SS Waves Research Articles

A Taxonomy of Upper‐Mantle Stratification in the US

AbstractThe investigation of upper mantle structure beneath the US has revealed a growing diversity of discontinuities within, across, and underneath the sub‐continental lithosphere. As the complexity and variability of these detected discontinuities increase—for example, velocity increase/decrease, number of layers and depth—it is hard to judge which constraints are robust and which explanatory models generalize to the largest set of constraints. Much work has been done to image discontinuities of interest using S‐waves that convert to P‐waves (or top‐side reflected SS waves). A higher resolution method using P‐to‐S scattered waves is preferred but often obscured by multiply reflected waves trapped in a shallower layer, limiting the visibility of deeper boundaries. Here, we address the interference problem and re‐evaluate upper mantle stratification using filtered P‐to‐S receiver functions (Ps‐RFs) interpreted using unsupervised machine‐learning. Robust insight into upper mantle layering is facilitated with CRISP‐RF: Clean Receiver‐Function Imaging using Sparse Radon Filters. Subsequent sequencing and clustering organizes the polarity‐filtered Ps‐RFs into distinct depth‐based clusters. We find three types of upper mantle stratification beneath the old and stable continental US: (a) intra‐lithosphere discontinuities (paired or single boundary), (b) transitional discontinuities (single boundary or with a top layer), and (c) sub‐lithosphere discontinuities. Our findings contribute a more nuanced understanding of mantle discontinuities, offering new perspectives on the nature of upper mantle layering beneath continents.

QS‐wave decoupling equation for wavefield separation in transversely isotropic media

AbstractConsidering the anisotropy of the earth media is helpful in reducing the depth error between seismic and drilling and providing reliable imaging data for seismic interpretation and inversion. Transversely isotropic media with a vertical axis of symmetry are the most common type of anisotropic media and have been under constant study. The separation of the P‐ and S‐wavefields, which affects the accuracy of elastic wave imaging and the inversion in transversely isotropic media with a vertical axis of symmetry, is a hot research topic. Among the commonly used wavefield decoupling methods, the S‐wave is typically obtained by subtracting the P‐wave from the total wavefield. However, this kind of wavefield decoupling method often leads to severe P‐wave crosstalk in the separated S‐wavefield; thus, it needs further development. In this paper, the principle that the divergence of the S‐wave is zero is employed to solve the unresolved S‐wave elastic parameters of the S‐wave stiffness matrix by utilizing the modified zero‐order pseudo‐Helmholtz decomposition operator. The obtained S‐wave elastic parameters are employed to construct the qS‐wave stress and facilitate the derivation of the relationship between the qS‐wave particle velocity and the qS‐wave stress. Furthermore, a qS‐wave decoupling first‐order velocity–stress equation, which matches the qP‐wave decoupling first‐order velocity–stress equation, is derived. By jointly using the decoupling equations of qS‐ and qP‐wave, the separation of the P‐ and S‐wavefields in transversely isotropic media with a vertical axis of symmetry is realized. The efficiency of the proposed method is demonstrated via numerical tests conducted to assess the wavefield separation. Moreover, a complex model is employed to perform elastic reverse‐time migration, resulting in the acquisition of accurate imaging results for PP, PS, SP, and SS waves, without the presence of significant artefacts. The correctness of the qS‐wave decoupling equation in transversely isotropic media with a vertical axis of symmetry is confirmed by the comparative tests using decoupling methods in isotropic media.

Age‐Independent Oceanic Plate Thickness and Asthenosphere Melting From SS Precursor Imaging

AbstractThe Earth's asthenosphere is a mechanically weak layer characterized by low seismic velocity and high attenuation. The nature of this layer has been strongly debated. In this study, we process 12 years of seismic data recorded at the global seismological network stations to investigate SS waves reflected at the upper and lower boundaries of this layer in global oceanic regions. We observe strong reflections from both the top and the bottom of the asthenosphere, dispersive across all major oceans. The average depths of the two discontinuities are 120 and 255 km, respectively. The SS waves reflected at the lithosphere and asthenosphere boundary are characterized by anomalously large amplitudes, which require a ∼12.5% reduction in seismic velocity across the interface. This large velocity drop can not be explained by a thermal cooling model but indicates 1.5%–2% localized melt in the oceanic asthenosphere. The depths of the two discontinuities show large variations, indicating that the asthenosphere is far from a homogeneous layer but likely associated with strong and heterogeneous small‐scale convection in the oceanic mantle. The average depths of the two boundaries are largely constant across different age bands. In contrast to the half‐space cooling model, this observation supports the existence of a constant‐thickness plate in oceanic regions with a complex and heterogeneous origin.

Laboratory study of wave damping by porous breakwaters on mangrove mudflats in the Mekong River Delta

This paper deals with a laboratory study on wave transmission at breakwaters of OCT (Oyster CASTLE-type) blocks on mangrove mudflats. The aim is to examine characteristics of wave transmission in connection with conditions favourable for mangrove rehabilitation in the Mekong River Delta. Across-structure wave energy balance and exchange of fine sediment were examined in comparison with several existing types of permeable breakwaters. It is shown that porous-type structures like the OCT breakwater are more advantageous over hollow-type ones and are generally recommended for use in favour of mangrove re-colonization.The bulk porosity is shown to play an exceptional role in controlling wave transmission as well as exchange of fine sediment. Long-period/infra-gravity waves (IG) are not effectively dissipated by the structure, which is in contrast to short-period (SS) ones. Transmission of IG waves is found to virtually depend on the bulk porosity only, but not on geometric parameters like SS waves. The result consolidates the assumption that transport of mud through the structure is largely associated with IG waves. For the functional design of OCT breakwaters, an empirical formula of wave transmission has been derived, in which the structure permeability effect expressed in terms of the bulk porosity is explicitly incorporated.

Finite-frequency imaging of the global 410- and 660-km discontinuities using SS precursors

SUMMARY We report finite-frequency imaging of the global 410- and 660-km discontinuities using boundary sensitivity kernels for traveltime measurements made on SS precursors. The application of finite-frequency sensitivity kernels overcomes resolution limits in previous studies associated with large Fresnel zones of SS precursors and their interferences with other seismic phases. In this study, we calculate the finite-frequency sensitivities of SS waves and their precursors based on a single-scattering (Born) approximation in the framework of travelling-wave mode summation. The global discontinuity surface is parametrized using a set of triangular gridpoints with a lateral spacing of about 4°, and we solve the linear finite-frequency inverse problem (2-D tomography) based on singular value decomposition (SVD). The new global models start to show a number of features that were absent (or weak) in ray-theoretical back-projection models at spherical harmonic degree l > 6. The thickness of the mantle transition zone correlates well with wave speed perturbations at a global scale, suggesting dominantly thermal origins for the lateral variations in the mantle transition zone. However, an anticorrelation between the topography of the 410-km discontinuity and wave speed variations is not observed at a global scale. Overall, the mantle transition zone is about 2–3 km thicker beneath the continents than in oceanic regions. The new models of the 410- and 660-km discontinuities show better agreement with the finite-frequency study by Lawrence & Shearer than other global models obtained using SS precursors. However, significant discrepancies between the two models exist in the Pacific Ocean and major subduction zones at spherical harmonic degree >6. This indicates the importance of accounting for wave interactions in the calculations of sensitivity kernels as well as the use of finite-frequency sensitivities in data quality control.

Dynamic triggering of shallow slip on forearc faults constrained by monitoring surface displacement with the IPOC Creepmeter Array

The Mw=8.8 Maule Earthquake of February 27, 2010, remotely triggered minor surface displacement along the Atacama Fault System in Northern Chile located 1500–1800 km from the epicenter. Here we present evidence that surface displacement events recorded by the IPOC (Integrated Plate Boundary Observatory in N-Chile) Creepmeter Array are related to earthquakes on the underlying subduction zone interface or to large earthquakes around the world. Our observations document dynamic triggering of shallow slip on faults in a forearc setting. Continuous monitoring revealed that up to 30 displacement events per year occur on the monitored fault segments, >90% of them triggered by earthquakes in the near or far field synchronous to the passage of the seismic wave. We can clearly attribute far field triggering to the passage of the surface wave and near field triggering to the passage of the body wave. The most distant triggered displacement event monitored occurred after the 2011, Mw=9.0 Tohoku-Oki Earthquake located 16 468 km away following the passage of large amplitude SS waves. Comparing the triggered displacement events with other triggered phenomena like seismicity, volcanic eruptions, hydrological changes as well as with data from the California Creepmeter Array we find that the seismic energy density appropriately describes a magnitude–distance scaling relationship of slip triggering. Finally, the observed surface displacement is found to not occur by continuous creep, but rather through nearly exclusively discrete displacement events.

Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii

Global seismic discontinuities near 410 and 660 km depth in Earth’s mantle are expressions of solid-state phase transitions. These transitions modulate thermal and material fluxes across the mantle and variations in their depth are often attributed to temperature anomalies. Here we use novel seismic array analysis of SS waves reflecting off the 410 and 660 below the Hawaiian hotspot. We find amplitude–distance trends in reflectivity that imply lateral variations in wavespeed and density contrasts across 660 for which thermodynamic modeling precludes a thermal origin. No such variations are found along the 410. The inferred 660 contrasts can be explained by mantle composition varying from average (pyrolitic) mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. Such compositional segregation was predicted, from petrological and numerical convection studies, to occur near hot deep mantle upwellings like the one often invoked to cause volcanic activity on Hawaii.

A hot midmantle anomaly in the area of the Indian Ocean Geoid Low

AbstractWe investigate the upper mantle seismic discontinuities at 410 and 660 km depth beneath the Indian Ocean Geoid Low (IOGL). To map the discontinuities' topography, we use differential travel times of PP and SS waves and their precursors. Our final data set consists of 37 events with Mw ≥ 5.8, which densely cover our investigation area, also with crossing ray paths. We use array methods to detect the low‐amplitude precursor signals. The best quality data show a deepened 410 km discontinuity in the center of the IOGL as well as a mostly elevated 660 km discontinuity beneath the northern Indian Ocean, which we interpret as a hot anomaly currently residing in the mantle transition zone. We conclude that the largest negative geoid anomaly might be caused by a combined effect of hot material in the midmantle below the innermost IOGL and cold material below 660 km farther south.

Shear velocity structure and mineralogy of the transition zone beneath the East Pacific Rise

Models of seismic velocity as a function of depth through the upper mantle provide some of the strongest constraints on the mineralogy and composition of the mantle. Although receiver function studies have provided new information on the depths of upper mantle discontinuities they do not provide as much information on seismic gradients between discontinuities and absolute velocities within the upper mantle. The waveforms and travel times of upper mantle turning waves provide the strongest constraints on vertical variations in upper mantle velocity although in the past they suffered from the lack of dense profiles of data sampling a single part of the upper mantle that would minimize effects of 3D variations in velocity. Here we model three dense profiles of triplicated upper mantle broadband S and SS waves recorded by USArray, Canadian and NARS-Baja stations located in western North America. Earthquakes along the East Pacific Rise were recorded along profiles within 5° back azimuth windows and with stations at a maximum of 0.5° separation. The distance range covered is from 35° to 60° and thus the waves sample the mantle from the shallow upper mantle to depths near 1000 km. The data were inverted using a conjugate gradient algorithm that utilizes the reflectivity synthetic technique. The results show a much smaller gradient within the transition zone than the PREM model with larger jumps in velocity across the 410 km and 660 km depth discontinuities. These results are consistent with velocities predicted for a pyrolite composition mantle transition zone. Compositional models with lower olivine content, such as piclogite, are not consistent with our seismic model.

Lithospheric cooling trends and deviations in oceanic PP‐P and SS‐S differential traveltimes

The thermal and compositional structure of oceanic lithosphere, which exerts an important control on plate behavior, is still debated. Our set of 60,000 PP‐P and SS‐S traveltime differences with oceanic PP and SS bounce points provides a good constraint on both compressional‐ and shear‐wave velocity. By calculating traveltimes for thermal models that are converted to seismic structures with a thermodynamic approach, we test whether lithospheric cooling can explain PP‐P and SS‐S traveltime variations with plate age. The PP‐P and SS‐S traveltimes have substantial scatter but, on average, decrease by 0.2 and 0.7 s/Myr½, respectively, when the PP and SS waves reflect off progressively older oceanic crust. Both a half‐space and a plate cooling model with a mid‐ocean ridge basalt‐source mantle potential temperature (1315° ± 50°C) explain the average values of the PP‐P and SS‐S anomalies and their decrease with plate age. Residual PP‐P and SS‐S anomalies relative to a cooling model reveal large‐scale patterns. Along a few paths (e.g., Tonga–Fiji to western North America), seismic heterogeneity in the deep mantle is responsible for a significant fraction of the PP‐P and SS‐S traveltime variation. Most anomalies probably correspond to broad temperature variations in the upper mantle, such as a very slow central–northern Pacific (which may require a 100°C excess temperature) and high‐ and low‐velocity anomalies along the ridges that correlate with deep and shallow bathymetry, respectively.

Triggering of tremors and slow slip event in Guerrero, Mexico, by the 2010 Mw 8.8 Maule, Chile, earthquake

We investigate the triggering of seismic tremor and slow slip event in Guerrero (Mexico) by the February 27, 2010 Maule earthquake (Mw 8.8). Triggered tremors start with the arrival of S wave generated by the Maule earthquake, and keep occurring during the passing of ScS, SS, Love and Rayleigh waves. The Rayleigh wave dispersion curve footprints the high frequency energy envelope of the triggered tremor, indicating a strong modulation of the source of tremors by the passing surface wave. This correlation and modulation by the passing waves is progressively lost with time over a few hours. The tremor activity continues during the weeks/months after the earthquake. GPS time series suggest that the second sub‐event of the 2009–2010 SSE in Guerrero is actually triggered by the Maule earthquake. The southward displacement of the GPS stations starts coincidently with the earthquake and tremors. The long duration of tremors indicate a continuing deformation process at depth, which we propose to be the second sub‐event of the 2009–2010 SSE. We show a quasi‐systematic correlation between surface displacement rate measured by GPS and tremor activity, suggesting that the NVT are controlled by the variations in the slip history of the SSE. This study shows that two types of tremors emerge: (1) Those directly triggered by the passing waves and (2) those triggered by the stress variations associated with slow slip. This indicates the prominent role of aseismic creep in the Mexican subduction zone response to a large teleseismic earthquake, possibly leading to large‐scale stress redistribution.

Study of the lithospheric and upper-mantle discontinuities beneath eastern Asia by SS precursors

SUMMARY We analyse broad-bandSS waveform data recorded by several networks in Europe with sources mainly in the west Pacific to study the underside reflections of teleseismic SS waves in the lithosphere and the upper mantle beneath eastern Asia and the NW Pacific ocean. SS bounce points sample a corridor from the Aleutian, Kamchatka and Japan subduction zones through the North China Craton and Central Asian Orogenic Belt to the Tibetan plateau. The corridor passes through different tectonic units such as subduction zones, an old continental shield, a fold belt and a high plateau. We investigate the seismic structure of the lithosphere and the mantle transition zone beneath the different geotectonic units along the profile and infer the correlation of geodynamic processes at different depths. We explore the short period frequency content in the SS waveform data and use moveout correction and common midpoint stack to acquire profiles with high lateral and depth resolution from the crust to the mantle transition zone. Clear SS precursors of the 410 and 660 km discontinuities show the effects of the interaction between the subducted oceanic lithosphere and the mantle transition zone beneath the NW Pacific subduction zones. A low-velocity layer has also been detected beneath the 410 km discontinuity and can be traced along the entire profile. Due to the improved resolution acquired by the method presented here we have been able to study the shallower structures such as the Moho and the lithosphere–asthenosphere boundary by SS precursors. The continental Moho can be clearly seen along this corridor. The depth variation agrees well with earlier receiver function results. We also see negative reflectors along the profile at varying depths, which can be interpreted as the lithosphere–asthenosphere boundary.

Frequency-dependent effects on global S-wave traveltimes: wavefront-healing, scattering and attenuation

We present a globally distributed data set of ∼400 000 frequency-dependent SH-wave traveltimes. An automated technique is used to measure teleseismic S, ScS and SS traveltimes at several periods ranging from 10 to 51 s. The targeted seismic phases are first extracted from the observed and synthetic seismograms using an automated time window algorithm. Traveltimes are then measured at several periods, by cross-correlation between the selected observed and synthetic filtered waveforms. Frequency-dependent effects due to crustal reverberations beneath each receiver are handled by incorporating crustal phases into WKBJ synthetic waveforms. After correction for physical dispersion due to intrinsic anelastic processes, we observe a residual traveltime dispersion on the order of 1–2 s in the period range of analysis. This dispersion occurs differently for S, ScS and SS, which is presumably related to their differing paths through the Earth. We find that: (1) Wavefront-healing phenomenon is observed for S and to a lesser extent SS waves having passed through very low velocity anomalies. (2) A preferred sampling of high velocity scatterers located at the CMB may explain our observation that ScS waves travel faster at low-frequency than at high-frequency. (3) A frequency-dependent attenuation q(ω) ∝q0×ω−α, with α∼ 0.2, is compatible with the globally averaged dispersion observed for S waves.

Computing traveltime and amplitude sensitivity kernels in finite-frequency tomography

The efficient computation of finite-frequency traveltime and amplitude sensitivity kernels for velocity and attenuation perturbations in global seismic tomography poses problems both of numerical precision and of validity of the paraxial approximation used. We investigate these aspects, using a local model parameterization in the form of a tetrahedral grid with linear interpolation in between grid nodes. The matrix coefficients of the linear inverse problem involve a volume integral of the product of the finite-frequency kernel with the basis functions that represent the linear interpolation. We use local and global tests as well as analytical expressions to test the numerical precision of the frequency and spatial quadrature. There is a trade-off between narrowing the bandpass filter and quadrature accuracy and efficiency. Using a minimum step size of 10km for S waves and 30km for SS waves, relative errors in the quadrature are of the order of 1% for direct waves such as S, and a few percent for SS waves, which are below data uncertainties in delay time or amplitude anomaly observations in global seismology. Larger errors may occur wherever the sensitivity extends over a large volume and the paraxial approximation breaks down at large distance from the ray. This is especially noticeable for minimax phases such as SS waves with periods >20s, when kernels become hyperbolic near the reflection point and appreciable sensitivity extends over thousands of km. Errors becomes intolerable at epicentral distance near the antipode when sensitivity extends over all azimuths in the mantle. Effects of such errors may become noticeable at epicentral distances>140°. We conclude that the paraxial approximation offers an efficient method for computing the matrix system for finite-frequency inversions in global tomography, though care should be taken near reflection points, and alternative methods are needed to compute sensitivity near the antipode.

Comment on ‘On sensitivity kernels for ‘wave-equation’ transmission tomography’ by de Hoop and van der Hilst

SUMMARY The finite-frequency sensitivity kernels developed by Dahlen et al. are an appropriate tool for inverting traveltimes of non-triplicated P, S, PP and SS waves, measured by cross-correlation with a synthetic pulse. The critique of this theoretical methodology in a recent paper by de Hoop & van der Hilst is based upon the incorrect notion that one can account for errors in the

Frequency-dependent effects on travel times and waveforms of long-period S and SS waves

The spectral properties of upper-mantle velocity perturbations are dominantly determined from regional and global tomographic models. In this paper we study frequency-dependent effects of l3ng-period S and SS waves of SH-type through random upper-mantle models with specified spectral properties. In particular, we investigate the information content of the power spectra of travel time perturbations as a function of model spectrum and frequency band. Wave propagation is simulated by a finite-difference (FD) approximation to the axi-symmetric wave equation in spherical coordinates. For global models with cylindrical symmetry and constant angular increment Δθ, the use of spherical coordinates leads to an effective lateral grid spacing (arc length) decreasing with depth. This is contrary to the requirements of global models with low velocities at the top of the mantle, which necessitate a dense grid spacing at small depths and a wider grid spacing at the base of the mantle. We introduce a grid with depth-dependent lateral grid spacing to overcome this inconsistency. Our main findings are that (1) the properties of spatial power spectra of travel time fluctuations are frequency-dependent; (2) power spectra of models obtained from long-period tomography may underestimate the power at intermediate scales; (3) frequency-dependent effects on the waveform are sensitive to the scales and amplitudes of perturbations present in the upper mantle.

Depth of the upper mantle discontinuities beneath the East Pacific Rise

We observe discontinuities at depths of 415 and 655 km beneath the northern East Pacific Rise (EPR). The data consist of precursors to SS waves from 71 tangential component seismograms ranging in epicentral distance from 107° to 167°. The mid‐points of the paths are beneath the northern EPR region with about half the data bouncing beneath ocean younger than 9 Ma. The SS phases were aligned and then time shifted to account for the ocean age at the bounce point. The data were then stacked to remove theoretical moveout for underside reflections from the transition zone. The stacked results show clear underside reflections. These values are slightly more shallow than recently derived average values for oceans but the depth difference between the discontinuities is almost identical to the average for all oceans (Shearer, 1996). Although the absolute depths are dependent on the velocity model used in the estimate, the depth difference in the discontinuities is not sensitive to velocity structure. Our results suggest there are not unusually hot temperatures within the transition zone beneath the EPR relative to average ocean. This implies that deep upwelling beneath the ridge, if it exists, is passive.

Can the differential sensitivity of body wave, mantle wave, and normal mode data resolve the trade-off between transition zone structure and boundary topography?

Abstract Large-scale seismic models of the three-dimensional (3D) variations in elastic properties will be biased by topography on mantle boundaries to the extent that volumetric and topographic structures produce similar effects in the data. To date, seismic inversions for global-scale 3D elastic models of the mantle have largely ignored the effect that topography on the major mantle discontinuities would have on estimating these models. In this paper we address three questions: (1) to what extent does unmodeled structure on the 410 and 660 km boundaries bias volumetric structure in inversions based on normal mode-mantle wave structure coefficients, absolute S-wave travel times, and differential SS-S travel times? (2) Can the differences in the sensitivity of S waves, SS-S phase pairs, and normal mode-mantle wave data be exploited to estimate transition zone volumetric models that are relatively unbiased by topographic structure? (3) Have current volumetric models resolved the trade-off that exists between volumetric and topographic structures? To address question (1), synthetic experiments were performed which show that volumetric models inferred from normal mode-mantle wave data can be biased by an average value of 20–25% in r.m.s. amplitude in the transition zone relative to current aspherical Earth models, depending on the model parametrization employed. The average transition zone volumetric bias of models inverted from absolute travel times from both transition zone and lower-mantle bottoming S rays that are imprinted with the same topographic signatures is reduced by at least by a factor of five relative to models inverted from normal mode-mantle wave data alone. A reduction in bias by a factor of about four is obtained using SS-S phase pairs in which the SS legs bottom in both the transition zone and lower mantle. The use of lower-mantle bottoming S rays or SS-S phase pairs with the SS legs bottoming in the lower mantle reduces the bias only by an average factor of about two to three relative to the normal mode-mantle wave inversion. These estimates of bias reduction can vary with the type of damping or smoothness constraints that are applied in the inversion. With respect to question (2), these results suggest that, in principle, absolute S-wave travel time data can be used to desensitize volumetric inversions to the bias caused by topography on the transition zone boundaries. In practice, however, near-source structure that contaminates S-wave travel times would diminish this capability. The use of SS-S differential times for SS waves which bottom in the transition zone and mantle waves sensitive to transition zone structure but insensitive to the 660 km boundary, should be the most effective means of overcoming the trade-off. To address question (3), we adopt the criterion that the combination of an unbiased volumetric model with an accurate topographic model should provide a better fit to normal mode structure coefficients than the volumetric model alone. The boundary model Topo660a when added to the volumetric model S12_WM13 fits the normal mode structure coefficients significantly better than the volumetric model alone. However, more recent models of 410 and 660 km boundary topography degrade the fit of the volumetric models S12_WM13 and SH.10c.17 to the normal mode structure coefficients, suggesting that there is not yet a conclusive answer to this question.

On PP and SS reflection point anomalies

Reflection point residuals of PP and SS waves obtained from the residuals of PP-P and SS-S with respect to the J–B tables are shown to be in need of corrections. Recent velocity models of the Earth's mantle predict residuals that differ considerably amongst themselves and from the J–B tables. Before useful reflection point residuals can be determined by this method a reference model has to approximate closely the actual velocity distribution of the Earth between the source and receiver. Lateral variations, layering in velocity, and scattering in the vicinity of the reflection point of short-period PP also contribute large uncertainties in defining the reflection points of PP and its precursors. All of the foregoing problems suggest that interpretations of PP and SS wave residuals in terms of structure near the symmetric reflections points, as far as they have been determined up to now, are subject to serious errors.

Differential shear‐wave attenuation (δt*) across the East Pacific Rise

SS phases from earthquakes on fracture zones near the Easter Island Cordillera and the West Chile Rise which are recorded in the United States have reflection points on either side of the East Pacific Rise (EPR) near the equator. The east‐west records from seven WWSSN stations of seven events in this region were used to obtain spectral amplitudes of horizontally polarized S and SS waves. SS‐to‐S amplitude ratios were formed, and differential attenuation (δt*) computed within the frequency band 0.01 to 0.11 Hz. The values of δt* vary between −0.1 sec and +35.8 sec for the 23 station‐event pairs used. However, the change in δt* with distance from the axis of the EPR does not reflect the smooth variation expected using a model of a simple cooling slab.