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Continent-scale observations of seismic phenomena have provided multi-scale constraints of the Earth’s interior. Of those analysed, array-based observations of slowness vector properties (backazimuth and horizontal slowness) and multipathing have yet to be made on a continental scale. Slowness vector measurements give inferences on mantle heterogeneity properties such as velocity perturbation, velocity gradient strength and quantify their effect on the wavefield. Multipathing is a consequence of waves interacting with strong velocity gradients resulting in two arrivals that arrive with different slowness vector properties and times. The mantle structure beneath the contiguous Unites States has been thoroughly analysed by seismic studies and is data-rich, making it an excellent testing ground to analyse mantle structure with our approach and compare with other techniques. We apply an automated array-analysis technique to an SKS dataset to create the first continent-scale dataset of multipathing and slowness vector measurements. We analyse the divergence of the slowness vector deviation field to highlight seismically slow and fast regions in our data. Our results resolve several slow mantle anomalies beneath Yellowstone, the Appalachian mountains and fast anomalies throughout the mantle. Many of the anomalies cause multipathing in frequency bands 0.15–0.30 and 0.20–0.40 Hz which suggests velocity transitions over at most 500 km exist. Comparing our observations to synthetics created from tomography models, we find model NA13 \citeA{bedle_continental_2021} fits our data best but differences still remain. We therefore suggest slowness vector measurements should be used as an additional constraint in tomographic inversions and will lead to better-resolved models of the mantle.

SUMMARYHorizontal slowness vector measurements using array techniques have been used to analyse many Earth phenomena from lower mantle heterogeneity to meteorological event location. While providing observations essential for studying much of the Earth, slowness vector analysis is limited by the necessary and subjective visual inspection of observations. Furthermore, it is challenging to determine the uncertainties caused by limitations of array processing such as array geometry, local structure, noise and their effect on slowness vector measurements. To address these issues, we present a method to automatically identify seismic arrivals and measure their slowness vector properties with uncertainty bounds. We do this by bootstrap sampling waveforms, therefore also creating random sub arrays, then use linear beamforming to measure the coherent power at a range of slowness vectors. For each bootstrap sample, we take the top N peaks from each power distribution as the slowness vectors of possible arrivals. The slowness vectors of all bootstrap samples are gathered and the clustering algorithm DBSCAN (Density-Based Spatial Clustering of Applications with Noise) is used to identify arrivals as clusters of slowness vectors. The mean of slowness vectors in each cluster gives the slowness vector measurement for that arrival and the distribution of slowness vectors in each cluster gives the uncertainty estimate. We tuned the parameters of DBSCAN using a data set of 2489 SKS and SKKS observations at a range of frequency bands from 0.1 to 1 Hz. We then present examples at higher frequencies (0.5–2.0 Hz) than the tuning data set, identifying PKP precursors, and lower frequency by identifying multipathing in surface waves (0.04–0.06 Hz). While we use a linear beamforming process, this method can be implemented with any beamforming process such as cross correlation beamforming or phase weighted stacking. This method allows for much larger data sets to be analysed without visual inspection of data. Phenomena such as multipathing, reflections or scattering can be identified automatically in body or surface waves and their properties analysed with uncertainties.

Qualitative and quantitative analysis of seismic waveforms sensitive to the core–mantle boundary (CMB) region reveal the presence of ultralow-velocity zones (ULVZs) that have a strong decrease in compressional (P) and shear (S) wave velocity, and an increase in density within thin structures. However, understanding their physical origin and relation to the other large-scale structures in the lowermost mantle are limited due to an incomplete mapping of ULVZs at the CMB. The SKS and SPdKS seismic waveforms is routinely used to infer ULVZ presence, but has thus far only been used in a limited epicentral distance range. As the SKS/SPdKS wavefield interacts with a ULVZ it generates additional seismic arrivals, thus increasing the complexity of the recorded wavefield. Here, we explore utilization of the multi-scale sample entropy method to search for ULVZ structures. We investigate the feasibility of this approach through analysis of synthetic seismograms computed for PREM, 1-, 2.5-, and 3-D ULVZs as well as heterogeneous structures with a strong increase in velocity in the lowermost mantle in 1- and 2.5-D. We find that the sample entropy technique may be useful across a wide range of epicentral distances from 100° to 130°. Such an analysis, when applied to real waveforms, could provide coverage of roughly 85% by surface area of the CMB.

Ultralow-velocity zones (ULVZs) at the core&ndash, mantle boundary (CMB) represent some of the most preternatural features in Earth&rsquo, s mantle. These zones most likely contain partial melt, extremely high iron content ferropericlase, or combinations of both. We analyzed a new collection of 58,155 carefully processed and quality-controlled broadband recordings of the seismic phase SPdKS in the epicentral distance range from 106&deg, to 115&deg, These data sample 56.9% of the CMB by surface area. From these recordings we searched for the most anomalous seismic waveforms that are indicative of ULVZ presence. We used a Bayesian approach to identify the regions of the CMB that have the highest probability of containing ULVZs, thereby identifying sixteen regions of interest. Of these regions, we corroborate well-known ULVZ existence beneath the South China Sea, southwest Pacific, the Samoa hotspot, the southwestern US/northern Mexico, and Iceland. We find good evidence for new ULVZs beneath North Africa, East Asia, and north of Papua New Guinea. We provide further evidence for ULVZs in regions where some evidence has been hinted at before beneath the Philippine Sea, the Pacific Northwest, and the Amazon Basin. Additional evidence is shown for potential ULVZs at the base of the Caroline, San Felix and Galapagos hotspots.

Measuring the elastic modulus of in situ rock masses over scales of tens of meters remains an important challenge in experimental rock mechanics. Here we present a new approach using ambient resonance measurements of freestanding rock landforms to identify vibrational modes, which are then matched with 3D numerical models implementing bulk, globally representative material properties. The result is an experimentally determined, albeit numerically calibrated, estimate of rock mass elastic modulus. We demonstrate the approach at five natural rock arches in southern Utah, each formed in Navajo Sandstone, where we have acquired resonance data and matched experimental resonant modes using 3D numerical modal analysis. Two material properties can be varied to match experimental data: density and modulus. We hold density constant, applying measured or reference values, and solve for elastic modulus using a forward approach. The resolved modulus is representative of the global small-strain dynamic behavior, integrating rock mass heterogeneity over the scale of the feature. The technique works well for freestanding geological landforms that exhibit clear vibrational modes. Errors arise with uncertain mechanical boundary conditions or strong material anisotropy. The resolved modulus values add relevant information describing the variation of elastic properties over scales from lab samples to in situ rock masses.

The natural modes of vibration of bedrock landforms, as well as the sources and effects of stimulated resonance remain poorly understood. Here we show that seismic energy created by an induced earthquake and an artificial reservoir has spectral content coincident with the natural modes of vibration of a prominent rock bridge. We measured the resonant frequencies of Rainbow Bridge, Utah using data from two broadband seismometers placed on the span, and identified eight distinct vibrational modes between 1 and 6 Hz. A distant, induced earthquake produced local ground motion rich in 1 Hz energy, stimulating a 20 dB increase in measured power at the bridge's fundamental mode. Moreover, we establish that wave action on Lake Powell, an artificial reservoir, generates microseismic energy with peak power ~1 Hz, also exciting resonance of Rainbow Bridge. These anthropogenic sources represent relatively new energy input for the bridge with unknown consequences for structural fatigue.

Ultra-low velocity zones (ULVZs) are thin patches of material with strongly reduced seismic wave speeds situated on top of the core-mantle boundary (CMB). A common phase used to detect ULVZs is SPdKS (SKPdS), an SKS wave with a short diffracted P leg along the CMB. Most previous efforts have examined ULVZ properties using 1D waveform modeling approaches. We present waveform modeling results using the 2.5D finite difference algorithm PSVaxi allowing us better insight into ULVZ structure and location. We characterize ULVZ waveforms based on ULVZ elastic properties, shape, and position along the SPdKS raypath. In particular, we vary the ULVZ location (e.g. source or receiver side), ULVZ topographical profiles (e.g. boxcar, trapezoidal, or Gaussian) and ULVZ lateral scale along great circle path (2.5o, 5o, 10o). We observe several waveform effects absent in 1D ULVZ models and show evidence for waveform effects allowing the differentiation between source and receiver side ULVZs. Early inception of the SPdKS/SKPdS phase is difficult to detect for receiver-side ULVZs with maximum shifts in SKPdS initiation of ∼3o in epicentral distance, whereas source-side ULVZs produce maximum shifts of SPdKS initiation of ∼5o, allowing clear separation of source- versus receiver-side structure. We present a case study using data from up to 300 broadband stations in Turkey recorded between 2005 and 2010. We observe a previously undetected ULVZ in the southern Atlantic Ocean region centered near 45o S, 12.5oW, with a lateral scale of ∼3°, VP reduction of 10%, VS reduction of 30%, and density increase of 10% relative to PREM.

We analyzed the resonance characteristics of a prominent natural arch in Canyonlands National Park, Mesa Arch, as measured from ambient seismic data. Evaluating spectral and polarization attributes, we distinguished the first four resonant frequencies of the arch, 2.9, 6.0, 6.9, and 8.5 Hz, as well as basic properties of the associated mode shapes. We then affirmed experimental data using 3-D numerical modal analysis, providing estimates of material properties and clarifying vibrational mode shapes. Monitoring resonant frequencies over time, we searched for shifts associated with changing environmental conditions and long-term progressive damage. We measured ~3% direct daily variation in resonant frequency associated with changing rock temperature, thermal stress, and stiffening of the rock matrix. Independent tilt data showed similar diurnal cycles associated with thermoelastic stresses and deformation of the arch. We observed no permanent resonant frequency shifts related to irreversible damage of Mesa Arch during our study period.

We analyzed transverse and radial component recordings from the 2010 M6.3 southern Spain earthquake (depth = 620 km) recorded on 370 broadband stations in North America. We grouped these seismograms into subarrays and applied fourth root vespa processing (vespagram analysis) in order to enhance low-amplitude arrivals. These vespagrams show clear Scd arrivals which indicate the existence of the D″ discontinuity beneath the North Atlantic Ocean (45–60°N, 45–55°W). These observations are best fit with a +2–4% velocity increase at the top of the D″ discontinuity at a height above the core-mantle boundary of 304 ± 14 km. We do not observe Scd arrivals at the eastern end of our study region which is consistent with the presence of the easternmost edge of the ancient Farallon plate.

We analyzed vertical component short-period ScP waveforms for 26 earthquakes occurring in the Tonga-Fiji trench recorded at the Alice Springs Array in central Australia. These waveforms show strong precursory and postcursory seismic arrivals consistent with ultralow-velocity zone (ULVZ) layering beneath the Coral Sea. We used the Viterbi sparse spike detection method to measure differential travel times and amplitudes of the postcursor arrival ScSP and the precursor arrival SPcP relative to ScP. We compare our measurements to a database of 340,000 synthetic seismograms finding that these data are best fit by a ULVZ model with an S wave velocity reduction of 24%, a P wave velocity reduction of 23%, a thickness of 8.5km, and a density increase of 6%. This 1:1 VS:VP velocity decrease is commensurate with a ULVZ compositional origin and is most consistent with highly iron enriched ferropericlase.

We collected a new data set of 1354 broadband SPdKS waveforms sampling the western Pacific Ocean region. These data indicate that multiple ultralow-velocity zones (ULVZs) exist in this region. We compared these data to 2.5-D synthetic seismograms computed with the PSVaxi method for a suite of 517 ULVZ models. The region beneath the North and South Philippine Sea shows no evidence for ULVZ presence. The region beneath the Coral Sea shows a large ULVZ which is approximately 700 × 700 km in lateral dimensions and up to 20 km thick. Multiple small-scale ULVZs approximately 180 × 180 km and up to 10 km thick are inferred beneath the South China Sea. Our findings are consistent with previous efforts utilizing the ScP seismic phase and provide additional constraints on ULVZ position and size. Key Points Collected a new dataset of 1354 waveforms We used new mapping technique to locate ULVZs We found previously undetected ULVZs.

The technology currently available in university classrooms often includes interactive whiteboards, high‐definition displays and projectors; furthermore, many students are equipped with tablet computers. Nevertheless, teaching material that can take advantage of these technologies is time consuming to produce and there is need for additional available material for courses in geophysics. The use of dynamic materials, such as animations and movies in the classroom has been shown to have the ability to enhance learning (e.g., see Mayer and Moreno, 2002, for a review). However, traditional methods of teaching about the global seismic wave field typically involve static imagery. Usually this has been achieved through a combination of drawing ray paths and wave fronts at distinct timesteps. Thus, students have had to build the mental connections between the underlying wave propagation and the static imagery being displayed. But the recent availability of cheap computer clusters has made numerical computation of the seismic wave field in realistic Earth models readily accessible. As a result, animating seismic‐wave propagation has become relatively commonplace. Some of the first global animations of seismic‐wave propagation were produced by summing torsional mode free oscillations in the mantle to show the SH wave field at periods down to 12 s (Wysession and Shore, 1994). This set of animations has been a valuable educational resource and has inspired us to create a new collection. Here, we present a new series of global seismic‐wave animations that includes both the SH and P / SV wave field. In addition, recent developments to the TauP Toolkit (Crotwell et al. , 1999) have made it possible to animate seismic ray paths and wavefronts. Thus, our collection combines the seismic wave field with dynamically drawn ray paths and/or wavefronts. In what follows, we describe a few of the animations that are available on the global scale and …

Accurate interpretation of seismic traveltimes and amplitudes in the exploration and global scales is complicated by the band-limited nature of seismic data. We discovered a stochastic method to reduce a seismic waveform into a most probable constituent spike train. Model waveforms were constructed from a set of candidate spike trains convolved with a source wavelet estimate. For each model waveform, a profile hidden Markov model (HMM) was constructed to represent the waveform as a stochastic generative model with a linear topology corresponding to a sequence of samples. Each match state in the HMM represented a sample in the model waveform, in which the amplitude was represented by a Gaussian distribution. Insert and delete states allowed the underlying source wavelet to dilate or contract, accounting for nonstationarity in the seismic data and errors in the source wavelet estimate. The Gaussian distribution characterizing each sample’s amplitude accounted for random noise. The Viterbi algorithm was employed to simultaneously find the optimal nonlinear alignment between a model waveform and the seismic data and to assign a score to each candidate spike train. The most probable traveltimes and amplitudes were inferred from the alignments of the highest scoring models. The method required no implicit assumptions regarding the distribution of traveltimes and amplitudes; however, in practice, the solution set may be limited to mitigate the nonuniqueness of solutions and to reduce the computational effort. Our analyses found that the method can resolve closely spaced arrivals below traditional resolution limits and that traveltime estimates are robust in the presence of random noise and source wavelet errors. The method was particularly well suited to fine-scale interpretation problems such as thin bed interpretation, tying seismic images to well logs, and the analysis of anomalous waveforms in global seismology.

[1] Intermittent seismic discontinuities near 250–300 km depth beneath South America and the Pacific basin are detected with high-resolution seismic array methods that use SS and PP precursors recorded at the High Lava Plains Seismic Experiment and the EarthScope Transportable Array. The transformation of coesite to stishovite in an eclogite-rich mantle composition produces a seismic discontinuity near 300 km depth; lateral changes in basalt fraction of the upper mantle will thus produce an intermittent seismic discontinuity. The sensitivity of the precursors to intermittent seismic structure is addressed using an axisymmetric finite difference model of wave propagation in the mantle. These numerical experiments find that the precursors are sensitive to structures ≥500 km in lateral extent and that the observations of this discontinuity are plausibly tied to lateral variations in basaltic composition of the upper mantle related to dynamics, such as plumes and subduction.

[1] In this study, we analyzed the seismic phases S, SKS, and SKKS from 31 deep-focus earthquakes in the Tonga-Fiji region recorded in North America between epicentral distances of 85° and 120°. The differential traveltimes and amplitude ratios for these phases reveal clear epicentral distance trends not predicted by standard one-dimensional (1-D) reference Earth models. The increase of the S/SKS amplitude ratio up to a factor of 10 is accompanied by an increase of the S-SKS differential traveltime of up to 10 s. SKKS-SKS differential traveltimes of 2–3 s and SKKS/SKS amplitude ratios of a factor of 2–4 across the epicentral range have maxima near 107°. We examined these observations using full (1-D and 3-D) waveforms for three 1-D seismic velocity profiles for the central Pacific region and for the tomographic model S40RTS including modifications: different regularization parameters, great-circle path azimuthal variation, strength of S wave velocity perturbations, S wave velocity gradients in the lower mantle, and ultra–low velocity zones. To explain these data, we constructed a hybrid model that combines both features of S40RTS and short-wavelength features from the 1-D profiles. The large-scale seismic structure is represented by S40RTS. Embedded within S40RTS are a 20 km thick ultra–low velocity zone at the core-mantle boundary near the source side and a 200 km thick negative velocity gradient zone near the receiver side of the paths. Our analysis demonstrates that the S wave velocity structure of the Pacific large low shear-velocity province cannot be interpreted solely by global tomographic or regional modeling approaches in exclusion of each other.

Mantle flow in Earth's interior has been inferred from a variety of geo-disciplines. Two continental-scale, nearly antipodal, large low shear velocity provinces (LLSVPs) at the base of the mantle, thought to be dense and chemically distinct likely play a significant role in mantle dynamics and plume generation, and hence are targeted in a high-resolution seismic study. We analyze broadband SPdKS waveforms using a 2.5D axi-symmetric finite difference wave propagation algorithm PSVaxi. Here we find patches of greatly reduced seismic wave speeds at the core–mantle boundary, i.e., ultra-low velocity zones (ULVZs), within the Pacific LLSVP, including the largest ULVZ detected to date, roughly 250×800 km in lateral dimension and 10–15 km thick, in an apparent hole in the LLSVP. The presence of this ULVZ in the LLSVP hole is well explained by dynamically merging, chemically-distinct piles containing ULVZs at their margins. The consequence of these merging piles may be to initiate anomalously large, infrequent plumes, as well as to provide a means to transfer isotopes to the surface.

SUMMARY We infer shear attenuation in the lower mantle by using the method of instantaneous frequency matching to calculate differential t* between core-reflected ScS and direct S (δt*ScS-S). The instantaneous frequency at the envelope peak of a seismic phase is related to the average Fourier spectral frequency of that phase. To estimate δt*ScS-S for a given trace, we first calculate the instantaneous frequency at the envelope peak of S and ScS. The trace is then attenuated through convolution with a suite of t* operators until the instantaneous frequency at the envelope peak of the seismic phase with the initially larger instantaneous frequency matches the value of the smaller instantaneous frequency from the initial calculation. The differential t* operator required to accomplish the match is then δt*ScS-S. We also calculate δt*ScS-S from the slope of the spectral ratio of windowed ScS and S. Both the spectral ratio and instantaneous frequency methods produce consistent results for high signal-to-noise ratio synthetic waveforms with S and ScS well separated in time, and where there are no other interfering phases. The instantaneous frequency method gives more stable results for low signal-to-noise ratio waveforms, and where S and/or ScS are affected by other interfering seismic phases. The instantaneous frequency matching method is applied to broadband data from South American earthquakes recorded in California that sample the lower mantle beneath Central America and the Cocos plate. δt*ScS-S ranges from approximately –4 to 2 s, but are predominately negative, suggesting S is more attenuated than ScS for these data. We estimate the possibly contaminating effects of 3-D velocity heterogeneity on δt*ScS-S through analysis of synthetic seismograms computed for a cross-section through a tomographically derived model of global shear wave heterogeneity, using an axisymmetric finite difference algorithm. Synthetics for path geometries of our data predict a δt*ScS-S of ∼0.2 s. We investigate the effect of seismic anisotropy by comparing δt*ScS-S before and after a subset of the data were corrected using splitting parameters obtained by linearizing the particle motion of the S and ScS phases. The rms error of the residuals between the corrected and uncorrected δt*ScS-S is ∼0.2 s. Neither of these efforts, however, match the large negative observed δt*ScS-S values, suggesting the mid-mantle beneath western Central America is in fact much more attenuating than the lowermost mantle below it, or S may be broadened by out-of-plane propagation effects, involving the remains of the Farallon plate containing stronger velocity heterogeneity than is imaged by seismic tomography.

Broadband USArray recordings of the 21 July 2007 western Brazil earthquake (Mw=6.0; depth = 633 km) include high-amplitude signals about 40 s, 75 s, and 100 s after the P wave arrival. They are consistent with S wave to P wave conversions in the mantle beneath northwestern South America. The signal at 100 s, denoted as S1750P, has the highest amplitude and is formed at 1,750 km depth based on slant-stacking and semblance analysis. Waveform modeling using axisymmetric, finite difference synthetics indicates that S1750P is generated by a 10 km thick heterogeneity, presumably a fragment of subducted mid-ocean ridge basalt in the lower mantle. The negative polarity of S1750P is a robust observation and constrains the shear velocity anomaly δVS of the heterogeneity to be negative. The amplitude of S1750P indicates that δVS is in the range from −1.6% to −12.4%. The large uncertainty in δVS is due to the large variability in the recorded S1750P amplitude and simplifications in the modeling of S1750P waveforms. The lower end of our estimate for δVS is consistent with ab initio calculations by Tsuchiya (2011), who estimated that δVS of eclogite at lower mantle pressure is between 0 and −2% due to shear softening from the poststishovite phase transition.

SUMMARY We extended a high-order finite-difference scheme for the elastic SH-wave equation in axisymmetric media for use on parallel computers with distributed memory architecture. Moreover, we derive an analytical description of the implemented ring source and compare it quantitatively with a double couple source. The restriction to axisymmetry and the use of high performance computers and PC networks allows computation of synthetic seismograms at dominant periods down to 2.5 s for global mantle models. We give a description of our algorithm (SHaxi) and its verification against an analytical solution. As an application, we compute synthetic seismograms for global mantle models with additional stochastic perturbations applied to the background S-wave velocity model. We investigate the influence of the perturbations on the SH wavefield for a suite of models with varying perturbation amplitudes, correlation length scales, and spectral characteristics. The inclusion of stochastic perturbations in the models broadens the pulse width of teleseismic body wave arrivals and delays their peak arrival times. Coda wave energy is also generated which is observed as additional energy after prominent body wave arrivals. The SHaxi method has proven to be a valuable method for computing global synthetic seismograms at high frequencies and for studying the seismic waveform effects from models where rotational symmetry may be assumed.

Geometric spreading of Pn and Sn waves in a spherical Earth model is different than that of classical headwaves and is frequency dependent. The behavior cannot be fully represented by a frequency-independent power-law model, as is com- monly assumed. The lack of an accurate representation of Pn andSn geometric spread- ing in a spherical Earth model impedes our ability to characterize Earth properties including anelasticity. We conduct numerical simulations to quantify Pn and Sn geometric spreading in a spherical Earth model with constant mantle-lid velocities. Based on our simulation results, we present new empirical Pn and Sn geometric- spreading models in the form Gr;f ��� 10 n3� f� =r0�� r0=rn1� flogr0=r�� n2� fand nif �� ni1� logf=f0�� 2 � ni2 logf=f0 �� ni3, where i � 1 ,2 , or 3;r is epicentral distance; f is frequency; r0 � 1 km; and f0 � 1 Hz. We derive values of coefficients nij by fitting the model to computed Pn and Sn amplitudes for a spherical Earth model having a 40-km-thick crust, generic values of P and S velocities, and a constant-ve- locity uppermost mantle. We apply the new spreading model to observed data in Eur- asia to estimate average Pn attenuation, obtaining more reasonable results compared to using a standard power-law model. Our new Pn and Sn geometric-spreading models provide generally applicable reference behavior for spherical Earth models with con- stant uppermost-mantle velocities.

Summary We use an axisymmetric, spherical Earth finite difference algorithm to model SH-wave propagation through cross-sections of laterally varying lower mantle models beneath the Cocos Plate derived from recent data analyses. Synthetic seismograms with dominant periods as short as 4 s are computed for several models: (1) a D″ reflector 264 km above the core-mantle boundary with laterally varying S-wave velocity increases of 0.9–2.6 per cent, based on localized structures from a 1-D double-array stacking method; (2) an undulating D″ reflector with large topography and uniform velocity increase obtained using a 3-D migration method and (3) cross-sections through the 3-D mantle S-wave velocity tomography model TXBW. We apply double-array stacking to assess model predictions of data. Of the models explored, the S-wave tomography model TXBW displays the best overall agreement with data. The undulating reflector produces a double Scd arrival that may be useful in future studies for distinguishing between D″ volumetric heterogeneity and D″ discontinuity topography. Synthetics for the laterally varying models show waveform variability not observed in 1-D model predictions. It is challenging to predict 3-D structure based on localized 1-D models when lateral structural variations are on the order of a few wavelengths of the energy used, particularly for the grazing geometry of our data. Iterative approaches of computing synthetic seismograms and adjusting model characteristics by considering path integral effects are necessary to accurately model fine-scale D″ structure.

For decades, seismic data stacking has been used to improve the signal-to-noise ratio (SNR) of coherent arrivals in seismic recordings. Seismic arrays are especially well-suited for this task and have been used to detect and identify typically weak seismic arrivals ( e.g., Birtill and Whiteway 1965; Green et al. 1965). Seismic arrays are deployments of no fewer than three seismometers with uniform instrumentation that allow the recorded time series to be stacked as an ensemble. Typical array apertures range from 2 km to 200 km (Rost and Garnero 2004) and consist of 10 to 100 stations. All seismic array elements receive a centralized time signal, minimizing possible timing errors. Stations are deployed in specialized configurations to ensure a high coherence of transient signals in the wavefield, while the statistical noise component is uncorrelated between stations (Haubrich 1968). Special techniques have been developed to process seismic array data (for a review, see Rost and Thomas 2002), enabling the use of subtle arrivals in the seismic wavefield for seismic analysis and thus holding promise for higher resolution snapshots of the structure of the Earth's interior ( e.g. , Vidale and Benz 1993; Vidale and Earle 2000; Rost et al. 2005b). With increasing numbers of temporary and permanent array installations, data are becoming readily available for further high-resolution studies of the Earth's interior. In this paper we present a method to extract array-specific information ( e.g., vertical incidence angle-slowness) from a large number of array recordings. Slowness, time, and amplitude information from array recordings of individual earthquakes are combined into a single summary trace per earthquake, then stacked with other summary traces from a multitude of earthquakes recorded at the same seismic array. As a test case we process short-period array data from the Canadian Yellowknife Array (YKA) to highlight seismic phases …

Hot spot volcanism may originate from the deep mantle in regions exhibiting the Earth’s most pronounced lateral S-wave velocity gradients. These strong gradient regions display an improved geographic correlation over S-wave velocities to surface hot spot locations. For the lowest velocities or strongest gradients occupying 10% of the surface area of the core–mantle boundary (CMB), hot spots are nearly twice as likely to overlie the anomalous gradients. If plumes arise in an isochemical lower mantle, plume initiation should occur in the hottest (thus lowest velocity) regions, or in the regions of strongest temperature gradients. However, if plume initiation occurs in the lowest velocity regions of the CMB lateral deflection of plumes or plume roots are required. The average lateral deflections of hot spot root locations from the vertical of the presumed current hot spot location ranges from ∼300 to 900 km at the CMB for the 10–30% of the CMB covered by the most anomalous low S-wave velocities. The deep mantle may, however, contain strong temperature gradients or be compositionally heterogeneous, with plume initiation in regions of strong lateral S-wave velocity gradients as well as low S-wave velocity regions. If mantle plumes arise from strong gradient regions, only half of the lateral deflection from plume root to hot spot surface location is required for the 10–30% of the CMB covered by the most anomalous strong lateral S-wave velocities. We find that strong gradient regions typically surround the large lower velocity regions in the base of the mantle, which may indicate a possible chemical, in addition to thermal, component to these regions.

Synchrotron X-ray computed microtomography (CμT) is a non-destructive technique for imaging porous and compositionally heterogeneous samples in three dimensions at the microscale. In this study, we report a package of FORTRAN algorithms for digital image optimization and three-dimensional analysis of porosity, pore connectivity, and pore structure within a CμT volume. The algorithm Tomo_optimize optimizes digital data by utilizing a series of matrix filters and contrast transforms. Tomo_classify labels individual voxels within the data set as solid, internal pore space, or external void space, thus defining virtual volume boundaries. Tomo_analysis calculates total porosity, porosity from interior pores (completely surrounded by solid), and porosity from connected pores (open to external void space), and provides an output of each pore and its pore size (number of voxels per pore). The algorithms were tested on two natural samples from hydrothermal vent chimneys. Physical volume was 116 and 72 mm 3 for each sample and CμT spatial resolution was estimated to be 57 μm. Porosity determined by the CμT algorithms was 14.1% and 15.4%, respectively. The majority of porosity (>98%) was due to connected pores rather than isolated pores, and most of the pore volume contributing to total porosity of both samples (>90%) was from one large interconnected pore. While total porosity was similar for both samples, three-dimensional visual reconstructions showed a more channelized pore structure in one sample. Sensitivity analyses were performed to test the effect of different cut-off values for air, internal pore space, and solid entered by the user before and after image processing on porosity calculations. These algorithms provide an integrated image processing and analysis package for synchrotron CμT data that should be useful for the analysis of microporous structures as this technique gains popularity.

[1] Using an unusually large earthquake near the big island of Hawaii, we study the core mantle boundary (CMB) beneath the north-northeastern Pacific between Hawaii and North America. A dense sampling of the CMB is achieved using the core-reflected phase PcP recorded at a large number of high-quality stations in North America, including networks in California, Oregon, Washington, and Alaska, as well as at EarthScope's USArray stations. We apply an adaptive stacking technique for optimal record alignment on specific phases (namely P and PcP) and subsequently stack seismograms to obtain summation traces possessing relatively high signal-to-noise ratios of PcP and P. Anomalous ultralow-velocity zone (ULVZ) layering at the CMB has been noted to exist in various parts of the Pacific beneath the lowermost mantle large low shear velocity province imaged by tomography. ULVZ structure produces anomalous PcP waveform variations in the form of precursors to PcP. These PcP data, however, lack precursory energy, indicating either that (1) ULVZ layering is lacking or (2) that a ULVZ is present and thinner than our detection threshold, i.e., less than a few kilometers thick. We use synthetic waveform modeling to establish the sensitivity and utility of investigating the time window ahead of PcP for precursors generated from fine-scale CMB layering. These results, combined with evidence for ULVZ structure in other parts of the Pacific, suggest that ULVZs are intermittent and possibly only detectable in regions where mantle currents collect ULVZ material, whether or not partially molten, presumably beneath (or near) upwellings or plumes.

[1] Between 105–115° degrees, the SKS waveform is complicated by the formation of SPdKS, a wave that has segments of P diffraction along the core mantle boundary. While previous studies have primarily focused on the move-out of SPdKS from SKS, we analyze the concomitant reduction of the SKS amplitude. Long-period SKKS/SKS amplitude ratios present a coherent global pattern. SKKS/SKS is relatively large in North and South American recordings of deep Tonga-Fiji earthquakes but PREM-like in European recordings of earthquakes in South America and North American recordings of earthquakes in Indonesia. Modeling of SKKS/SKS indicate that Ultra-Low Velocity Zones (ULVZs), layers at the base of the mantle with a thickness of about 10–20 km and a shear velocity reduction between 20–30%, are required to explain high SKKS/SKS ratios and the early move-out of SPdKS.

Ultra-low velocity layering at the Earth’s core-mantle boundary (CMB) has now been detected using a variety of seismic probes. P- and S-wave velocity reductions of up to 10’s of percent have been mapped in a thin (5–50 km) layer, which commonly underlies reduced seismic shear wave speeds in the overlying few 100 km of the mantle. Ultra-low velocity zones (ULVZ) contain properties consistent with partial melt of rock at the very base of the mantle. Strong evidence now exists for a significant density increase in the layer (∼5–10% greater than reference models), which must be included in dynamical scenarios relating ULVZ partial melt to deep mantle plume genesis. 3-D geodynamical calculations involving an initially uniform dense layer in the lowermost few 100 km of the mantle result in thermo-chemical piles that are geographically well-correlated with seismic tomography low velocities, when past plate motions are imposed as a surface boundary condition. The hottest lower mantle regions underlay edges of the dense thermo-chemical piles. A scenario is put forth where these piles geographically correlate with ultra-low velocity zones, and subsequent mantle plume genesis.

Temperature gradients in a low-shear-velocity province in the lowermost mantle ( D″ region) beneath the central Pacific Ocean were inferred from the observation of a rapid S -wave velocity increase overlying a rapid decrease. These paired seismic discontinuities are attributed to a phase change from perovskite to post-perovskite and then back to perovskite as the temperature increases with depth. Iron enrichment could explain the occurrence of post-perovskite several hundred kilometers above the core-mantle boundary in this warm, chemically distinct province. The double phase-boundary crossing directly constrains the lowermost mantle temperature gradients. Assuming a standard but unconstrained choice of thermal conductivity, the regional core-mantle boundary heat flux (∼85 ± 25 milliwatts per square meter), comparable to the average at Earth's surface, was estimated, along with a lower bound on global core-mantle boundary heat flow in the range of 13 ± 4 terawatts. Mapped velocity-contrast variations indicate that the lens of post-perovskite minerals thins and vanishes over 1000 kilometers laterally toward the margin of the chemical distinct region as a result of a ∼500-kelvin temperature increase.

[1] Anomalous boundary layer structure at the core-mantle boundary (CMB) is investigated using a global set of broadband SKS and SPdKS waves from permanent and portable broadband seismometer arrays. SPdKS is an SKS wave that intersects the CMB at the critical angle for ScP, thus initiating a diffracted P wave (Pdiff) along the CMB at the core entry and exit locations. The waveshape and timing of SPdKS data are analyzed relative to SKS, with some SPdKS data showing significant delays and broadening. Broadband data from several hundred deep focus earthquakes were analyzed; retaining data with simple sources and high signal-to-noise ratios resulted in 53 high-quality earthquakes. For each earthquake an empirical source was constructed by stacking pre-SPdKS distance range SKS pulses (∼90°–100°). These were utilized in our synthetic modeling process, whereby reflectivity synthetic seismograms are produced for three classes of models: (1) mantle-side ultralow-velocity zones (UVLZ), (2) underside CMB core rigidity zones, and (3) core-mantle transition zones. For ULVZ structures, ratios of P-to-S velocity reductions of 1:1 and 1:3 are explored, where 1:3 is appropriate for a partial melt origin of ULVZ. Over 330 unique CMB boundary layer models have been constructed and tested, corroborating previous work suggesting strong trade-offs between the three model spaces. We produce maps of inferred boundary layer structure from the global data and find evidence for extremely fine-scale heterogeneity where our wave path sampling is the densest. While uncertainties are present relating to the source versus receiver sides of the SPdKS wave path geometry, our data are consistent with the hypothesis that ULVZ presence (or absence) correlates with reduced (or average) heterogeneity in the overlying mantle.