HR: 1340h
AN: S23B-1394    [Abstracts]
TI: Imaging lateral heterogeneity in the northern Apennines from time reversal of reflected surface waves
AU: * Stich, D
AF: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Via Donato Creti 12, Bologna, 40128, Italy
AU: Danecek, P
AF: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Via Donato Creti 12, Bologna, 40128, Italy
AU: Morelli, A
AF: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Via Donato Creti 12, Bologna, 40128, Italy
AU: Tromp, J
AF: Seismological Laboratory, California Institute of Technology, 1200 East California Boulevard, Pasadena, 91125, United States
AB: Seismic surface waves are partially reflected when their phase velocity changes abruptly, which makes reflected surface waves a natural choice to study lateral heterogeneity in the lithosphere. We investigate surface wave reflections at the northern Apennines (Italy), which can be observed as characteristic late wave packages in the intermediate-period coda in near-regional seismograms in the wider Alpine area, a few 100 km north of the Apennines. Forward modelling of the timing and amplitudes of late arrivals suggests that they can be attributed to the combined effects of a Moho discontinuity and different crustal velocities across the mountain chain. Here, we image the northern Apennines' reflector by time reversal of the surface wave coda. We make use of the concept of adjoints, and compute three-dimensional sensitivity kernels numerically by the time correlation between the simulation of the original forward wavefield and the simulation of the corresponding back-propagating adjoint wavefield. Wave propagation is simulated with the spectral element method, and a three-dimensional regional earth model is involved. The sources for the adjoint wavefield are the time-reversed displacement waveforms of the three-component intermediate-period coda applied at the receiver locations, representing the misfit compared to an earth where reflection did not occur. From this set-up, we expect that our sensitivity kernels illuminate lateral heterogeneity. We compute event kernels for five moderate earthquakes in the Southern Alps and up to ten receiver locations. We obtain clear images of the reflectivity associated with the Northern Apennines especially in kernels for density and S-wave speed, consistent with the fact that reflected Love waves represent the most prominent coda arrivals. The kernels show that surface wave reflections originate from the axial zone of the mountain chain. Apart from the Apennines, sensitivity is low, and no other comparable source of coherent surface wave reflections becomes evident.
DE: 3285 Wave propagation (0689, 2487, 4275, 4455, 6934)
DE: 7218 Lithosphere (1236)
DE: 7255 Surface waves and free oscillations
DE: 7290 Computational seismology
SC: Seismology [S]
MN: 2007 Fall Meeting