A lower mantle S-wave triplication and the shear velocity structure of D''

We have determined subsurface structure using the refraction microtremor (ReMi) method at the Ngatamariki geothermal field, Central North Island, New Zealand. The local geology is such that refraction and reflection studies are hindered by energy scattering and attenuation in the near-surface layers. The ReMi method uses surface waves from ambient noise and active sources to determine S-wave velocities in the shallow subsurface. We have deployed two lines of 72-channel, 10 Hz vertical geophones with 10 m spacing, and we were able to model near-surface S-wave velocity to depths of 57–93 m for 2D profiles and as much as 165 m for 1D profiles. Shear-velocity anomalies were detected on one line that were spatially correlated with a fault. The location of the fault was previously inferred from stratigraphic offset in the geothermal wells, suggesting that the ReMi method can provide important constraints on near-surface geology in noisy geothermal settings.

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Lowermost mantle shear-velocity structure from hierarchical trans-dimensional Bayesian tomography

10.31223/x5mg8v ◽

2021 ◽

Author(s):

Sima Mousavi ◽

Hrvoje Tkalčić ◽

Rhys Hawkins ◽

Malcolm Sambridge

Keyword(s):

Velocity Structure ◽

Spatial Scales ◽

Model Space ◽

Shear Velocity ◽

Outer Core ◽

Travel Times ◽

S Wave ◽

Low Velocity ◽

Spatial Coverage ◽

Lowermost Mantle

The core-mantle boundary (CMB) is the most extreme boundary within the Earth where the liquid, iron-rich outer core interacts with the rocky, silicate mantle. The nature of the lowermost mantle atop the CMB, and its role in mantle dynamics, is not completely understood. Various regional studies have documented significant heterogeneities at different spatial scales. While there is a consensus on the long scale-length structure of the inferred S-wave speed tomograms, there are also notable differences stemming from different imaging methods and datasets. Here we aim to overcome over-smoothing and avoid over-fitting data for the case where the spatial coverage is sparse and the inverse problem ill-posed. Here we present an S-wave tomography model at global scale for the Lowermost Mantle (LM) using the Hierarchical Trans-dimensional Bayesian Inversion (HTDBI) framework, LM-HTDBI. Our HTDBI analysis of ScS-S travel times includes uncertainty, and the complexity of the model is deduced from the data itself through an implicit parameterization of the model space. Our comprehensive resolution estimates indicate that short-scale anomalies are significant and resolvable features of the lowermost mantle regardless of the chosen mantle-model reference to correct the travel times above the D’’ layer. The recovered morphology of the Large-Low-Shear-wave Velocity Provinces (LLSVPs) is complex, featuring small high-velocity patches among low-velocity domains. Instead of two large, unified, and smooth LLSVPs, the newly obtained images suggest that their margins are not uniformly flat.

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Seismological expression of the iron spin crossover in ferropericlase in the Earth’s lower mantle

Nature Communications ◽

10.1038/s41467-021-26115-z ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Grace E. Shephard ◽

Christine Houser ◽

John W. Hernlund ◽

Juan J. Valencia-Cardona ◽

Reidar G. Trønnes ◽

...

Keyword(s):

Lower Mantle ◽

Spin Crossover ◽

Shear Velocity ◽

Oceanic Lithosphere ◽

P Wave ◽

Depth Range ◽

S Wave ◽

Seismic Profiles ◽

P Waves ◽

Wave Tomography

AbstractThe two most abundant minerals in the Earth’s lower mantle are bridgmanite and ferropericlase. The bulk modulus of ferropericlase (Fp) softens as iron d-electrons transition from a high-spin to low-spin state, affecting the seismic compressional velocity but not the shear velocity. Here, we identify a seismological expression of the iron spin crossover in fast regions associated with cold Fp-rich subducted oceanic lithosphere: the relative abundance of fast velocities in P- and S-wave tomography models diverges in the ~1,400-2,000 km depth range. This is consistent with a reduced temperature sensitivity of P-waves throughout the iron spin crossover. A similar signal is also found in seismically slow regions below ~1,800 km, consistent with broadening and deepening of the crossover at higher temperatures. The corresponding inflection in P-wave velocity is not yet observed in 1-D seismic profiles, suggesting that the lower mantle is composed of non-uniformly distributed thermochemical heterogeneities which dampen the global signature of the Fp spin crossover.

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A new 3-D mantle density model from recent normal-mode measurements

10.5194/egusphere-egu21-9852 ◽

2021 ◽

Author(s):

Rûna van Tent ◽

Arwen Deuss ◽

Andreas Fichtner ◽

Lars Gebraad ◽

Simon Schneider ◽

...

Keyword(s):

Normal Mode ◽

Lower Mantle ◽

Velocity Structure ◽

Normal Modes ◽

Shear Velocity ◽

Boundary Region ◽

Splitting Function ◽

The Pacific ◽

Compositional Variations ◽

Body Tides

Constraints on the 3-D density structure of Earth&#8217;s mantle provide important insights into the nature of seismically observed features, such as the Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle under Africa and the Pacific. The only seismic data directly sensitive to density variations throughout the entire mantle are normal modes: whole Earth oscillations that are induced by large earthquakes (Mw&#160;> 7.5). However, their sensitivity to density is weak compared to the sensitivity to velocity and different studies have presented conflicting density models of the lower mantle. For example, Ishii & Tromp (1999) and Trampert et al. (2004) have found that the LLSVPs have a larger density than the surrounding mantle, while Koelemeijer et al. (2017) used additional Stoneley-mode observations, which are particularly sensitive to the core-mantle boundary region, to show that the LLSVPs have a lower density. Recently, Lau et al. (2017) have used tidal tomography to show that Earth's body tides prefer dense LLSVPs.A large number of new normal-mode splitting function&#160;measurements has become available since the last density models of the entire mantle were published.&#160;Here, we show the models from&#160;our inversion of these recent data and compare our results to previous studies. We find areas of high as well as low density at the base of the LLSVPs and we find that inside the LLSVPs density varies on a smaller scale than velocity, indicating the presence of compositionally distinct material. In fact, we find low correlations between the density and velocity structure throughout the entire mantle, revealing that compositional variations are required at all depths inside the mantle.

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