High-resolution imaging of subduction zones through SKS splitting intensity tomography: first results from test on numerically modelled complex settings

Abstract

<p>Measurements of seismic anisotropy provide a lot of information on the deformation and structure of the upper mantle, as well as flow in the Earth’s interior. High-resolution seismic anisotropy in the upper mantle can be obtained with several methods that may catch anisotropy at different depths depending on the nature of used seismic waves, including analyses based on surface waves and SKS splitting measurements. Taken together, these anisotropic measurements yield insights into the structure and dynamics of the crust and upper mantle. Nevertheless, mantle images resulting from surface waves result in poor lateral resolution. On the other hand, nearly vertically propagating SKS waves, when interpreted in a ray-based framework, have little or no depth resolution, not allowing to easily image the distribution of the anisotropy through depth. Albeit seismic anisotropic structure of the upper mantle has been demonstrated by a wealth of observational research, most of standard teleseismic body-wave tomography studies overlook P- and S-wave anisotropy, thus producing artefacts in tomographic models in terms of amplitude and localization of heterogeneities. To overcome this problem much effort has been done to implement tomographic methods for inverting SKS splitting observations for anisotropic structures, based on finite-frequency sensitivity kernels that relate elastic model perturbations to splitting observations. A promising approach to track the seismic anisotropy distribution with depth is the splitting intensity (SI): this seismic observable is a measure of the amount of energy on the transverse component of the waveform. At first order, SI is linearly related to the elastic perturbations of the medium through the 3-D sensitivity kernels, and therefore can be inverted allowing a high-resolution image of the upper-mantle anisotropy. The advantage of using splitting intensity is that its amplitude is related directly to the back azimuth, so fast polarization direction and time delay can be determined by fitting the azimuthal variation of splitting intensities with a sine function, with amplitude and phase shift respectively related to delay time and fast velocity direction. Here we present an application of the splitting intensity tomography approach to a synthetic subduction setting. Starting from synthetic SKS waveforms, we first derived high-quality SKS synthetic SI measurements; then we used the obtained results as input for the tomographic inversion based on the algorithm of Chevrot (2006) and Montellier & Chevrot (2011). This approach enables high‐resolution tomographic imaging of upper‐mantle anisotropy through recovering vertical and lateral changes in anisotropy and represents a propaedeutic step to the imaging of real cases of subduction settings.</p>