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Seismic tomographic models based only on wave velocities have limited ability to distinguish between a thermal or compositional origin for Earth’s 3D structure1 . Complementing wave velocities with attenuation observations can make that distinction, which is fundamental for understanding mantle convection evolution. However, global 3D attenuation models are only available for the upper mantle at present2–5. Here we present a 3D global model of attenuation for the whole mantle made using whole-Earth oscillations, constraining even spherical harmonics up to degree four. In the upper mantle, we find that high attenuation correlates with low velocity, indicating a thermal origin, in agreement with previous studies6,7. In the lower mantle, we find the opposite and observe the highest attenuation in the ‘ring around the Pacific’, which is seismically fast, and the lowest attenuation in the large low-seismic-velocity provinces (LLSVPs). Comparing our model with wave speeds and attenuation predicted by a laboratory-based viscoelastic model8 suggests that the circum-Pacific is a colder and small-grain-size region9, surrounding the warmer and large-grain-size LLSVPs. Viscosities calculated for the inferred variations in grain size and temperature confirm LLSVPs as long-lived, stable features10
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Lateral Variations in Teleseismic Attenuation of the Conterminous U.S. and New Insights Derived From Its Relationship to Mantle Seismic Velocity
Much of our knowledge of the North American lithosphere comes from imaging seismic velocities. Additional constraints on the subsurface can be gained by studying seismic attenuation, which has different sensitivity to physical properties. We produce a model of lateral variations in attenuation across the conterminous U.S. by analyzing data recorded by the EarthScope Transportable Array. We divide the study area into 12 overlapping tiles and differential attenuation is measured in each tile independently; and twice for four of the tiles. Measurements are combined into a smooth map using a set of linear inversions. Comparing results for adjacent tiles and for repeated tiles shows that the imaged features are robust. The final map shows generally higher attenuation west of the Rocky Mountain Front than east of it, with significant small length scale variations superimposed on that broad pattern. In general, there is a strong anticorrelation between differential attenuation and shear wave velocities at depths of 80–250 km. However, a given change in velocity may correspond to a large or small change in attenuation, depending on the area; suggesting that different physical mechanisms are operating. In the western and south‐central U.S., as well as the Appalachians, velocity variations are large compared to attenuation changes, while the opposite is true in the north‐central and southeastern U.S. Calculations with the Very Broadband Rheology calculator show that these results are consistent with the main source of heterogeneity being temperature and melt fraction in the former regions and grain size variability in the latter ones.
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- Award ID(s):
- 1944998
- PAR ID:
- 10504266
- Publisher / Repository:
- American Geophysical Union / Wiley
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 128
- Issue:
- 12
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2023JB027299
