Mantle Q Structure from S-wave Amplitude Measurements

Abstract:

The seismic quality factor (Q) of the Earth’s mantle is of great importance for the understanding of thermal and chemical properties that control mantle anelasticity (e.g., temperature and water content). The radial structure of the Earth’s Q is less well resolved compared to its velocity structure, and large discrepancies exist among global 1-D Q models. In this study, we build a global dataset of amplitude measurements of S, SS, SSS and SSSS waves using earthquakes that occurred between 2009 and 2017 with moment magnitudes ranging from 6.5 to 8.0. Synthetic seismograms for those events are computed in a 1-D reference model PREM, and amplitude ratios between observed and synthetic seismograms are calculated in the frequency domain by spectra division, with measurement windows determined based on visual inspection of seismograms. The dataset includes about 5500 S and SS measurements that are not affected by mantle transition zone triplications (multiple ray paths), and those measurements are applied in t* inversion to obtain a preliminary 1-D Q model. This model improves the overall fit to the S and SS data but does not fully explain the amplitudes of triplicated SS, SSS and SSSS waves. We simulate wave propagation in a global velocity model S40RTS using SPECFEM and show that the average amplitude ratio as a function of epicentral distance is not sensitive to 3-D focusing and defocusing, based on the source-receiver configuration of the same dataset. Using forward modeling, we modify the preliminary 1-D Q model iteratively to reduce the average amplitude misfit of the entire dataset. The final model reveals a high Q region in the uppermost lower mantle, in general agreement with viscosity layering inferred from geoid observations.