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SUMMARY The seismic quality factor (Q) of the Earth’s mantle is of great importance for the understanding of the physical and chemical properties that control mantle anelasticity. The radial structure of the Earth’s Q is less well resolved compared to its wave speed structure, and large discrepancies exist among global 1-D Q models. In this study, we build a global data set 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. We simulate wave propagation in a global velocity model S40RTS based on SPECFEM3D and show that the average amplitude ratio as a function of epicentral distance is not sensitive to 3-D focusing and defocusing for the source–receiver configuration of the data set. This data set 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 linear inversions to obtain a preliminary 1-D Q model QMSI. This model reveals a high Q region in the uppermost lower mantle. While model QMSI improves the overall datafit of the entire data set, it does not fully explain SS amplitudes at short epicentral distances or the amplitudes of the SSS and SSSS waves. Using forward modelling, we modify the 1-D model QMSI iteratively to reduce the overall amplitude misfit of the entire data set. The final Q model QMSF requires a stronger and thicker high Q region at depths between 600 and 900 km. This anelastic structure indicates possible viscosity layering in the mid mantle.
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Can Traveltime Tomography Benefit From Wave Amplitudes?
Abstract Regularization of seismic inversions has a strong imprint on tomographic images. We analyze recorded and spectral‐element S, Sdiff, and SS waveforms to evaluate the benefit of body‐wave amplitudes in global tomography. L‐curve analysis for S40RTS models with recorded and synthetic waveforms show that SS‐S traveltimes and SS/S amplitude ratios have minima within the same damping parameter range. SS/S ratios for S40RTS and model GLAD‐M25 show the trade‐off between scale‐length and strength of lowermost‐mantle heterogeneities. The recorded SS/Sdiff ratios are lower than predicted by 3D mantle models which may be explained by a decrease in the mean shear velocity by at the lowermost 200 km of the mantle. Our results suggest that SS/S amplitude measurements made with 3D waveforms can be used to constrain damping in linearized inversions, and amplitudes are essential for studying the size of heterogeneities.
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- PAR ID:
- 10642946
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 52
- Issue:
- 17
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2025GL115464
