Further structural constraints and uncertainties of a thin laterally varying ultralow-velocity layer at the base of the mantle

Constraints and uncertainties are presented for modeling of an ultralow-velocity zone layer (ULVZ) at the base of Earth's mantle using an SKS wave with small segments of P wave diffraction at the SKS core entry and exit locations, called SP_dKS. Source or receiver effects are ruled out as causes for the SP_dKS anomalies used to map ULVZ structure, since systematic SP_dKS-SKS travel time moveout behavior is present in profiles of recordings of a given earthquake at many seismographic stations and also for many events recorded at one station. The southwest Pacific region produces strong variability in observed SP_dKS/SKS amplitude ratios (compared to synthetic seismograms), which geographically corresponds to an anomalous ULVZ region. Accurate determination of absolute ULVZ thicknesses requires knowledge of, in addition to magnitude of P wave velocity (V_p) reduction in the layer, the magnitude of S wave velocity (V_S) reduction and density (ρ) perturbation (if any). Synthetic seismogram experiments demonstrate several key points regarding uncertainties and constraints in modeling ULVZ structure: (1) thicker layers (up to 300 km thick) with mild reductions (e.g., −2.5 to −5.0%) cannot reproduce the anomalous SP_dKS behavior seen in the data; (2) for ULVZ layers less than 10 km thick, strong trade-offs exist between discontinuous velocity reductions and linear gradient reductions over a thicker zone; (3) uncertainties preclude precise determination of magnitude of δV_P and δV_S reductions, as well as the δV_S:δV_P ratio; (4) large density increases within the ULVZ (e.g., up to 60% and more) can efficiently broaden and delay the peak of the energy that we identify as SP_dKS for models with strong velocity reductions in the layer; (5) models with extreme Q reductions in the ULVZ can affect SP_dKS waveforms, and dampen spurious ringing energy present in Sd waveshapes due to the ULVZ; and (6) the minimum required V_p reduction for the most anomalous data (around −10%) trades off with thinner ULVZ structures containing larger velocity reductions (with possible density increases as well)

Seismic detection of a thin laterally varying boundary layer at the base of the mantle beneath the central-Pacific

We explore lowermost mantle structure beneath the Pacific with long‐period recordings of the seismic core phases SKS, SP_dKS, and SKKS from 25 deep earthquakes. SP_dKS and SKKS are anomalously delayed relative to SKS for lower mantle paths beneath the southwest Pacific. Late SP_dKS arrivals are explained by a laterally varying mantle‐side boundary layer at the CMB, having P‐velocity reductions of up to 10% and thickness up to 40 km. This layer is detected beneath a tomographically resolved large‐scale low velocity feature in the lower mantle beneath the central‐Pacific. SKS, SP_dKS, and SKKS data for the generally faster‐than‐average circum‐Pacific lower mantle are well‐fit by models lacking any such low‐velocity boundary layer. The slow boundary layer beneath the central Pacific may be a localized zone of partial melt, or perhaps a chemically distinct layer, with its location linked to overlying upwelling motions

Seismic Detection of the Layers of the Lunar Core

This slide presentation reviews the analysis of Apollo-era seismic data and indirect geophysical measurements (i.e., moment of inertia, lunar laser ranging and electromagnetic induction) and concludes that significant questions still remain. The Apollo deep moonquake seismograms using terrestrial array processing methods is analyzed to infer the structure of the lunar core. The results indicate the presence of a solid inner and fluid outer core

GRAIL Refinements to Lunar Seismic Structure

A method to enhance and detect subtle seismic arrivals typically used in terrestrial seismology, is to stack seismograms that have been time shifted to the predicted arrival time of a hypothetical phase of interest. We previously applied this array processing approach to the Apollo lunar seismic data, providing the first direct constraint on the size and state of the Moon's core. The method used travel time predictions made from pre-existing estimates of the crust and mantle velocities and densities and assumed that each of the Moons layers ia a uniform shell with no lateral variation or heterogeneity. In reality the structural properties of the Moon are likely inhomogeneous and vary both laterally and with depth

Multiparameter Phospho-Flow Analysis of Lymphocytes in Early Rheumatoid Arthritis: Implications for Diagnosis and Monitoring Drug Therapy

The precise mechanisms involved in the initiation and progression of rheumatoid arthritis (RA) are not known. Early stages of RA often have non-specific symptoms, delaying diagnosis and therapy. Additionally, there are currently no established means to predict clinical responsiveness to therapy. Immune cell activation is a critical component therefore we examined the cellular activation of peripheral blood mononuclear cells (PBMCs) in the early stages of RA, in order to develop a novel diagnostic modality.PBMCs were isolated from individuals diagnosed with early RA (ERA) (n = 38), longstanding RA (n = 10), osteoarthritis (OA) (n = 19) and from healthy individuals (n = 10). PBMCs were examined for activation of 15 signaling effectors, using phosphorylation status as a measure of activation in immunophenotyped cells, by flow cytometry (phospho-flow). CD3+CD4+, CD3+CD8+ and CD20+ cells isolated from patients with ERA, RA and OA exhibited activation of multiple phospho-epitopes. ERA patient PBMCs showed a bias towards phosphorylation-activation in the CD4+ and CD20+ compartments compared to OA PBMCs, where phospho-activation was primarily observed in CD8+ cells. The ratio of phospho (p)-AKT/p-p38 was significantly elevated in patients with ERA and may have diagnostic potential. The mean fluorescent intensity (MFI) levels for p-AKT and p-H3 in CD4+, CD8+ and CD20+ T cells correlated directly with physician global assessment scores (MDGA) and DAS (disease activity score). Stratification by medications revealed that patients receiving leflunomide, systemic steroids or anti-TNF therapy had significant reductions in phospho-specific activation compared with patients not receiving these therapies. Correlative trends between medication-associated reductions in the levels of phosphorylation of specific signaling effectors and lower disease activity were observed.Phospho-flow analysis identified phosphorylation-activation of specific signaling effectors in the PB from patients with ERA. Notably, phosphorylation of these signaling effectors did not distinguish ERA from late RA, suggesting that the activation status of discrete cell populations is already established early in disease. However, when the ratio of MFI values for p-AKT and p-p38 is >1.5, there is a high likelihood of having a diagnosis of RA. Our results suggest that longitudinal sampling of patients undergoing therapy may result in phospho-signatures that are predictive of drug responsiveness

Travel Time and Waveform Measurements of Global Multibounce Seismic Waves Using Virtual Station Seismogram Stacks

Abstract We construct geographically localized bin stacks of waveforms, called virtual stations, to enhance signal‐to‐noise ratios (SNRs) for travel time and waveform measurements of multibounce S and ScS phases (S up to S6 and ScS up to ScS5), as well as direct S, ScS, and Sdiff, on tangential component data. Major arc S and ScS multibounce waves were also measured. Virtual station data are referenced to empirical wavelets constructed from direct S waves for each event. The virtual station approach is useful for low SNR data, bolstering wave path coverage in the southern hemisphere. Goodness of fit measurements between the adapted empirical wavelet and virtual station waveforms are documented, as well as SNRs, allowing for objective definition of travel time measurement quality. From a data set of 360 earthquakes and 8,407 seismographic stations, nearly 4 million records were utilized to construct 248,657 virtual station stacked seismograms, which were compared to best‐fitting empirical wavelets. After human inspection of virtual station results, 8,871 travel time measurements were retained from 19 different minor and major arc seismic wave types. Higher multibounce data improve sampling of the southern hemisphere. From 188,003 single seismograms, 3,331 multibounce wave measurements were also made. Comparisons of single seismogram and virtual station stack measurements show a consistent bias: Virtual stack onset times are systematically early due to a broadening effect from stacking records with arrival time differences, which we correct for. The travel time and waveform measurements are publicly available

On seismic resolution of lateral heterogeneity in the Earth's outermost core

Issues concerning resolution of seismically determined outermost core properties are presented with an example from three earthquakes in the Fiji-Tonga region. Travel time behavior of the commonly used family of SmKS waves, which travel as S in the mantle, P in the core, reflecting m − 1 times at the underside of the core-mantle boundary (CMB), are analyzed over a large distance range (125–165°). Data having wavepaths through an area of known D″ heterogeneity (±2%) exhibit systematic anomalies in SmKS differential times. Two-dimensional wave propagation experiments demonstrate how large-scale lower-mantle velocity perturbations can explain long-wavelength behavior of such anomalous SmKS times, though heterogeneity on smaller scales may be responsible for the observed scatter about these trends. If lower-mantle heterogeneity is not properly accounted for in deriving a core model, misfit of the mantle model maps directly into core structure. The existence of outermost core heterogeneity is difficult to resolve at present, owing to uncertainties in global lower-mantle structure. Resolving a one-dimensional chemically stratified outermost core also remains difficult, owing to the same uncertainties. Inclusion of the slowly accruing broadband data should help in this regard. Restricting study to higher multiples of SmKS (m = 2, 3, 4) can help reduce the effect of mantle heterogeneity, because of the closeness of the mantle legs of the wavepaths. SmKS waves are ideal in providing additional information on the details of lower-mantle heterogeneity

Seismic Resolution of the Earth's Outermost Core in Relation to Lower Mantle Heterogeneity

Issues concerning resolution of seismically determined outermost core properties are presented. Particular attention is given to effects of both large and small scale lower mantle heterogeneity on seismic phases most commonly used for determining outermost core properties. The family of SmKS waves, which travel as S in the mantle, P in the core, reflecting (m-1) times at the underside of the core-mantle boundary (CMB), are the outer core's equivalent to upper mantle multiple S waves (S, SS, SSS, ... ), and are well-suited for studying outermost core structure. The higher multiples of SmKS have outer core wave paths restricted to the outermost few hundred km of the core (see figure). Travel time and waveform behavior of SmKS waves are analyzed over a large distance range (125° - 165°) and correlated to overlying mantle structure. Long-period (LP) World Wide Seismographic Station Network data are utilized due to the presently unsurpassed ≈20 year time span of operation for global station coverage. This data set is augmented by available broadband data. In regions where lower mantle heterogeneity is predicted small, SmKS observations are well predicted by the PREM reference model, with the addition of a slight reduction in Vp in the top 50 km of the core (1.5%). Such a reduction implies chemical stratification in this 50 km zone, though this model feature is not uniquely resolved. Data having wave paths through areas of known D" heterogeneity (± 2%) exhibit systematic anomalies in SmKS differential times. 2-D wave propagation experiments using a modified WKBJ method demonstrate how large scale lower mantle velocity perturbations can explain long wavelength behavior of such anomalous SmKS times, though heterogeneity on smaller scales may be responsible for the observed scatter about these trends. A 2-D model having anomalously slow lower mantle velocities beneath the Indonesia region produces SmKS differential time residuals that agree with observations of Fiji-Tonga events recorded in Eurasia and Africa. In general, information from previously published 3-D maps of mantle heterogeneity can be used to construct starting models of 2-D cross sections appropriate for source-receiver geometries of interest

A very slow basal layer underlying large-scale low-velocity anomalies in the lower mantle beneath the Pacific: evidence from core phases

A multi-phase analysis using long-period World Wide Standardized Seismograph Network and Canadian Network data has been conducted using core-phases for deep focus events from the southwest Pacific. These include SKS, S2KS, SVd_(iff), and SP_dKS. The last phase emerges from SKS near 106° and is associated with a P-wave diffracting along the bottom of the mantle. Patterns in S2KS - SKS differential travel times (T_(S2KS-SKS)) correlate with those in SP_dKS - SKS (T_(SPdKS-SKS)). T_(S2KS-SKS) values strongly depend on variations in V_S structure in the lower third of the mantle, whereas T_(SPdKS-SKS) values mainly depend on V_P structure and variations in a thin zone (100 km or less) at the very base of the mantle. Anomalously large T_(S2KS-SKS) and T_(SPdKS-SKS) values (relative to the Preliminary Reference Earth Model (PREM)) are present for Fiji-Tonga and Kermadec events (recorded in North and South America), along with anomalously large SV_(diff) amplitudes well into the core's shadow. More northerly paths beneath the Pacific to North America for Indonesian and Solomon events display both PREM-like and anomalous times. A model compatible with the observations is presented, and contains a thin very-low-velocity layer at the base of the mantle that underlies the large volumetric lower-mantle low-velocity regions in the southwest Pacific. A low-velocity layer of 20–100 km thickness with reductions of up to 5–10% (relative to PREM) can reproduce T_(SPdKS-SKS) as well as SV_(diff) amplitudes. Large-scale (more than 1000 km) lower-mantle V_S heterogeneity (2–4%) can explain long-wavelength trends in T_(S2KS-SKS). The exact thickness and velocity reduction in the basal layer is uncertain, owing to difficulties in resolving whether anomalous structure occurs on the source- and/or receiver-side of wavepaths (at the CMB)