Rayleigh-wave focal spot imaging of the European Alps and surrounding areas

SUMMARY We apply Rayleigh-wave focal spot imaging to seismic records collected by $\sim$800 stations in the European Alps and surrounding areas and demonstrate the effectiveness of the method for lithospheric imaging in a complex plate boundary environment. We reconstruct ZZ-component focal spots from AlpArray and related network data and estimate fundamental-mode Rayleigh-wave phase velocities in the 20 to 200 s period range using spatial autocorrelation (SPAC) models. Using a short data distance on the order of $1\lambda$ allows us to study Rayleigh waves at longer periods compared to the limits of ambient-noise tomography. We implement SPAC models corrected for the anisotropic background illumination to mitigate the observed strong surface-wave energy incidence in the SW-NE direction. The comparison between our focal spot imaging results and two regional tomography studies demonstrates the resolution power of the method for non-isotropic wavefield conditions and a spatially variable array shape. We invert the Rayleigh-wave dispersion curves using the Neighbourhood Algorithm to construct a pseudo 3-D shear-wave velocity model between 20 and 200 km depth and to image the Moho topography. The agreement of the obtained velocity features with reference shear-wave models illustrates the potential of focal spot imaging to resolve relevant structural information and helps to establish the local time domain spatial autocorrelation analysis as a complementary approach to ambient-noise tomography.