Seismic structure of the southern Apennines as revealed by waveform modelling of regional surface waves

SUMMARY We investigate the crust and upper‐mantle structures beneath the southern Apennine mountain chain using three‐component seismograms from the Calabria‐Apennine‐Tyrrhenian/Subduction‐Collision‐Accretion Network (CAT/SCAN) array. Surface wave waveforms from three moderate‐sized ( M w > 5.0) regional earthquakes are modelled using multiple frequencies (0.03–0.06 and 0.05–0.2 Hz) and both forward and linearized‐inversion algorithms. Our best‐fitting shear velocity models clearly reflect the major tectonic units where, for example, the average seismic structure at depths above 50 km beneath Apulia is substantially faster than beneath the Apennine mountain chain. We identify a prominent low‐velocity channel under the mountain belt at depths below ∼25–30 km and a secondary low‐velocity zone at 6–12 km depth near Mt Vulture (a once active volcano). Speed variations between Love and Rayleigh waves provide further constraints on the fabric and dynamic processes. Our analysis indicates that the crustal low‐velocity zones are highly anisotropic (maximum 14 per cent) and allow transversely polarized shear waves to travel faster than vertically polarized shear waves. The upper crustal anomaly reveals a layer of highly deformed rocks caused by past collisions and by the active normal faults cutting across the thrust sheets, whereas hot mantle upwelling may be responsible for a high‐temperature, partially molten lower crust beneath the southern Apennines.