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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS011 Poster presentation 2165 SKS splitting measurements beneath Northern Apennines region: a case of oblique trench retreat Mrs. Silvia Pondrelli Sezione di Bologna INGV IASPEI Salimbeni Simone, Margheriti Lucia, Park Jeffrey, Levin Vadim We present here the new observations of seismic anisotropy obtained from SKS corerefracted shear waves analysis. We studied 34 teleseismic earthquakes recorded by the temporary seismic network of RETREAT project in the Northern Apennines region. For each single-event couple we calculate the anisotropic parameters (delay time and fast polarization direction) by minimizing the energy in the transverse component. Our measurements confirm the existence of two domains. The Tuscany domain, on the west with respect to the Apennines, shows mostly NW-SE fast axes directions, with a rotation toward E-W direction moving toward the Tyrrhenian Sea. The Adria domain, east of the Apennines orogen, shows more scattered measurements, with prevailing N-S to NNESSW directions; also a backazimuthal dependence is evidenced. The transition between the two domains is abrupt in the northern part of the study region while southward it is more gradual. The detected anisotropy is located principally in the asthenosphere. Only beneath the Adria domain, where the presence of a double layer structure seems possible, a lithospheric contribution is not excluded. An interpretation of the anisotropy pattern as produced by mantle deformation is used to describe a differential evolution of the trench retreat process along the Northern Apennines orogen. The orogen-parallel anisotropy in the study region is beneath the inner part of the chain instead of beneath the crest, as occurs all along the rest of the Apennines, and no orogen-normal measurements are found. Compared to the anisotropy pattern of the typical slab retreat taking place perpendicular to the slab strike (as seen in the Northern Central Apennines), in the northernmost part of the orogen the anisotropy pattern suggests that a more oblique retreat occurred in the most recent part of the orogen s history. Keywords: seismic anisotropy, apennines, subduction

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS011 Poster presentation 2166 3D models of the upper mantle developed using the coupled spectral element method Mr. Vedran Lekic Department of Earth and Planetary Science University of California Seismic waveforms contain information on the elastic and anelastic structure of the earth. High quality data from global seismic networks combined with approximate waveform modeling techniques that rely on first-order perturbation theory have made possible the development of high resolution global models of shear wave velocity and, recently, radial anisotropy (e.g. Panning and Romanowicz, 2006), with reliable details of wavelength 1000 km or less. However, mapping the 3D distribution of seismic attenuation has lagged behind due to difficulties in accounting for purely elastic effects of scattering and (de)focusing that result from often poorly constrained gradients of elastic structure. Furthermore, even when elastic structure is known, inaccuracies in forward modeling can obscure the anelastic signal. Therefore, anelastic 3D models only exist presently for theupper mantle and only resolve wavelengths larger than 2000 km (degree12 in spherical harmonics). The models agree qualitatively, and, inparticular, show correlation with velocity structure in the uppermost ~250 km of the mantle. However, the quantitative agreement is not as good, and the amplitudes of lateral variations, which are critical for interpretation in terms of physical processes, are not well constrained. Therefore, development of high resolution global models of attenuation is predicated upon both better retrieval of gradients of elastic structure and implementation of a forward modeling theory that accurately predicts the effects of elastic structure on seismic waveforms. In the first step of the development of a new generation 3D Q model of the upper mantle, we have applied the coupled Spectral Element Method (cSEM, Capdeville et al., 2003) which allows a complete description of the seismic wavefield to the forward modeling of long period (60s) waveforms. We present preliminary models of elastic structure developed from a waveform dataset of 3 component surface waves and overtones recorded at more than 100 stations of the IRIS/GSN, GEOSCOPE, GEOFON and various regional networks. In particular, cSEM allows accurate modeling of crustal effects, especially beneath continental shields, where conventional methods that rely on first order perturbation theory are inadequate. We explore the contamination of elastic models of the mantle that can result from inadequate forward modeling theory. Keywords: mantle, tomography, attenuation

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