Full microtremor H/V and phase velocity dispersion curves inversion for S-wave velocity profiles of subaqueous slopes in Lake Lucerne (Switzerland)

The assessment of seismic hazard and potential earthquake secondary effects such as tsunamis in offshore environments requires, similar to onshore environments, a good knowledge of the subsurface shear-wave velocity (Vs) distribution with depth. Between 2018 and 2020, a seismological campaign with Ocean Bottom Seismometers (OBS) was performed on selected subaqueous slopes with unconsolidated sediment cover in Lake Lucerne (Switzerland) to measure ambient vibrations and to monitore earthquakes. OBS were deployed in the single station and array configurations. First, we use the single station ambient vibration data to estimate the microtremor horizontal-to-vertical (H/V) spectral ratio. Second, we follow a robust preprocessing workflow to extract the phase velocity dispersion curves (DC) from OBS ambient vibration array measurements. The estimated H/V spectral ratios and the DC are used as targets in an inversion process for the Vs profile determination. The microtremor H/V spectral ratio forward modelling routine uses a new model that is based on the seismic interferometry principles under the diffuse field assumption and considers the presence of the water layer.

Keywords: Microtremor H/V spectral ratios, Phase velocity dispersion curves, Ocean
Bottom Seismometer, Site effects, Shear-wave velocity