Ambient noise analysis in the Ligurian Sea, a Mediterranean back-arc basin

Abstract:

The Liguro-Provencal Basin is located in the north-western Mediterranean Sea. It was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is highly debated whether ‘normal’ oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea using the LOBSTER array. LOBSTER consists of 29 broadband Ocean Bottom Seismometers (OBS) that were installed in the Ligurian Sea for eight months between June 2017 and February 2018, as part of the AlpArray seismic network.

Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data and calculate cross-correlation functions (CCF) for the LOBSTER array and surrounding land stations. For some station pairs we observe that the correlation functions are dominated by higher modes. Using only fundamental modes, we have to exclude those pairs. Additionally, we correlate short time windows that include strong earthquake events. These CCFs allow us to derive surface wave group velocities for longer periods. We use both sets of CCF to manually pick dispersion curves for a surface wave tomography to obtain 2D group velocity maps. Further, we use these maps to calculate 1D depth inversions. Also, the teleseismic events are used in a receiver function analysis to map the discontinuities beneath the Ligurian Basin.

Our group velocity maps show strong heterogeneities for short periods (5-15 s, corresponding to shallow depths). In general, the velocities increase with depth and the velocity anomalies can be related to varying sediment thickness and magmatism. The longer periods (20-90 s) show a smoother velocity structure that reveals mantle velocities in the vicinity of the Ligurian margin, north of the basin centre. However, the resolution is limited along the Corsican margin due to less station coverage. Our results do not indicate an oceanic spreading centre, however, may hint to an asymmetric opening of the Ligurian Basin.