Reflection of seismic surface waves at the northern Apennines

Seismic surface waves with periods of 15 to 20 s are reflected laterally at the northern Apennines. For earthquakes originated and recorded in the wider Alpine area, a few hundred kilometres north of the Apennines, we can directly observe late arrivals of reflected regional surface waves. These have a characteristic polarization of particle motion and often dominate the intermediate-period surface wave coda. Love waves are the most prominent coda arrivals, while reflected Rayleigh waves show smaller amplitudes, and become clear only after rotation of the recordings to the reflection incidence direction. We can track the development of the Love wave reflection along a temporary broad-band transect across the northern Apennines, indicating the location of the reflector near to the highest topography. From spectral element simulations of three-dimensional wave propagation, we attribute the reflection to a continuous, ∼ 270 km long offset in the crust-mantle boundary under the mountain belt with vertical throw of ∼ 20 km, thus supporting a deep crustal root under the outer side of the Apennines fold and thrust belt, and significant crustal thinning on the inner side. Amplitude reflection coefficients for near-normal incidence can be estimated as ∼ 7% for both Love and Rayleigh waves.