Improved Characterization of Ultralow‐Velocity Zones Through Advances in Bayesian Inversion of ScP Waveforms

Pachhai, Surya; Thorne, M. S.; Rost, Sebastian

doi:10.1029/2023jb026415

Cited by 4 publications

(1 citation statement)

References 89 publications

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“…That said, the differences in ULVZ thickness indicated by our robust bins suggest lateral ULVZ variability across the CMB. The fact that many of the ULVZ+ and complex bins are interspersed with the robust bins (Figures 6 and 10) may also suggest complicated, internal ULVZ structure (e.g., Pachhai et al., 2023). For example, the two distinct peaks displayed in our ULVZ+ bins may be reflections from a two‐layered ULVZ anomaly.…”

Section: Discussion: Possible Ulvz Originsmentioning

confidence: 99%

Ultra‐Low Velocity Zones Beneath the Southern Hemisphere Imaged With Double‐Array Stacking of PcP Waveforms

Agboola,

Hansen,

Garnero

et al. 2024

JGR Solid Earth

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Ultra‐low velocity zones (ULVZs) are anomalous structures, generally associated with decreased seismic velocity and sometimes an increase in density, that have been detected in some locations atop the Earth's core‐mantle boundary (CMB). A wide range of ULVZ characteristics have been reported by previous studies, leading to many questions regarding their origins. The lowermost mantle beneath Antarctica and surrounding areas is not located near currently active regions of mantle upwelling or downwelling, making it a unique environment in which to study the sources of ULVZs; however, seismic sampling of this portion of the CMB has been sparse. Here, we examine core‐reflected PcP waveforms recorded by seismic stations across Antarctica using a double‐array stacking technique to further elucidate ULVZ structure beneath the southern hemisphere. Our results show widespread, variable ULVZs, some of which can be robustly modeled with 1‐D synthetics; however, others are more complex, which may reflect 2‐D or 3‐D ULVZ structure and/or ULVZs with internal velocity variability. Our findings are consistent with the concept that ULVZs can be largely explained by variable accumulations of subducted oceanic crust along the CMB. Partial melting of subducted crust and other, hydrous subducted materials may also contribute to ULVZ variability.

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Section: Discussion: Possible Ulvz Originsmentioning

confidence: 99%

Ultra‐Low Velocity Zones Beneath the Southern Hemisphere Imaged With Double‐Array Stacking of PcP Waveforms

Agboola,

Hansen,

Garnero

et al. 2024

JGR Solid Earth

Self Cite

View full text Add to dashboard Cite

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Investigating Ultra‐Low Velocity Zones as Sources of PKP Scattering Beneath North America and the Western Pacific Ocean: Potential Links to Subducted Oceanic Crust

Thorne,

Pachhai,

et al. 2024

AGU Advances

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Seismic energy arriving before the compressional (P) wave passing through the core (PKP), called PKP precursors, have been detected for decades, but the origin of those arrivals is ambiguous. The largest amplitude arrivals are linked to scattering at small‐scale lowermost mantle structure, but because these arrivals traverse both source and receiver sides of the mantle, it is unknown which side of the path the energy is scattered from. To address this ambiguity, we apply a new seismic array method to analyze PKP waveforms from 58 earthquakes recorded in North America that allows localization of the origin of the PKP precursors at the core‐mantle boundary (CMB). We compare these measurements with high frequency 2.5‐D synthetic predictions showing that the PKP precursors are most likely associated with ultra‐low velocity zone structures beneath the western Pacific and North America. The most feasible scenario to generate ULVZs in both locations is through melting of mid‐ocean ridge basalt in subducted oceanic crust.

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Examining the influence of 2.5-D ultra-low velocity zone morphology on ScP waveforms and estimated elastic parameters

Pachhai,

Thorne,

Rost

2024

Geophysical Journal International

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Summary Ultra-low velocity zones (ULVZs) have been identified as regions of extremely low velocity anomalies in the Earth's lowermost mantle using seismic observations from reflected, refracted, and diffracted arrivals along the mantle side of the core-mantle boundary (CMB). Estimation of ULVZ geometrical (i.e., shape and size) and elastic (i.e., velocity and density) parameters with uncertainties is crucial in understanding the role of ULVZs in the ongoing dynamic processes within the Earth's mantle; however, these parameters are still poorly known due to uncertainties and tradeoffs of the seismically resolved ULVZ geometries and elastic parameters. Computation of synthetic waveforms for 2-D and 3-D ULVZs shapes is currently computationally feasible, but past studies utilize higher dimensional waveform modeling of mostly only low-frequency diffracted waves. Most studies focusing on high-frequency core-reflected waveforms (e.g., ScP) still use 1-D modeling approaches to determine ULVZ properties. This approach might lead to wrong results if the imaged structures have inherently 3-D geometries. This study investigates high-frequency synthetic ScP waveforms for various 2.5-D ULVZ geometries showing that additional seismic arrivals are generated even when the ScP geometrical ray path does not directly strike the location of the ULVZ. The largest amplitude additional phases in the 2.5-D models are post-cursor arrivals that are generated at the edges of the finite-length ULVZs. These newly identified ScP post-cursors can arrive within the ScsP post-cursor time window traditionally analyzed in 1-D ULVZ studies. These post-cursors might then be misidentified or constructively/destructively interfere with the ScsP postcursor, leading to incorrect estimation of ULVZ parameters. In this study we investigate the bias introduced by the 2.5-D morphologies on the 1D estimated ULVZ elastic properties in a Bayesian waveform inversion scheme. We further expand the Bayesian method by including the data noise covariance matrix in the inversion and compare it to an autoregressive noise model that was utilized in previous studies. From the application to the observed ScP data, we find that the new approach converges faster, particularly for the inversion of data from multiple events, and the new algorithm retrieves ULVZ parameters with more realistic uncertainties. The inversion of 2.5-D synthetic ScP waveforms suggests that the retrieved ULVZ parameters can be misleading with unrealistically high confidence if we do not consider the data noise covariance matrix in the inversion. Our new approach can also retrieve the shape of a multi-dimensional Gaussian ULVZ if its length is 12o or longer in the great circle arc direction. However, 2.5-D synthetic waveforms show additional waveform complexity which can constructively interfere with the ScP wavefield. Hence, in many cases the estimation of ULVZ properties using 1-D forward modeling can provide incorrect ULVZ parameters. Hence previous ULVZ modeling efforts using 1-D parameter estimation methods may be incorrect.

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Improved Characterization of Ultralow‐Velocity Zones Through Advances in Bayesian Inversion of ScP Waveforms

Cited by 4 publications

References 89 publications

Ultra‐Low Velocity Zones Beneath the Southern Hemisphere Imaged With Double‐Array Stacking of PcP Waveforms

Ultra‐Low Velocity Zones Beneath the Southern Hemisphere Imaged With Double‐Array Stacking of PcP Waveforms

Investigating Ultra‐Low Velocity Zones as Sources of PKP Scattering Beneath North America and the Western Pacific Ocean: Potential Links to Subducted Oceanic Crust

Examining the influence of 2.5-D ultra-low velocity zone morphology on ScP waveforms and estimated elastic parameters

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