Seismic anisotropy in the lowermost mantle beneath North America from SKS-SKKS splitting intensity discrepancies

Geochem Geophys Geosyst

Karabinos

et al. 2020

Self Cite

Introduction 1.1. Tectonic Background The bedrock geology of southern New England is extraordinary in its complexity, and reflects a range of plate tectonic processes, including subduction and terrane accretion during the Appalachian Orogeny and extension and rifting during the breakup of the Pangea supercontinent. Neoproterozoic rifting of Rodinia created the Laurentian passive margin preserved in western New England (Figure 1). Mesoproterozoic Grenvillian crust is exposed in the Green Mountain and Berkshire massifs in Vermont and Massachusetts (Kara

Section: Multichannel Methods Resultsmentioning

confidence: 74%

Section: Resultsmentioning

confidence: 93%

Section: Resultsmentioning

confidence: 99%

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SKS Splitting and Upper Mantle Anisotropy Beneath the Southern New England Appalachians: Constraints From the Dense SEISConn Array

Lopes

Geochem Geophys Geosyst

Karabinos

et al. 2020

Self Cite

“…Before estimating the shear wave splitting due to the lowermost mantle, we corrected each PKS waveform by using the SKS upper mantle modeling from Figure 3. There is some scatter in the observed PKS splitting measurements, as is typical for shear wave splitting studies (e.g., Asplet et al, 2020;Creasy et al, 2017;Deng et al, 2017;Grund & Ritter, 2020;Long, 2009;Lutz et al, 2020;Niu & Perez, 2004). Therefore, we stacked each PKS error surface (after upper mantle correction) using Stacksplit (Grund, 2017) and found best-fitting PKS splitting parameters (due to D″ seismic anisotropy) of ϕ = −85° ± 10° (ϕ = −41° in ray-centered coordinates, using the convention of Wookey et al, 2005 andNowacki andWookey, 2010) and δt = 0.4 s ± 0.1 s (Figure 5).…”

Section: Shear Wave Splitting Resultsmentioning

confidence: 99%

Modeling of Seismic Anisotropy Observations Reveals Plausible Lowermost Mantle Flow Directions Beneath Siberia

Creasy

Pisconti

Geochem Geophys Geosyst

et al. 2021

Self Cite

Observations of seismic anisotropy are a powerful tool for mapping deformation within the Earth (e.g., Long & Becker, 2010), and are often used to study deformation and flow in the upper mantle (e.g., Skemer & Hansen, 2016). The lowermost mantle, also known as the D″ layer, also clearly exhibits seismic anisotropy (e.g., Garnero & Lay, 1997;Silver, 1996, and references within Nowacki et al., 2011;Romanowicz & Wenk, 2017), and seismic anisotropy observations can be used to map deformation at the base of the mantle. However, this requires thorough knowledge of the mechanism for D″ seismic anisotropy and the relationship between deformation and strain, which can be established by experiments and theoretical modeling. There are several proposed mechanisms for D″ seismic anisotropy, including the shape preferred orientation (SPO) of melt inclusions (or other elastically distinct material) and the crystallographic preferred orientation (CPO) of bridgmanite (bm), ferropericlase (fp), post-perovskite (ppv), or some mixture of these minerals (e.g., Nowacki et al., 2011).A major obstacle in the interpretation of D″ seismic anisotropy measurements is our imprecise knowledge of the mechanism responsible, along with the non-uniqueness of data sets that are based on a limited number of measurements in a given region. A recent synthetic modeling study (Creasy et al., 2019) demonstrated that tighter constraints on seismic anisotropy at the base of the mantle can be obtained by combining different types of observations of body wave anisotropy than by a single type of data alone. Specifically, Creasy et al. ( 2019) examined reflection polarity measurements (P or S waves that reflect off the D″ discontinuity-PdP and SdS) and shear wave splitting (seismic wave birefringence). In this study, we apply the insights gained from the synthetic modeling of Creasy et al. (2019) and combine observations of shear wave splitting (for both ScS and PKS phases) due to D″ seismic anisotropy with observations of PdP/SdS polarities (and their variation with direction) to obtain tight constraints on geometry in a single target region. We apply a novel modeling approach that is based on previous work (Creasy et al., 2017;Ford et al., 2015) and has been

“…However, in some cases, there may be some contribution from anisotropy in the deeper mantle. The pres-ence of lowermost mantle anisotropy is often inferred from differential SKS-SKKS splitting (e.g., Niu & Perez, 2004;Restivo & Helffrich, 2006;Long, 2009;Deng et al, 2017;Grund & Ritter, 2018;Wolf et al, 2019;Reiss et al, 2019;Lutz et al, 2020;Asplet et al, 2020). The argument for this analysis technique is that SKS and SKKS raypaths are very similar in the upper mantle, but they sample different portions of the deep mantle and have different propagation directions (see Figure 1a).…”

Section: Introductionmentioning

confidence: 99%

Observations of mantle seismic anisotropy using array techniques: shear-wave splitting of beamformed SmKS phases

Wolf

Frost

et al. 2022

Preprint

Shear-wave splitting measurements are commonly used to resolve seismic anisotropy in both the upper and lowermost mantle. Typically, such techniques are applied to SmKS phases that have reflected (m-1) times off the underside of the core-mantle boundary before being recorded. Practical constraints for shear-wave splitting studies include the limited number of suitable phases as well as the large fraction of available data discarded because of poor signal-to-noise ratios (SNRs) or large measurement uncertainties. Array techniques such as beamforming are commonly used in observational seismology to enhance SNRs, but have not been applied before to improve SmKS signal strength and coherency for shear wave splitting studies. Here, we investigate how a beamforming methodology, based on slowness and backazimuth vespagrams to determine the most coherent incoming wave direction, can improve shear-wave splitting measurement confidence intervals. Through the analysis of real and synthetic seismograms, we show that (1) the splitting measurements obtained from the beamformed seismograms (beams) reflect an average of the single-station splitting parameters that contribute to the beam; (2) the beams have (on average) more than twice as large SNRs than the single-station seismograms that contribute to the beam; (3) the increased SNRs allow the reliable measurement of shear wave splitting parameters from beams down to average single-station SNRs of 1.3. Beamforming may thus be helpful to more reliably measure splitting due to upper mantle anisotropy. Moreover, we show that beamforming holds potential to greatly improve detection of lowermost mantle anisotropy by demonstrating differential SKS-SKKS splitting analysis using beamformed USArray data.