Lithospheric structure of the southern French Alps inferred from broadbandanalysis

Broadband receiver functions analysis is commonly used to evaluate the fine -scale S-velocity structure of the lithosphere. We analyse teleseismic P-wa ves and their coda from 30 selected teleseismic events recorded at three se ismological stations of to the French TGRS network in the Alpes Maritimes. Receiver functions are computed in the time domain using an SVD matrix inve rsion method. Dipping Moho and lateral heterogeneities beneath the array ar e inferred from the amplitude, arrival time and polarity of locally-generat ed P-S phases. We propose that the Moho dips 11 degrees towards 25 degrees +/- 10 degreesN below station CALF, in the outer part of the Alpine belt. A t this station, we determine a Moho depth of about 20 +/- 2 km; the same de pth is suggested below SAOF station also located in the fold-trust belt. Be neath station STET located in the inner part of the Alpine belt, the Moho d epth increases to 30 km and dips towards the N-NW. Moreover, 1D-modelling o f summed receiver function from STET station constrains a crustal structure significantly different from that observed at stations located in the oute r part of the Alps. Indeed, beneath CALF and SAOF stations we need a 2 km t hick shallow low velocity layer to fit best the observed receiver functions whereas this layer seems not to be present beneath STET station. Because r ecent P-coda studies have shown that near-receiver scattering can dominate teleseismic P-wave recordings in tectonically complicated areas, we account for effect of scattering energy in our records from array measurements. As the array aperture is wide relative to the heterogeneity scale length in t he area, the array analysis produces only smooth imaging of scatterers bene ath the stations. (C) 2000 Elsevier Science B.V. All rights reserved.