Imaging lateral heterogeneity in the northern Apennines from time reversal of reflected surface waves

Abstract

SUMMARY Prominent arrivals in the coda of seismograms from the wider Alpine area can be associated with lateral reflections of Love waves at the northern Apennines mountain chain (Italy), where structural heterogeneity causes an abrupt contrast in phase velocity. We discuss an approach to image lateral heterogeneity from reflected surface waves using intermediate‐period, three‐component coda waveforms as sources for an adjoint wavefield that propagates the reflections backward in time. We numerically compute three‐dimensional sensitivity kernels for the dependence of coda waveforms on P velocity, S velocity and density, based upon correlations between the adjoint and the regular forward wavefields. We consider synthetic coda waveforms for a simplified model of the northern Apennines, as well as real coda observations from five moderate magnitude earthquakes (MW 4.6–5.6) in the southern Alps. Wave propagation is simulated using the spectral‐element method, for which a 3‐D regional earth model is used in the case of real data. Single and combined event sensitivity kernels provide clear images of the reflectivity associated with the northern Apennines in kernels for density and S‐wave speed. The kernels show that surface wave reflections occur near the axial zone of the mountain chain. Apart from the Apennines, the approach is able to image other smaller reflectivity patches from the coda waveforms, like the Ivrea zone in the southern Alps. Our coda misfit kernels can be integrated in a gradient‐based waveform tomography, where they could enhance the sharpness of the model at lateral discontinuities.

Time reverse location of seismic long-period events recorded on Mt Etna

“…2008), imaging the great Sumatra earthquake (Larmat et al . 2006), the scattering of regional surface waves (Stich et al 2009) and location of synthetic tremor from a 2‐D model of Mt Etna (Lokmer et al . 2009).…”

Spectral-Element Simulations of Seismic Waves Generated by the 2009 L'Aquila Earthquake

“…If need be, the SEM can be implemented as a discontinuous Galerkin method (Kopriva et al, 2002;Kopriva, 2006;Acosta Minolia and Kopriva, 2011). The SEM has been used to study 2D and 3D local, regional, and global seismic-wave propagation problems (e.g., Cohen et al, 1993;Priolo et al, 1994;Faccioli et al, 1997;Komatitsch, 1997;Komatitsch and Vilotte, 1998;Komatitsch and Tromp, 2002a,b;Chaljub and Valette, 2004;Komatitsch et al, 2004Komatitsch et al, , 2005Liu et al, 2004;Stich and Morelli, 2007;Lee et al, 2008;Stich et al, 2009;Stupazzini et al, 2009;Tape et al, 2010;Zhu et al, 2012a,b). In particular, in the recent work of Smerzini and Villani (2012), the SEM implemented in the code GeoELSE (Stupazzini et al, 2009) has been used to simulate near-fault ground motions produced by the 2009 L'Aquila mainshock.…”

Application of Surface-Wave Methods for Seismic Site Characterization

“…Geophys (2011) 32:777-825 813 All of these approaches, however, do not directly deal with the problem of a complex wave field where surface wave propagation is affected by the presence of sharp lateral heterogeneities. The presence of reflection/refraction phenomena induced by such heterogeneities has been recognized in large scale seismology and has led to the development of specific inversion procedures devoted to imaging bodies responsible for observed patterns in Rayleigh waves (e.g., Meier et al 1997) and coda waves (e.g., Stich et al 2009) registrations.…”