which is most pronounced beneath the peripheral regions, and which is generally underlain by a transition zone at the base of which the top of the lower crust occurs. The top of the lower crust has been modelled with velocities ranging from 6.25‐6.5 km s‐1 beneath the peripheral regions to 6.5‐6.65 km s‐1 beneath the central Alps. Velocity uncertainties at the top of the lower crust are estimated to be around ±0.2 km s‐1. Beneath the central Alps a low velocity layer has also been modelled in the lower crust. The top 6 km of the mantle has been modelled with velocities of 8.0–8.2 km s‐1 with individual estimates having uncertainties of ±0.15 km s‐1. Below this, the upper mantle down to almost 100 km depth has been modelled as a series of high velocity layers embedded in material with lower velocities of 8.0–8.2 km s‐1. The combined effects of tectonic overthrusting and uplift and erosion have probably led to the lower crust being apparently thicker than the upper crust in the central Alps at the present day.