Minimum 1D Velocity Model from Local Earthquake Data in the Provence Region,South-Eastern France

We computed a one-dimensional (1D) velocity model and station corrections refering to the Provence region (South-eastern France) by inverting P-wave arrival times recorded on an eight-station local seismic network. Using this velocity model and the program HYPOELLIPSE (Lahr, 1989), we relocated a set of 108 local events. The quality comparison between previous earthquake locations and new relocated shows a good improvement.The obtained Minimum 1D velocity model can be used in a better-quality routine for earthquake location and represent a first step towards more detailed seismic studies in the Provence region.     

Imaging of the Moho depth in Madagascar through the inversion of gravity data: geodynamic implications

Several E-W profiles of Bouguer anomalies across the Precambrian basement were inverted by applying a linearized inversion procedure in the spectral domain and using a simplified two-layer model with a crust-upper mantle interface of constant density contrast. From these inverted profiles, a contour map of Moho topography has been constructed which covers the area of exposed Precambrian basement in the eastern and central parts of the island. Such imaging of the Moho depth in particular exhibits a N20°E-trending zone of substantially thinned crust along the axis of the island. This parallels the east coast margin which resulted from the northward motion of India relative to Madagascar in the Upper Cretaceous. From combined analysis of geological, tectonic and metamorphic observations, this prominent crustal feature in the Precambrian basement of Madagascar is interpreted as the relic of continental lithospheric extension and thinning possibly related to Panafrican collapse tectonics on the eastern edge of the Mozambique belt. This extensional feature would have been reactivated only in part during the rifting and dislocation stages of the Madagascar and India continental blocks as evidenced by an axis of Cretaceous volcanism emphasized by the Ankaratra and Itasy massifs. Final separation nevertheless occurred further east, at the site of the east coast margin.     

Accurate hypocentre locations in the Middle-Durance Fault Zone, South-Eastern France

A one-dimensional velocity model and station corrections for the Middle-Durance fault zone (south-eastern France) were computed by inverting P-wave arrival times recorded on a local seismic network of 8 stations. A total of 93 local events with a minimum of 6 P-phases, RMS 0.4 s and a maximum gap of 220° were selected. Comparison with previous earthquake locations shows an improvement for the relocated earthquakes. Tests were carried out to verify the robustness of inversion results in order to corroborate the conclusions drawn from our findings. The obtained minimum 1-D velocity model can be used to improve routine earthquake locations and represents a further step toward more detailed seismotectonic studies in this area of south-eastern France.