Abstract
Author(s): Karaoglu, H; Romanowicz, B | Abstract: We present the results of synthetic tests that aim at evaluating the relative performance of three different definitions of misfit functionals in the context of 3-D imaging of shear wave attenuation in the earth's upper mantle at the global scale, using long-period full-waveform data. The synthetic tests are conducted with simple hypothetical upper-mantle models that contain Qμ anomalies centred at different depths and locations, with or without additional seismic velocity anomalies. To build synthetic waveform data sets, we performed simulations of 50 events in the hypothetical (target) models, using the spectral element method, filtered in the period range 60-400 s. The selected events are chosen among 273 events used in the development of radially anisotropic model SEMUCB-WM1 and recorded at 495 stations worldwide. The synthetic Z-component waveforms correspond to paths and time intervals (fundamental mode and overtone Rayleigh waves) that exist in the real waveform data set. The inversions for shear attenuation structure are carried out using a Gauss-Newton optimization scheme in which the gradient and Hessian are computed using normal mode perturbation theory. The three different misfit functionals considered are based on time domain waveform (WF) and waveform envelope (E-WF) differences, as well as spectral amplitude ratios (SA), between observed and predicted waveforms.We evaluate the performance of the three misfit functional definitions in the presence of seismic noise and unresolved S-wave velocity heterogeneity and discuss the relative importance of physical dispersion effects due to 3-D Qμ structure. We observed that the performance of WF is poorer than the other two misfit functionals in recovering attenuation structure, unless anelastic dispersion effects are taken into account in the calculation of partial derivatives. WF also turns out to be more sensitive to seismic noise than E-WF and SA. Overall, SA performs best for attenuation imaging. Our tests show that it is important to account for 3-D elastic effects (focusing) before inverting for Qμ. Additionally, we show that including high signal-to-noise ratio overtone wave packets is necessary to resolve Qμ structure at depths greater than 250 km.
Highlights
Intrinsic attenuation is a material property that strongly depends on temperature, water content and partial melt (e.g. Karato 1993, 2003; Jackson et al 2004; Dalton & Faul 2010), mapping it can advance our understanding of mantle structure and dynamics significantly
This study focuses on the selection of a proper misfit functional for attenuation imaging within the scope of a full-waveform inversion methodology that relies on the comparison of individual energy wave packets
We have carried out a comparative assessment of three misfit functionals based on waveform (WF), waveform envelope (E-WF) and spectral amplitude (SA) differences, within the framework of a hybrid full-waveform inversion scheme for attenuation imaging
Summary
Intrinsic attenuation is a material property that strongly depends on temperature, water content and partial melt (e.g. Karato 1993, 2003; Jackson et al 2004; Dalton & Faul 2010), mapping it can advance our understanding of mantle structure and dynamics significantly. Karato 1993, 2003; Jackson et al 2004; Dalton & Faul 2010), mapping it can advance our understanding of mantle structure and dynamics significantly This task is a challenging one, as it requires addressing uncertainties related to the seismic wave amplitude measurements. Dziewonski & Anderson 1981; Widmer et al 1991; Durek & Ekstrom 1996; Cammarano & Romanowicz 2008) While this structure is not known in as great detail as for elastic velocities, there is consensus on the presence of a high attenuation zone in the upper mantle, that more-or-less coincides with the well-established seismic lowvelocity zone, and the average shear quality factors (Qμ) in the upper- and lower-mantle are relatively well constrained. The proposed models lag behind the global seismic velocity models both in resolution and
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