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Authors: Stich, D.* 
Danececk, P.* 
Morelli, A.* 
Tromp, J.* 
Title: Imaging lateral heterogeneity in the northern Apennines from time reversal of reflected surface waves
Issue Date: 2008
Keywords: Prominent arrivals in the coda of seismograms from the wider Alpine
time-reversal imaging
sensitivity kernel
surface waves
lithospheric heterogeneity
Subject Classification04. Solid Earth::04.01. Earth Interior::04.01.99. General or miscellaneous 
Abstract: 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 based upon the spectral-element method, for which a three-dimensional regional earth model is used in the case of real data. Single event sensitivity kernels and their stacks 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 unravel other smaller reflectivity patches from the coda waveforms, located for example at 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 heterogeneities.
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