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Variable azimuthal anisotropy in Earth’s lowermost mantle
Edward J. Garnero1, Valérie Maupin2, Thorne Lay3, Matthew J. Fouch1
1 Department
of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404, USA
2 Department
of Geosciences, University of Oslo PO Box 1047, Blindern, Oslo 0316, Norway
3
Institute of Geophysics and Planetary Physics and Earth Sciences Department, University of
California, Santa Cruz, CA 95064, USA
A persistent reversed polarity onset for vertically polarized shear waves (SV) that
graze the lowermost mantle (the D" layer) in many regions around the globe initiates
at the arrival time of horizontally polarized shear waves (SH). Such SV-SH coupling
cannot be explained by the current paradigm for D" anisotropy; transverse isotropy
with a vertical axis of symmetry (VTI). Full waveform modeling of the split shear waves
for paths beneath the Caribbean, a region with the most abundant high quality
broadband data, requires azimuthal anisotropy at the base of Earth’s mantle. Data are
well fit by models with laterally coherent patterns of transverse isotropy with the
hexagonal symmetry axis tilted from the vertical by up to 20º. Small-scale patterns in
convection of the overlying mantle may cause the observed variations by inducing
laterally variable crystallographic or shape-preferred orientation in mineralogical
components in the boundary layer.
The deepest mantle likely plays as critical of a role in Earth’s dynamical systems
as its near-surface counterpart, the lithosphere-asthenosphere system (1,2). Both of these
major internal boundary layers possess extensive seismic wave anisotropy (3-5), which
holds promise to reveal mineral and structural fabric alignment (6). High-resolution
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investigations of D" anisotropy are restricted to a few key regions due to geographical
limitations in the global distribution of earthquake sources and seismic sensors; these
include beneath the Caribbean Ocean, Alaska, the central Atlantic, the central and
southern Pacific, central Eurasia, and southern India. Figure 1 shows representative
seismograms from several regions. Ground velocity recordings (Fig 1A,B) allow picking
of an impulsive SV downward arrival having a polarity opposite to that of SKS (as
predicted by VTI or isotropic models) and arriving seconds after the SH onset (7,8).
However, on the displacement trace there is clear SV energy with the same polarity as
SKS, arriving at the same time as the initial SH motion. This is consistent with a
projection of the fast polarization arrival onto SH and SV components, a clear indication
of azimuthal anisotropy (9). The SH arrival involves two impulsive ground velocity
signals, and a correspondingly complex displacement record.
This feature can be
accounted for by triplication of the S wave due to an abrupt 2-3% velocity discontinuity
at the top of D" (8,10); the first arrival involves energy refracting below the discontinuity,
the second is energy reflected from the discontinuity (10-11). Figure 1C shows examples
of similar reversed onsets in SV signals (shaded upwards pulse) for other D" regions, all
of which have been characterized in past studies as having VTI anisotropy (12).
Source complexity or deconvolution problems are ruled out as the cause of the SV
waveform complexity since SKS signals show no corresponding reversed onsets. SV-toP conversions at the receiver are precluded by the rotation to the incident wavefront and
the lack of any precursors for SKS arrivals. Models with complex D" lamellae and VTI
structures (12), and with 3D isotropic heterogeneities (13), show that it is very difficult to
produce reversed SV onsets without introducing azimuthal anisotropy. We explore
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models in which the D" discontinuity coincides with the onset of anisotropy 250 km
above the core-mantle boundary, such that azimuthal anisotropy affects the first arrival,
which penetrates into the D" layer.
We focus on D" structure beneath the Caribbean, the region with the greatest
sampling by high quality broadband data, provided by S waves from intermediate and
deep focus South American earthquakes recorded by the Canadian National
Seismographic Network (CNSN). The epicentral distance range (87-112°) is such that
waves graze or diffract horizontally through the D" layer. Broadband data were
deconvolved by the instrument responses to obtain ground displacement traces, and then
rotated into the reference frame of the incoming S wavefront to obtain SH and SV
motion, minimizing crustal SV-to-P conversions (14). Data with simple, impulsive
signals and favorable signal-to-noise ratios for both SH and SV energy were retained,
reducing our initial dataset from 120 to 16 earthquakes recorded at 19 broadband CNSN
stations, comprising 89 individual high-quality recordings. Data were corrected for upper
mantle anisotropy using shear wave splitting parameters published for CNSN stations
(15,16), and were discarded when corrections did not adequately remove SKS energy
from the SH component.
Past studies (7,8) in this region have reported SV delays relative to SH. This type
of splitting, with purported decoupling of SV and SH components, has led to the
interpretation of extensive VTI in the D" layer (7,8,17,18). VTI can arise from preferred
orientation of hexagonal crystal lattices with vertical symmetry axes or preferred
orientation in the horizontal plane of azimuthally random crystals with a more general
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anisotropy. Alternatively, it can arise from fine-scale lamellae of strongly contrasting
material properties (12,19).
A full wave theory method (9) was used to construct synthetic seismograms for
general forms of D" anisotropy in the first comprehensive modeling of a large data set.
Introduction of azimuthal anisotropy dramatically increases the model space; limited
azimuthal sampling restricts unique resolution to two anisotropic parameters: the
direction of polarization of the fast and slow waves in the plane perpendicular to the
propagation direction, and the amplitude of the anisotropy. We also have clear
observations of delayed SV signals with no apparent SH-SV coupling (similarly with
other regions).
These features led us to adopt a model space involving transverse
isotropy where the symmetry axis ranges from vertical to tilted, essentially setting VTI as
the reference state.
Synthetic seismograms are shown in Figure 2 for an isotropic reference model, a
VTI model, and tilted transverse isotropy (TTI) models where the tilt angle is measured
from the vertical and azimuth varies in all directions. TTI models produce reversed onset
SV signals for suitable orientations. The precursory behavior is dependent on initial S
wave polarization upon entry into the anisotropic D" layer, thus every record was
individually modeled using the correct source mechanism. We exclude waveforms for
which uncertainty in the source radiation pattern gives rise to uncertainty in the predicted
SV waveform. While the overall strength of anisotropy is fairly well resolved, as it
basically controls the arrival time difference between fast and slow polarizations, the
depth distribution of D" anisotropy is less constrained. The difference in overall SV and
SH wave shape is partly a consequence of interference with ScSV and ScSH core-
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reflections arriving a few seconds after S, which cause destructive interference on the SV
components and constructive interference on the SH components.
Individual waveform modeling for 6 example observations is shown in Figure 3
for a range of isotropic, VTI and TTI models. Small tick marks indicate SH arrival times
and ‘correctly polarized’ SV waves, which guides the choice of anisotropy strength for
the VTI models. Due to the mixing of fast and slow polarization energy onto the SV and
SH waveforms, the relative amplitudes are very dependent on the anisotropic properties,
and not explained by isotropic models. Two of the data are relatively well modeled by
the VTI model, but those two, as well as the other four data are well modeled by TTI
models. In the latter four cases there is clear preference for either east or west tilting
symmetry axes. We focus on precise modeling of the early part of the waveforms; the
latter portions depend strongly on poorly constrained details of the discontinuity structure
and the individual receiver crusts.
The map in Figure 4 summarizes our modeling. The first-order result is the
predominance of east dipping TTI in the southeast of the study area, and west dipping
TTI toward the northwest. There is an intermingling of observations consistent with VTI
throughout the study area, with numerous waveforms being compatible with both TTI
and VTI predictions.
Only a few waveforms are best explained by isotropy.
A
reasonable interpretation is that there are spatial fluctuations in the macroscopic fabric of
the boundary layer, deviating from a VTI average as a consequence of complexity in the
flow field.
Prior studies have proposed the existence of azimuthal anisotropy in localized
regions of D", primarily below the Pacific (9,20,21), based on early arrivals of SV or
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ScSV signals. Those observations can be explained by TTI models with horizontal
symmetry axes (90° tilt) sampled by raypaths at azimuths perpendicular to the symmetry
axes. The current lack of azimuthal sampling for any one region of
D" prevents
resolution of more complex anisotropy systems such as those expected for lattice
preferred orientation (LPO) of likely lower mantle minerals. MgO has been shown to
have anisotropic properties and dominant slip systems that should favor SH velocities
being faster than SV velocities in horizontally sheared portions of D", but with a 4
azimuthal variation in relative velocities (22-25). We calculated synthetics for the cubic
symmetry of ferropericlase (Mg,Fe)O using published elastic parameters (22), finding
that specific crystallographic orientations can be found that reproduce the waveform
behavior we observe here. The azimuthal dependence for this system may explain why
our data show variations at a rather small scale, but lacking good azimuthal coverage this
cannot be robustly resolved.
Deep mantle anisotropy may be related to mantle circulation, with mid-mantle
downwelling currents beneath subduction zones possibly inducing LPO in cooled, high
stress regions of D" (such as beneath the Caribbean), while hot upwelling regions, such
as beneath groupings of surface hot spots, could give rise to shape preferred orientation
(SPO) by, for example, aligning melt inclusions (6,12,18,19). Three-dimensional flow
patterns could account for the spatial pattern in tilting of the symmetry axis imaged here,
whether the fundamental cause of the anisotropy is LPO or SPO.
Further modeling of
data for other regions is needed to assess the overall variability in azimuthal anisotropy.
The apparent onset of anisotropy with the velocity jump at the top of D" suggests
a link between the two, which could arise if there is a chemically distinct layer (1), a
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transition to a sheared zone of enhanced velocity heterogeneity (26), a change in iron
partitioning associated with pressure-induced change in spin-state (27), or a major phase
change (28,29). Improved azimuthal constraints will be required to distinguish between
these possibilities, but we now have unequivocal evidence that extensive regions of D"
anisotropy contain more complex structure than simple VTI, with high resolution seismic
analysis being required to detect and map the structure. The lateral variations in the
observed azimuthal anisotropy are undoubtedly coupled to small scale dynamics in the
deepest mantle.
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References
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31. Acknowledgements. We thank Michael Bostock for assistance in accessing some of
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U.S. National Science Foundation. VM was on leave to Institut de Physique du Globe
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Figure captions
Figure 1. Broadband shear wave data from station FCC for the events of (A) 29 April
1994 and (B)10 May 1994. SV and SH ground velocity recordings (Vel.) are shown
along with ground displacements (Disp.). SKS is cleanly isolated on the SV components.
The predicted polarity of the SV component of the S arrival is downward in the figure,
and the short vertical line indicates an arrival that could be picked as a late SV on the
velocity records (delay times noted on records), but in the displacement record there is
clearly SV energy arriving at the same time as the strong SH onset (shaded, peak noted
by inverted triangle). The SH waveforms show two arrivals, which correspond to
interactions with a shear velocity increase at the top of D". (C). Examples from other D”
regions. D" sampling of data are indicated by thick black lines in map, with shading
indicating past D" anisotropy study regions.
Figure 2. (top) Tilt (T) and azimuth (AZ) parameters of the symmetry axis (dashed line),
for models of tilted transverse isotropy (depicted by sheets). (below) Full wave theory
synthetics (SV-solid line; SH-dashed line) for varying T (left column) and AZ (right
column) values computed for the parameters of station FRB at a distance of 91.8° for the
event 29 April 1994 (see corresponding data in Figure 3).
ISO indicates isotropic
reference model, with a 2.7% D" discontinuity 250 km above the CMB. AZ=90° for the
varying T calculations, and T=20° for the varying AZ calculations. Several examples of
SV signals having a reversed polarity onset (shaded) are present. The strength of the
anisotropy for all TTI models is 2.2%.
Figure 3. Observed SH (blue) and SV (red) displacement traces for six event-station
pairs (columns a through f). The data are shown with original polarities, normalized to
the peak SH amplitudes, with the top row having small markers at the onset of SH and
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correctly polarized SV arrivals. The data are compared with synthetics (dotted) for four
models: top row, W(20) synthetics are for tilted transverse isotropy with T=20°, Az=270°;
second row, E(20) synthetics are for tilted transverse isotropy with T=20°, Az=90°; third
row, VTI synthetics are for transverse isotropy with vertical symmetry axis; bottom row,
ISO synthetics are for isotropic D” structure. Asterisks (*) indicate models that fit the
data very well.
Figure 4. (Upper left) Ray paths from earthquakes to stations showing the portion of
each raypath within a 250 km thick D" region (thick lines). Contours indicate the pattern
of volumetric shear velocity heterogeneity in the D" portion of a tomographic model (30),
with higher velocities in blue and lower velocities in red (contour interval is 1%). (Main
panel) Map of raypaths and best fitting anisotropic models. Ray path segments are for
the deepest 50 km of the raypath, with the color-coded portions being for the deepest 10
km of each path. ISO indicates paths for which no anisotropy is needed to match the
data, EAST indicates paths for which easterly azimuths of the tilted transverse isotropy
symmetry axis match the data (AZ=90°, T=20°), WEST is for paths for which westerly
azimuths of the tilted transverse isotropy symmetry axis match the data (AZ=270°,
T=20°), and VTI is for paths for which vertical transverse isotropy fits the data.