D″ discontinuity structure beneath the North Atlantic from <i>Scd</i> observations

Yao, Yao; Whittaker, Stefanie; Thorne, M. S.

doi:10.1002/2015gl063989

Cited by 18 publications

(29 citation statements)

References 30 publications

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“…These velocity jumps range from 0.48% (in mod7) to 2.52% (in mod11). The strength of these velocity discontinuities is similar to that reported for downwelling regions (e.g., Helmberger et al, 2005; Sun et al, 2016; Yao et al, 2015; Young & Lay, 1987a, 1987b). …”

Section: Apparent Splitting For a Deep Earthquake Sourcesupporting

confidence: 87%

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

2018

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Observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D ′′ layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) polarized shear waves. By computing waveforms at a period > 6 s for a wide range of 1‐D and 3‐D Earth structures, we illustrate that a time shift (i.e., apparent splitting) between SV and SH may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near‐surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection sS. For deep earthquakes, apparent splitting can be due to the S wave triplication in D ′′, reflections off discontinuities in the upper mantle, and 3‐D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations.

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Section: Apparent Splitting For a Deep Earthquake Sourcesupporting

confidence: 87%

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

2018

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“…Nonetheless, the trade‐off between velocity and height implies that our discontinuity height in this region may be somewhat elevated as the upgoing Scd paths will be sped up in the presence of the topographic high. Midmantle seismic heterogeneity can also affect the Scd‐S differential times used in estimating discontinuity height [e.g., Yao et al ., ]. In our study region there are significant regions of midmantle high wave speeds due to the subducting Farallon slab.…”

Section: Discussionmentioning

confidence: 99%

“…Hence, we do not use differential slowness to constrain height. Previous modeling efforts have demonstrated that to match δ u S‐Scd a negative velocity gradient immediately above the D ″ discontinuity is required [ Yao et al ., ]. There is no correlation between measured amplitude ratios and discontinuity height.…”

Section: Methodsmentioning

confidence: 99%

Seismic array constraints on the D″ discontinuity beneath Central America

et al. 2016

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We analyzed 16,150 transverse component seismic recordings from 54 deep‐focus earthquakes in the South American and Caribbean regions recorded at broadband stations in North America between 2005 and 2012. We treated subgroups of seismic stations within 3° radius geographical bins as seismic arrays and performed vespagram analysis. We focused on the S, ScS, and Scd arrivals and collected data in the epicentral distance range from 55° to 90°. In particular, we searched for D″ discontinuity presence in the vespagrams in a 25° by 35° (or 1520 by 2130 km) area beneath Central America. Analysis of these data showed 125 clear Scd observations, 180 Scd observations of lesser quality, and 343 nonobservations. We produced a new map of the discontinuity height beneath Central America. Our map shows an average discontinuity height of 286 ± 6 km (σ = 76 km). The region is punctuated by a large topographic high centered at approximately 10°N and 90°W with a maximum height of 380 km. Two smaller topographic highs are located at approximately 4°N and 81°W (discontinuity height of 320 km) and at 4°N and 70°W (height of 315 km). The observation of multiple Scd arrivals collocated with the strongest gradients in inferred topography provides evidence for topographic variation on the discontinuity rather than multiple discontinuities. The regions where the discontinuity has the greatest height can be explained by localized enrichment of mid‐ocean ridge basalt from the subducted Farallon slab impinging on the core‐mantle boundary.

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“…In this paper, we are devoted to locate paleoslabs in D" near the Perm Anomaly. Here, we analyzed the Scd arrival—a lowermost mantle triplication occurring as the downgoing S wave refracts below the D" discontinuity (e.g., Sun et al, ; Yao et al, )—to investigate the D" discontinuity sampling the southeastern edge of the Perm Anomaly (Figure ); the Scd arrivals are recorded by a network comprising 57 stations in southeastern Europe from a deep earthquake with magnitude of Mw 6.5 that occurred on 4 September 2013 beneath the northwestern Pacific subduction zone. The event with moderate magnitude (5.8 < Mw < 7) was selected to limit the duration of source time function (Sun et al, ).…”

Section: Data Methods and Analysesmentioning

confidence: 99%

Seismic Evidence for a Paleoslab in the D" Layer Residing Adjacent to the Southeastern Edge of the Perm Anomaly

2019

Geochem Geophys Geosyst

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The Perm Anomaly, which seismologically resembles the large low‐shear velocity provinces (LLSVPs) in the lowermost mantle under Africa and the South Pacific, but is considerably smaller, has been identified beneath Eurasia. Numerical simulations have indicated that the LLSVPs dynamically interact with subducted slabs in D", producing strong deformation near the edges of the LLSVPs. The existence of deformation has been corroborated by seismic anisotropic observations in the D" layer nearby the borders of the slow anomalies such as the Perm Anomaly (Long & Lynner, 2015, https://doi.org/10.1002/2015GL065506; Thomas & Kendall, 2002, https://doi.org/10.1046/j.1365-246X.2002.01760.x). The existence of paleoslabs, however, has rarely been reported. Our purpose of this study, therefore, is to detect fast anomalies associated with paleoslabs near the edges of the Perm Anomaly. Here we find evidence of the existence of the D" discontinuity at a height above the core‐mantle boundary of 250 km with a +3% shear wave velocity increase; it is located adjacent to the southeastern edge of the Perm Anomaly, which is most likely due to a phase transition from perovskite to postperovskite in conjunction with the presence of slab remnants, by analyzing the Scd (the lower mantle triplication) phases recorded at stations in southeastern Europe. The existence of fast anomaly in the D" region (likely due to accumulation of a paleoslab) is further confirmed by analyzing differential traveltime residuals of SKS versus SKKS and SKKKS measured at stations in North America. The subducted slab—probably attributed to the Central China slab—at the base of the mantle may play a role in influencing the shape and location of the Perm Anomaly.

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D″ discontinuity structure beneath the North Atlantic from Scd observations

Cited by 18 publications

References 30 publications

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Seismic array constraints on the D″ discontinuity beneath Central America

Seismic Evidence for a Paleoslab in the D" Layer Residing Adjacent to the Southeastern Edge of the Perm Anomaly

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