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Imaging Upper‐Mantle Structure Under USArray Using Long‐Period Reflection Seismology

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Journal of Geophysical Research: Solid Earth
Authors:
Peter M. Shearer
Peter M. Shearer
Peter M. Shearer
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Janine Buehler
Janine Buehler
Janine Buehler
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Abstract and Figures

Topside reverberations off mantle discontinuities are commonly observed at long periods, but their interpretation is complicated because they include both near‐source and near‐receiver reflections. We have developed a method to isolate the stationside reflectors in large data sets with many sources and receivers. Analysis of USArray transverse‐component data from 3,200 earthquakes, using direct S as a reference phase, shows clear reflections off the 410‐ and 660‐km discontinuities, which can be used to map the depth and brightness of these features. Because our results are sensitive to the impedance contrast (velocity and density), they provide a useful complement to receiver‐function studies, which are primarily sensitive to the S velocity jump alone. In addition, reflectors in our images are more spread out in time than in receiver functions, providing good depth resolution. Our images show strong discontinuities near 410 and 660 km across the entire USArray footprint, with intriguing reflectors at shallower depths in many regions. Overall, the discontinuities in the east appear simpler and more monotonous with a uniform transition zone thickness of 250 km compared to the western United States. In the west, we observe more complex discontinuity topography and small‐scale changes below the Great Basin and the Rocky Mountains, and a decrease in transition‐zone thickness along the western coast. We also observe a dipping reflector in the west that aligns with the top of the high‐velocity Farallon slab anomaly seen in some tomography models, but which also may be an artifact caused by near‐surface scattering of incoming S waves.
(a) A global stack of long‐period S waves from shallow earthquakes (<50‐km depth) and transverse‐component, global seismic network data from 1976 to 2010, using direct S as a reference phase. Waveforms are aligned and normalized to unit amplitude using the maximum amplitude of direct S (or Sdiff), flipping the polarity as needed, and stacked in 0.5° bins in epicentral distance. Positive amplitudes are plotted as red, negative amplitudes as blue, with the maximum set to 0.05 of the direct S amplitude. The topside reverberations, Ss410S and Ss660s, appear as blue streaks following S by 2.5 to 4 min. Note that their polarity is reversed from S because of the discontinuity reflection. Other phases, including precursors to SS from underside reflections (S410S and S660S), are labeled. (b) The topside 660‐km reflected phase, Ss660s results from both near‐source and near‐receiver reverberations, as shown by raypaths for the iasp91 velocity model (Kennett & Engdahl, 1991).
(a) A global stack of long‐period S waves from shallow earthquakes (<50‐km depth) and transverse‐component, global seismic network data from 1976 to 2010, using direct S as a reference phase. Waveforms are aligned and normalized to unit amplitude using the maximum amplitude of direct S (or Sdiff), flipping the polarity as needed, and stacked in 0.5° bins in epicentral distance. Positive amplitudes are plotted as red, negative amplitudes as blue, with the maximum set to 0.05 of the direct S amplitude. The topside reverberations, Ss410S and Ss660s, appear as blue streaks following S by 2.5 to 4 min. Note that their polarity is reversed from S because of the discontinuity reflection. Other phases, including precursors to SS from underside reflections (S410S and S660S), are labeled. (b) The topside 660‐km reflected phase, Ss660s results from both near‐source and near‐receiver reverberations, as shown by raypaths for the iasp91 velocity model (Kennett & Engdahl, 1991).
… 
Sources and receivers used in this study. We examined 3,200 earthquakes of M ≥ 5.5 and depth ≤50 km (red stars) recorded by 1,688 USArray stations (blue dots) between 75° and 120°, with 183 earthquakes remaining after our selection screening (see text).
Sources and receivers used in this study. We examined 3,200 earthquakes of M ≥ 5.5 and depth ≤50 km (red stars) recorded by 1,688 USArray stations (blue dots) between 75° and 120°, with 183 earthquakes remaining after our selection screening (see text).
… 
A record‐section stack of our USArray transverse‐component data, aligned and normalized on direct S as in Figure 1. The topside reflected phases, Ss410s and Ss660s, are labeled, as well as the underside reflected precursors to SS, S410S, and S660S.
A record‐section stack of our USArray transverse‐component data, aligned and normalized on direct S as in Figure 1. The topside reflected phases, Ss410s and Ss660s, are labeled, as well as the underside reflected precursors to SS, S410S, and S660S.
… 
Fresnel zones for topside reverberations, as mapped by traveltime differences in seconds for lateral perturbations in the topside bounce points for reflectors at depths of 200, 400, and 600 km. The Snell's law reflection point is at zero time at the center of the 5‐s contour. For 20‐s period data, the Fresnel zone of constructive interference is defined by the 10‐s contour.
Fresnel zones for topside reverberations, as mapped by traveltime differences in seconds for lateral perturbations in the topside bounce points for reflectors at depths of 200, 400, and 600 km. The Snell's law reflection point is at zero time at the center of the 5‐s contour. For 20‐s period data, the Fresnel zone of constructive interference is defined by the 10‐s contour.
… 
Raypath geometries for 660‐km topside S wave reflections. (a) Near‐source reflections observed at source‐receiver distances of 75°, 87°, and 103°. (b) Near‐receiver reflections compared to direct S at source‐receiver distances of 81° and 97°. Raypaths are based on the iasp91 velocity model (Kennett & Engdahl, 1991).
+7
Raypath geometries for 660‐km topside S wave reflections. (a) Near‐source reflections observed at source‐receiver distances of 75°, 87°, and 103°. (b) Near‐receiver reflections compared to direct S at source‐receiver distances of 81° and 97°. Raypaths are based on the iasp91 velocity model (Kennett & Engdahl, 1991).
… 
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Imaging Upper-Mantle Structure Under USArray Using
Long-Period Reflection Seismology
Peter M. Shearer1and Janine Buehler2
1Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA, 2Now at Swiss National
Science Foundation, Berne, Switzerland
Abstract Topside reverberations off mantle discontinuities are commonly observed at long periods,
but their interpretation is complicated because they include both near-source and near-receiver
reflections. We have developed a method to isolate the stationside reflectors in large data sets with many
sources and receivers. Analysis of USArray transverse-component data from 3,200 earthquakes, using
direct Sas a reference phase, shows clear reflections off the 410- and 660-km discontinuities, which can be
used to map the depth and brightness of these features. Because our results are sensitive to the impedance
contrast (velocity and density), they provide a useful complement to receiver-function studies, which are
primarily sensitive to the Svelocity jump alone. In addition, reflectors in our images are more spread out in
time than in receiver functions, providing good depth resolution. Our images show strong discontinuities
near 410 and 660 km across the entire USArray footprint, with intriguing reflectors at shallower depths in
many regions. Overall, the discontinuities in the east appear simpler and more monotonous with a
uniform transition zone thickness of 250 km compared to the western United States. In the west, we
observe more complex discontinuity topography and small-scale changes below the Great Basin and the
Rocky Mountains, and a decrease in transition-zone thickness along the western coast. We also observe a
dipping reflector in the west that aligns with the top of the high-velocity Farallon slab anomaly seen in
some tomography models, but which also may be an artifact caused by near-surface scattering of incoming
Swaves.
1. Introduction
Stacks of long-period seismograms at teleseismic distances reveal a variety of secondary seismic phases
resulting from reflections and phase conversions from upper-mantle discontinuities (e.g., Shearer, 1991,
1990). Of these, converted phases from interfaces below the stations (receiver functions) and underside
reflections precursory to SS and PP have received the most attention and have been widely used to map the
topography of the 410- and 660-km discontinuities. In contrast, topside reverberations, such as Ss660s,are
obvious in data stacks (see Figure 1a) but have not been studied very much because their interpretation is
complicated by the fact that they include both near-source and near-receiver reflections (see Figure 1b).
In principle, this source-receiver ambiguity can be removed given many sources and receivers and spatially
variable discontinuity properties. That is, if each source is recorded by multiple stations and each station
records multiple earthquakes, then one can formulate an inverse problem to separately resolve near-source
and near-receiver discontinuity structure. We apply this approach here to image upper-mantle discontinu-
ities under 1688 stations of the USArray experiment across the contiguous United States using long-period
SH-wave reflections and a common-reflection-point (CRP) stacking method. We observe variations in the
properties of the 410- and 660-km discontinuities, as well as a negative polarity reflection (NPR) at 40- to
100-km depth. In unsmoothed data, we also observe reflections from a dipping feature that agrees in posi-
tion with the Farallon slab imaged in some tomography models, but which may also be an artifact related
to scattering from the western continental margin.
2. Data and Method
We obtained USArray data from 2004 to 2014 for 3,200 shallow earthquakes (M≥5, depth ≤50 km, see
Figure 2), rotated the horizontals to obtain transverse components and applied a 10-s low-pass filter. We then
required that each earthquake be recorded by at least five stations with a signal-to-prearrival-noise ratio of
RESEARCH ARTICLE
10.1029/2019JB017326
Key Points:
• Common-reflection-point stacking
applied to long-period teleseismic
SH waves images upper-mantle
discontinuities under USArray
• These images show the 410- and
660-km discontinuities as well as a
negative polarity reflection between
50 and 100 km under much of the
United States
• A steeply dipping structure is imaged
in the western United States that
could be related to the Farallon Slab
Supporting Information:
• Supporting Information S1
•TablesS1
•TablesS2
•TablesS3
•FigureS1
•FigureS2
•FigureS3
•FigureS4
Correspondence to:
P. Sh ea re r,
pshearer@ucsd.edu
Received 7 JAN 2019
Accepted 19 JUL 2019
Accepted article online 31 JUL 2019
©2019. American Geophysical Union.
All Rights Reserved.
SHEARER AND BUEHLER 9638
Published online 4 SEP 2019
Citation:
Shearer, P. M., & Buehler, J. (2019).
Imaging upper‐mantle structure under
USArray using long‐period reflection
seismology. Journal of Geophysical
Research: Solid Earth,124, 9638–9652.
https://doi.org/10.1029/2019JB017326
... Following this study, T. Liu and Shearer (2021) imaged shallow mantle structures under the contiguous US by analyzing only deep earthquakes to avoid contamination from source-side structures. In this study, we apply the S-reverberation method in Shearer and Buehler (2019) to the data collected by the local networks and all TA stations in Alaska to obtain high-resolution images of crustal and MTZ structures beneath the region. ...
... Next, we stack and normalize the traces in 1° increments in epicentral distance and 2-s increments in time with direct S as the reference phase, flipping the polarity when direct S has negative amplitude (Figures 1b and 1c). These record sections exhibit clear topside reflections Ss410s and Ss660s and are similar to Figure 1 in Shearer and Buehler (2019). Since there are strong SS and ScS arrivals interfering with the MTZ discontinuity phases at distances less than 60°, we only use traces between 60° and 120° for the 10-s data set. ...
... Note that these cells have equal N-S and E-W dimensions at 60°N and thus are approximately square across most of Alaska. Next we apply the common-reflection-point method and obtain the sum of receiver-side reflections within each cell by minimizing both source and receiver terms using an iterative least-square inversion approach (Shearer & Buehler, 2019). Finally, considering the credibility and resolution of the results (see Section 2.2 for more detail), 10.1029/2023JB026667 5 of 17 we smooth the results horizontally using a radially symmetric cos 2 taper with a 3° radius and treat only the grid cells contributed by more than 100 bounce points as robust. ...
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View
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... Pioneering work (Shearer, 1991) demonstrated that weak phases related to the mantle discontinuities can be extracted by stacking many seismograms. For example, topside reflexions of earthquake phases and SS precursors from underside reflections on the mantle discontinuities have small amplitudes, but they can be aligned and stacked so as to constrain lateral variations in arrival times of the two discontinuities, see for example Shearer and Buehler (2019) and references within. Surface wave overtones can also be used to constrain lateral variations of the mantle transition zone (Meier et al., 2009). ...
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  • Apr 2017
  • EARTH PLANET SC LETT
We report from converted seismic waves, a pervasive seismically anomalous layer above the transition zone beneath the western US. The layer, characterized by an average shear wave speed reduction of 1.6%, spans over an area of ∼1.8×106 km2∼1.8×106 km2 with thicknesses varying between 25 and 70 km. The location of the layer correlates with the present location of a segment of the Farallon plate. This spatial correlation and the sharp seismic signal atop of the layer indicate that the layer is caused by compositional heterogeneity. Analysis of the seismic signature reveals that the compositional heterogeneity can be ascribed to a small volume of partial melt (0.5 ± 0.2 vol% on average). This article presents the first high resolution map of the melt present within the layer. Despite spatial variations in temperature, the calculated melt volume fraction correlates strongly with the amplitude of P–S conversion throughout the region. Comparing the values of temperature calculated from the seismic signal with available petrological constraints, we infer that melting in the layer is caused by release of volatiles from the subducted Farallon slab. This partially molten zone beneath the western US can sequester at least 1.2×1017 kg1.2×1017 kg of volatiles, and can act as a large regional reservoir of volatile species such as H or C.
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Roughness of the Mantle Transition Zone Discontinuities Revealed by High Resolution Wavefield Imaging: Roughness of the Mantle Discontinuities
Article
  • Sep 2016
We processed two independent datasets: 2,458,973 three-component seismograms from all Earthscope Transportable Array (USArray) stations and 141,080 pairs of high quality receiver functions from the Earthscope Automated Receiver Survey (EARS) using the generalized iterative deconvolution method and shaped the output with a 0.1 Hz Ricker wavelet. We used these data as input to our 3D plane wave migration (PWMIG) method to produce an image volume of P to S scattering surfaces under all of the lower 48 states. Model simulations show that compared to the widely used common conversion point (CCP) stacking method, our method is capable of resolving not only dipping discontinuities but also subtler details of amplitude and topography variations at scales near the diffraction limit. Maps of attributes derived from these discontinuities reveal several previously unobserved features of the 410 and 660 discontinuities (d410 and d660). Topography with many 10s of km is resolved at a range of scales. Additionally, we observed large variations of amplitude on the radial component and in the radial to transverse amplitude ratio that correlate with inferred variations in discontinuity topography. We argue this combination of observations can be explained by roughness at a range of scales. The structural fabric we imaged is preferentially aligned orthogonal to the Farallon-North American relative motion. Two regions we infer to have exceptional roughness show spatial correlation with locations where tomography models indicate the subducted slab is passing through the transition zone. We suggest that these rough regions are indicators of vertical mantle flows through the transition zone.
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Midlithospheric discontinuities and complex anisotropic layering in the mantle lithosphere beneath the Wyoming and Superior Provinces: Craton Anisotropy
Article
  • Aug 2016
The observation of widespread seismic discontinuities within Archean and Proterozoic lithosphere is intriguing, as their presence may shed light on the formation and early evolution of cratons. A clear explanation for the discontinuities, which generally manifest as a sharp decrease in seismic velocity with depth, remains elusive. Recent work has suggested that mid-lithospheric discontinuities (MLDs) may correspond to a sharp gradient in seismic anisotropy, produced via deformation associated with craton formation. Here we test this hypothesis beneath the Archean Superior and Wyoming Provinces using anisotropic Ps receiver function (RF) analysis to characterize the relationship between MLDs and seismic anisotropy. We computed radial and transverse component RFs for 13 long-running seismic stations. Of these, six stations with particularly clear signals were analyzed using a harmonic regression technique. In agreement with previous studies, we find evidence for multiple MLDs within the cratonic lithosphere of the Wyoming and Superior Provinces. Our harmonic regression results reveal that 1) MLDs can be primarily explained by an isotropic negative velocity gradient, 2) multiple anisotropic boundaries exist within the lithospheric mantle, 3) the isotropic MLD and the anisotropic boundaries do not necessarily occur at the same depths, and 4) the depth and geometry of the anisotropic boundaries vary among stations. We infer that the MLD does not directly correspond to a change in anisotropy within the mantle lithosphere. Furthermore, our results reveal a surprising level of complexity within the cratonic lithospheric mantle, suggesting that the processes responsible for shaping surface geology produce similar structural complexity at depth.
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Evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath Africa
Article
  • Dec 1998
We present data which indicate that the broad, low shear velocity anomaly beneath southern Africa is stronger and more extensive than previously thought. Recordings of earthquakes in the southwestern Atlantic Ocean at an array of broadband seismic stations in eastern Africa show anomalously large propagation time delays of the shear phases S, ScS, and SKS which vary rapidly with epicentral distance. By forward modeling, we estimate that the low velocity anomaly extends from the core-mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3% lower than in standard models such as PREM. Strong velocity contrasts exist at its margins (2% over about 300 km). These seismic characteristic are consistent with recent numerical simulations of lower mantle mega-plume formation.
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The meaning of mid-lithospheric discontinuities: A case study in the northern U.S. craton
Article
Converted wave imaging has revealed significant negative velocity gradients, often termed midlithospheric discontinuities, within the thick, high-velocity mantle beneath cratons. In this study, we investigate the origins and implications of these structures with high-resolution imaging of mantle discontinuities beneath the Archean Wyoming, Superior and Medicine Hat, and Proterozoic Yavapai and Trans-Hudson terranes. Sp phases from 872 temporary and permanent broadband stations, including the EarthScope Transportable Array, were migrated into three-dimensional common conversion point stacks. Four classes of discontinuities were observed. (1) A widespread, near-flat negative velocity gradient occurs largely at 70-90 km depth beneath both Archean and Proterozoic cratons. This structure is consistent with the top of a frozen-in layer of volatile-rich melt. (2) Dipping negative velocity gradients are observed between 85 and 200 km depth. The clearest examples occur at the suture zones between accreted Paleoproterozoic Yavapai arc terranes and the Wyoming and Superior cratons. These interfaces could represent remnant subducting slabs, and together with eclogite in xenoliths, indicate that subduction-related processes likely contributed to cratonic mantle growth. (3) Sporadic positive velocity gradients exist near the base of the lithospheric mantle, perhaps due to laterally variable compositional layering. (4) In contrast to off-craton regions, clear Sp phases are typically not seen at lithosphere-asthenosphere boundary depths beneath Archean and Proterozoic terranes, consistent with a purely thermal contrast between cratonic mantle lithosphere and asthenosphere.
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