Download Seismic constraints on Ear th`s small-sc

Rost: Bullerwell lecture
Seismic constraints on Ear The small-scale structure of the
Earth’s interior is more difficult to
access than the big picture. In the
Bullerwell Lecture 2009, Sebastian
Rost discusses seismic studies
targeting small-scale structure
at the core–mantle boundary,
including areas containing partially
molten mantle material and the
remnants of subducted slabs,
and assesses how they influence
aspects of the Earth’s dynamics.
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M
ost of the Earth’s interior is com­
pletely inaccessible. Direct measure­
ments using boreholes have reached
depths of no more than 12 km, but compared to
the radius of the Earth – about 6371 km – these
samples are just scratching the surface. To study
the bulk of the Earth we must use remote sensing
techniques. Seismology is one of the best remote
sensing tools used to study the Earth’s interior,
and many major discoveries of structures in the
deep Earth (such as the existence of the inner
and outer core and the crust–mantle boundary)
have been made using seismological techniques.
Despite considerable success in imaging the
Earth’s interior, many unanswered questions
about the structure and dynamics of the interior
of our planet remain. Seismology has to play a
major role in answering many of these.
The Earth’s core–mantle boundary (CMB)
lies almost 3000 km beneath the surface of the
planet. It is the boundary between two major
convective systems with very different physical
properties. On the one side, the core consists of a
liquid iron alloy, convecting with a typical veloc­
ity in the range of kilometres per year and having
a viscosity, although poorly constrained, in the
range of 10 –2 Pas (Mound and Buffett 2007) at
the CMB – comparable to the viscosity of water
(10 –3 Pas) at room temperature. On the other
hand, the mantle contains silicate rocks with
a viscosity on the order of 1021 Pas (Moucha et
al. 2007) and convects with velocities of a few
centimetres per year. The CMB affects many
processes active in the Earth: coupling between
mantle and core influences the generation of the
magnetic field; it represents the lower thermal
boundary layer of mantle convection and is a
major driver of plate tectonics; thermal instab­
ilities from this boundary might form mantle
plumes generating intraplate volcanism such
as seen in Hawaii and Iceland. The chemical
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1: Topographical maps of Europe. Political boundaries are marked. Topography from the Etopo5
grid from the US National Geophysical Data Center (NGDC). (In: Data Announcement 88-MGG02, Digital relief of the Surface of the Earth. NOAA, National Geophysical Data Center, Boulder,
Colorado, 1988.) (a) Original dataset. The resolution of the 5-minute gridded dataset is about 9 km.
(b) Same dataset but filtered with a 100 km lowpass. Topographical features seem blurred, but
in general the nature of the topography is preserved. (c): Same dataset but filtered with a 500 km
lowpass. Main topographical features are changed, such as Britain being an independent island.
(d) Same dataset but filtered with a 1000 km lowpass. Major features of Europe such as Britain and
Ireland are lost in this filtered topography version.
differences across the boundary mean that little
mass transfer between core and mantle is likely
and the CMB might be the final resting place
for slabs making their long way down from the
surface. Therefore the CMB region is crucial for
our understanding of the dynamics and evolu­
tion of our planet.
As can be expected from such a dominant
feature, the CMB is not a simple boundary
between mantle and core. Indeed, seismology
resolves many structures within the CMB region
(for recent reviews see Garnero 2000 and Lay
2007). Seismic tomography resolves large-scale
variations in seismic velocities in the deep Earth
such as the broad regions with reduced S-wave
velocities at the CMB beneath Africa and the
central Pacific, dubbed large low shear velocity
provinces (LLSVPs). The so-called D″ (“dee-dou­
ble-prime”) region, ranging from the CMB itself
to a few hundred kilometres above the CMB,
shows changes in seismic velocity gradients that
have been related to the lower thermal boundary
of the mantle. The upper boundary of the D″
region is often marked by a sharp increase in
velocity, the D″ discontinuity, for S-waves, and
a weaker discontinuity for P-waves. There is also
evidence for strong seismic velocity anisotropy
within D″. These observations can be explained
by the phase transition from perovskite to postperovskite that has been detected in the appropri­
ate temperature and pressure range (Murakami
et al. 2004, Oganov and Ono 2004) although
other explanations are discussed. Further seis­
mic observations indicate increased small-scale
structures within the D″ region, which have been
related to the existence of chemical heteroge­
neities. Also, there is evidence for very thin lay­
ers (between 5 and 25 km thick) with strongly
reduced seismic velocities, called ultra-low veloc­
ity zones (ULVZs), atop the CMB (Garnero and
Helmberger 1995) and for layering at the top
of an otherwise well mixed core (Buffett et al.
2000, Rost and Revenaugh 2001). For many of
these observations a generally accepted model
A&G • April 2010 • Vol. 51
Rost: Bullerwell lecture
th’s small-scale structure
for their generation and influence on larger scale
mantle processes is still lacking.
The structures discussed above are detected
over many lengthscales – from the D″ discontinu­
ity being detected over thousands of kilo­metres,
to ULVZs being detected in areas as small as
100 km, and scattering indicating structure of
scales smaller than 10 km. This is similar to geo­
logical observations at the surface of our planet
showing heterogeneities on many scale lengths
from the difference between oceanic and conti­
nental crust to the crystal scale. The importance
of knowing the full spectrum of Earth heteroge­
neity is highlighted in figure 1: using only infor­
mation on the large-scale structure might lead to
overlooking some important information, while
only looking at small-scale structure might blur
our vision of the large-scale picture.
The origin, either thermal or chemical, of
many structures in the deep Earth remains
unclear. Large-scale structures such as LLSVP
have been explained as purely thermal features
or regions chemically distinct from the rest
of the mantle, but are likely to be a mixture
of these two end members, i.e. they will be of
thermo-chemical origin. Small-scale heteroge­
neities are more likely to have a chemical origin
due to their small size; a thermal structure on
this scale would equilibrate with its surround­
ings relatively quickly. We expect only chemi­
cal heterogeneities to survive in the convecting
mantle for long periods of time. Chemical het­
erogeneities require mechanical mixing to be
assimilated because chemical diffusion rates in
the lower mantle are very small. The extent and
the details of this mixing process are unknown.
Using estimates from geodynamical models of
the volume of chemical heterogeneity introduced
into the mantle and an estimate of the volume of
chemical heterogeneities in the mantle we might
be able to understand this process.
The major process that can introduce chemical
A&G • April 2010 • Vol. 51 (a)
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VS
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mantle
5 to 40km
remnants of
basal magma
ocean
ULVZ
depth (km)
2: (a) ULVZs are characterized by strong
velocity reductions of the order of 10 and 30%
for P- and S-waves in thin layers (<30 km) atop
the CMB. Several studies indicate a density
increase of the order of 10% in these layers.
(b) Possible models for ULVZs. The most
common explanation for ULVZs is the existence
of partial melt. Alternative models invoke
iron enrichment of the perovskite or postperovskite mantle material either through
material from the mantle, surface (banded iron
formations, BIFs) or core–mantle reactions.
It has also been speculated that ULVZs might
be the last remnant of the basal magma ocean
that existed during Earth’s accretion.
BIF
partial
melt
CMB
2900
Fe-enriched
pPv
0
4
8
12
velocity (km/s)
density (g/cm3)
16
heterogeneity into the mantle is plate tecton­
ics. Oceanic crust is constantly formed at
mid-oceanic ridges by partially melting the
uppermost mantle beneath the ridge. There­
fore, the crust is rich in the incompatible ele­
ments that partition into the melt, while the
upper part of the mantle from which the crust
was extracted is depleted. The oceanic crust
and lithosphere is then recycled into the mantle
through subduction. Therefore, the subduction
process constantly introduces chemical hetero­
geneities into the mantle. This process is imaged
both in geophysics and geochemistry.
Geochemistry and geophysics have very differ­
ent approaches to the study of the Earth. While
geochemistry tries to balance the interaction of
different geochemical reservoirs, geophysicists
try to understand the processes that lead to the
interaction from physical arguments (Helffrich
2006). Information from both disciplines is
necessary to understand processes in the deep
Earth, but comparing results is difficult due to
the different lengthscales of sampling. Compar­
ing tomography results sampling the Earth on
scales of a few hundred to thousand kilometres
with geochemical results sampling the Earth
at much shorter scales is challenging. Seismic
information about the small-scale structure of
the Earth will help to reconcile these datasets.
I will discuss below two examples of imaging
small-scale structure close to the CMB. I will
show evidence for the existence of partially mol­
ten material in the deep Earth and of piles of the
remnants of subducted oceanic lithosphere.
Ultra-low velocity zones
ULVZs are among the most enigmatic structures
found at the CMB. ULVZs are characterized by
strong decreases in P-wave and S-wave velocities
in the range of 10% to 30% relative to global 1D
Earth models in a thin veneer ranging in thick­
ness from a few kilometres to about 20 km (figure
core
CMB
core–
mantle
reactants
2a). There is also growing evidence for a density
increase on the order of 10% in these thin lay­
ers (Thorne and Garnero 2007). One puzzling
aspect of ULVZs is their distribution at the CMB.
Seismic studies detect areas of the CMB that are
apparently devoid of ULVZ structure in close
proximity to clear ULVZ detections (Thorne et
al. 2004, Rost et al. 2005). There seems to be
no ubiquitous ULVZ layer at the CMB within
the resolution of seismology (Persh and Vidale
2004), which would be able to detect ULVZs
as thin as 3 to 5 km with some studies report­
ing higher vertical resolution (Rost et al. 2009).
ULVZs seem to be strongest in regions that are
correlated with intraplate volcanism in the form
of hot spots (Williams et al. 1998, Helmberger et
al. 1998, Rost et al. 2005) and seem to prefer the
edges of large regions of low seismic velocities as
detected in tomographic studies (Garnero and
McNamara 2008) and strong lateral gradients in
the lowermost mantle (Thorne et al. 2004).
Several models to explain ULVZs have been
proposed (figure 2b). The most commonly used
explanation is the existence of partial melt in
thin layers (Williams and Garnero 1996, Rost et
al. 2005). The existence of partial melts within
ULVZs is supported by the observation that
S-wave velocity reduction is three times greater
than P-wave velocity reduction in ULVZs. This
can be explained by rocks containing between
5 and 30% of melt depending upon whether
the melt forms well-connected or poorly con­
nected melt pockets and the configuration of the
melt (Williams and Garnero 1996, Berryman
2000). The melt could either percolate down
from the overlying mantle (Rost et al. 2005),
be generated at the CMB, or be the remnant of
a global magma ocean (Labrosse et al. 2007).
Other models for ULVZs include accumulated
silicate sediments from the core (Buffett et al.
2000), iron-rich remnants of subduction (Dob­
son and Brodholt 2005), core–mantle reaction
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Rost: Bullerwell lecture
50
KS
SKS
P
Ps
c
ULVZ
P
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relative time (sec)
–1
0
PcpupP
PcP
PuP
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110°
PscP
ScP
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ScsP
SdP
SPcP
outer core
SPKS
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outer core
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products (Knittle and Jeanloz 1991) and local­
ized zones of iron-enriched post-perovskite
(Mao et al. 2006).
If the ULVZs are partially molten mantle
material, they should exist wherever the solidus
of the mantle material is reached since the CMB
is isothermal. Assuming an iso-chemical lower
mantle, this would predict a ubiquitous ULVZ
layer, although its thickness might vary laterally
and the ULVZs might be below the resolution
level for our current seismic probes.
Several seismic probes for ULVZs are used
regularly. They can be roughly divided into
probes using diffracted paths (e.g. SPdiff KS,
PKKPdiff [Garnero and Helmberger 1995, Rost
and Garnero 2004]) or core-reflected phases
(e.g. ScS, PcP, ScP: Reasoner and Revenaugh
2000, Rost et al. 2005, Mori and Helmberger
1995, Avants et al. 2006). Other probes using
scattering and atypical diffractions have also
been used (Xu and Koper 2009, Thomas et al.
2009). Ray paths for these probes are shown in
figure 3. The largest CMB areas can be sampled
for ULVZs by SPdiff KS, while the core-reflected
phases such as PcP and ScP give high lateral
resolution but are restricted to small lateral
areas. Overall, less than half of the CMB area
has been sampled for ULVZ structure. ULVZs
can be detected in the waveforms of these
phases with the interaction of SPdiff KS with a
ULVZ leading to a distinct post-cursor (a wave
arriving after the reference arrival) to SKS while
the inter­action of core-reflected phases leads to
dominant precursors (seismic energy arriving
before the reference arrival) to ScS, PcP and ScP
phases. For the core reflections, post-cursors
also exist but are complicated to analyse due
to the interference with coda waves generated
mainly in the crust beneath the station. Here I
will focus on some studies using seismic array
data to study waveforms of ScP.
Figure 4 shows ScP waveforms from the
Tonga–Fiji subduction zone as recorded at the
Australian Warramunga (WRA) array (Rost and
Revenaugh 2003, Rost et al. 2005). WRA is a
small-aperture array (approximately 20 km) with
20 vertical component, short-period stations
deployed along two roughly perpendicular legs.
Each trace represents the summation of the time
series of the 20 seismic stations located at WRA
recording the elastic waves from one earthquake
to increase the amplitude of the coherent signal
relative to that of the incoherent noise (SNR).
The time differences of the arrivals due to the
incidence angle of the seismic waves and the
distribution of the stations have been taken
into account in a process called beam-forming
(Rost and Thomas 2002). The array processing
allows very high signal quality for these shortperiod signals. All arrivals are aligned on the ScP
arrival time and sorted by source latitude which
represents a roughly north–south profile along
the CMB in a region east of Australia. The ScP
10
20
30
relative time (sec)
3: Raypaths for different phases used to sample the CMB for ULVZs. P-waves are marked as solid
lines and S-waves are marked as dashed lines. (a) Core-reflected phase ScP, an S-wave that
converts to a P-wave upon CMB reflection. Reflections and conversions at the top of the ULVZ will
lead to additional pre- and post-cursors of the ScP waveform including SdP (ULVZ top reflection),
ScsP (conversion on receiver side) and SPcP (conversion of source side). (b) Core reflection PcP. A
dominant precursor (PuP) is generated through a reflection at the ULVZ top. Other post-cursory
phases include PscP and multiples within the ULVZ. (c) SPdiffKS which is generated when SKS
reaches the critical ray parameter for P waves at the CMB and forms a distinct post-cursor to SKS.
Other rays influencing the SKS waveform include SPKS and multiples in the ULVZ. (Courtesy of Ed
Garnero, garnero.asu.edu)
waveforms show a distinct precursor from the
reflection from the top side of the ULVZ (SdP)
in a small CMB region. The precursor is very
coherent within this distance range (figure 4b),
indicating a laterally constant structure over
approximately 100 km of the CMB. ScP wave­
forms sampling outside of this region do not
show the distinct precursor (figure 4b).
Modelling the CMB
Using waveform forward modelling it is possible
to find a model of the CMB region that pro­
duces seismic waveforms in agreement with the
data (figure 4c). The best fitting model for the
ULVZ region sampled here shows a thickness
of 8.5 ± 1 km, with P-wave and S-wave velocity
reductions of 8 ± 2.5% and 25 ± 4% (Rost et al.
2005), respectively, relative to a 1D reference
Earth model. Higher frequency data allow a
resolution of possible internal structure in the
ULVZs (Rost et al. 2006). The thickness of
the ULVZ in this area seems constant. More
complicated waveforms sampling the edges of
the ULVZ indicate multi-pathing at the edges
of the ULVZ. Unfortunately, the seismic data
cannot resolve whether the ULVZ is indeed dis­
appearing at this edge or is thinning below the
resolution of the seismic probe. Nonetheless, the
observation of multi-pathing indicates that the
ULVZ is restricted to this small area.
The region studied by the data shown in fig­
ure 4 lies at the edge of a large area of reduced
seismic shear velocities, the LLSVP resolved in
several tomographic studies. This is in good
agreement with previous work observing a
correlation between ULVZ material and the
edges of the large low-shear velocity provinces
(Thorne and Garnero 2004). This correlation
can be understood from geodynamical model­
ling. Models including chemical and thermal
convection (McNamara and Zhong 2005) are
able to explain the distribution of the LLSVP as
anomalously dense lower mantle material piled
up by the convective flow driven by subduction
of oceanic crust (McNamara and Zhong 2005,
Garnero and McNamara 2008). Geodynamic
simulations show that these piles are internally
convecting and heat up due to a lack of convec­
tion across their boundaries and enrichment
in radioactive elements. The hottest regions of
these dense thermo-chemical piles (DTCP) can
be found near the edges and underneath upwell­
ings. These observations are in good agreement
with a partial melting model of ULVZs, both in
their location close to the edges of the LLSVP
and the necessary energy to invoke partial melt­
ing of the mantle material.
Some ULVZs show a P-wave to S-wave reduc­
tion ratio of 1:3, possibly indicative of partial
melt, but some ULVZs do not show this feature
(Hutko et al. 2009). This might indicate that
there are several mechanisms active at the CMB
that result in ULVZs. Unfortunately, as discussed
before, our coverage of the CMB is incomplete so
a full interpretation of ULVZ properties and their
connection to other mantle processes is currently
A&G • April 2010 • Vol. 51
Rost: Bullerwell lecture
realistic mantle materials at high pressures and
temperatures from experimental or computa­
tional mineral physics;
● understanding of the behaviour of melt at high
pressures;
● h igh-resolution geodynamical modelling to
understand the dynamics of ULVZs.
There is strong progress in all of these fields.
ScP
(a)
–24.00
source latitude (deg)
–24.25
–24.50
CMB scattering
–24.75
–25.00
–25.25
–25.50
–25.75
5s
(b)
precursor
ScP
(c)
synthetic
no precursors
5s
observed
5s
4: (a) Source longitude sorted ScP beam traces of Tonga/Fiji earthquakes recorded at the
Australian Warramunga array. This sorting is equivalent to a north–south profile along the CMB
south of New Caledonia. Blue traces show a coherent and distinct precursor to ScP indicative of
ULVZ structure at the ScP reflection point. This precursor is absent in purple traces. Green traces
show complicated waveforms indicative of multi-pathing at sharp ULVZ edges.
(b) Overlay of ScP precursor traces (top) illustrates the coherency of the precursor. Red trace
indicates the summation of all ScP precursor traces (also shown in the middle). Bottom trace
shows the summation trace of all ScP waveforms not showing a precursor.
(c) Synthetic ScP waveform for the best-fitting ULVZ model (top black line). The thick grey line
shows the ScP summation trace for the precursor traces which is the trace to be fitted – also
shown at the bottom. The ScP waveform can be fitted very accurately by a simple 8.5 km thick
ULVZ with 8% and 25% P-wave and S-wave reduction, respectively, and 10% density increase.
not possible. New dense station deployments and
new probes for ULVZ structure might give better
CMB coverage in the future.
Another problem arises from the need to
explain the apparent density increase in ULVZs
and how to keep dense partially molten ULVZs
stable. The required melt fraction to explain the
seismic velocities ranges from 5% to 30% (Wil­
liams and Garnero 1996, Berryman 2000) and
might be very close to the percolation threshold
(Rost et al. 2005). Dense interconnected melt
could therefore drain out quickly and spread
out at the CMB, forming a thin extended layer
below the seismic resolution (Hernlund and
Tackley 2007). Several mechanisms have been
invoked to prevent this, including interstitial
crystals blocking the flow of material towards
the CMB (Rost et al. 2005). A further challenge
involves the observed densities in a pure partial
A&G • April 2010 • Vol. 51 melt. Although silicic melt at lower mantle pres­
sures might be denser than the solid (Karato
and Karki 2001, Stixrude et al. 2009), it is not
clear whether this can explain the 10% density
increase observed. It seems likely that a combi­
nation of melt and enrichment (e.g. of iron) is
responsible for the density increase. Whether
the necessary iron for enrichment drains out of
the mantle (Rost et al. 2005) or is the product
of reaction products with the core (Knittle and
Jeanloz 1991) remains unclear.
ULVZs are enigmatic features detected at the
CMB which have so far eluded any comprehen­
sive explanation. Progress in several fields will
be necessary to solve this puzzle:
● b etter seismic resolution of ULVZ regions
and ULVZ properties and an understanding of
where ULVZs are not present;
● characterization of the material properties of
The high-frequency teleseismic wavefield at fre­
quencies around 1 Hz is dominated by extended
codas accompanying each arrival. These codas
are mainly generated through elastic scatter­
ing at small-scale heterogeneities with scales
on the order of a wavelength (~10 km in the
lower mantle). Therefore, the short-period seis­
mic wavefield contains information about the
small-scale structure of the Earth which is not
routinely used in seismology. Most of the coda
is produced when the seismic energy travels
through the crust to the receiver or in an area
close to the source. Nonetheless, several studies
indicate that scattered waves are also generated
in the deep Earth (Cleary and Haddon 1972,
Haddon and Buchbinder 1987). This scattered
wavefield has been used to resolve small-scale
structure in the mantle and core (Vidale and
Hedlin 1998, Hedlin and Shearer 2000, Vidale
and Earle 2000, Helffrich 2006).
The scattered seismic wavefield is sensitive to
velocity and density heterogeneities on the scale
of the wavelength of the sampling seismic wave.
This means that short-period teleseismic data
with a dominant frequency of about 1 Hz are
able to resolve heterogeneities on the order of a
few kilometres. This small scale makes a chemi­
cal origin of these heterogeneities likely. Thermal
disturbances would quickly equilibrate to ambi­
ent temperatures, while chemical heterogeneities
can survive much longer because of incomplete
mechanical mixing during convection (Becker
et al. 1999, Albarede 2005). Therefore the scat­
tered wavefield allows us to collect valuable
information about small-scale chemical differ­
ences that are most likely introduced into the
mantle through the recycling of subducted mate­
rial (Christensen and Hofmann 1994) or through
chemical interaction between mantle and core
(Knittle and Jeanloz 1991, Buffett and Seagle
2010). Therefore we can use seismology to gather
information about the chemical state of the lower
mantle on short scales (Helffrich 2006).
Several seismic probes have been used to study
scattering in the lower mantle. The most com­
mon seismic phase used to study scattering
from the lowermost mantle is PKP (Cleary and
Haddon 1972). The ray configuration of PKP
allows the separation of scattering from the
deep Earth from scattering in the crust beneath
the receiver. The scattered energy from the deep
Earth will arrive with a shorter traveltime (i.e.
as a precursor) to the reference (unscattered)
2.29
Rost: Bullerwell lecture
(a)
(b) 2100
B
2000
1900
P
time (sec)
B`
PKKP
1800
A
C
1700
(a)
P
PK•KP
0
KP
P•P
1600
50
PK•KP
P•KKP
100
distance (deg)
150
Pa•Pa
60
distance (deg)
50
40
30
20
0
1000
2000
3000
time (s)
0
30
330
0
30
60
300
330
300
(b)
60
1.0
0.8
90
270
90
270
0.6
0.4
power
6: (a) Beam traces of PKKP scattering for
the Yellowknife array (YKA) in Canada (Rost
and Earle 2010). Each trace represents a
summation of the output of the seismic sensors
at YKA. Traces are distance sorted. The large
first arrival is the first arriving P-wave. PK•KP
can be identified as an energy increase arriving
at the station more than 1720 s after the
earthquake. Scattering related to the phase
PKPPKP (P′P′) is also marked.
(b) PK•KP after array processing. Energy
of PK•KP is plotted in a slowness (vertical
incidence angle) and backazimuth (horizontal
incidence angle) coordinate system. Each
lobe represents scattering from one distinct
CMB location found by backtracing the energy
to the CMB using slowness and backazimuth
information. The energy plots show the distinct
arrival of energy off great circle path (marked
as dashed line) with slownesses between
2.2 s/deg (innermost white circle) and 4.4 s/deg
(middle white circle) indicative of the origin of
PK•KP at the CMB. Black circles indicate
2 s/deg slowness intervals. Note that for one
example the lobes do not show equal strength,
which indicates lateral variation in scattering
strength of the mantle material.
SK•KP
KK
P•
5: (a) Ray path for PKKP scattering from the
CMB (PK•KP). The special ray configuration
leads to scattering off the great circle path
(the shortest distance between source and
receiver).
(b) Observation time window for PK•KP.
Between 20 and 60° epicentral distance the
expected arrival time window of PK•KP is free
of any interfering body waves (green travel
time curves). The main PKKP arrival is marked
in blue. For reference other PKKP scattering
paths are also marked (red).
210
phase (in this case PKP). In contrast, the scat­
tered energy produced at small-scale hetero­
geneities beneath the station will arrive with a
longer traveltime (i.e. as a post-cursor). The PKP
studies are able to study small-scale heterogenei­
ties from the CMB to mid-mantle depths. Mod­
elling of the pre­cursory scattered energy then
allows the extraction of statistical properties
such as average velocity and density variation
and correlation length of the scattering medium.
These studies indicate lateral variations in scat­
tering strength in the lower mantle (Hedlin and
Shearer 2000, 2002).
A rarely used probe to lower mantle scatter­
2.30
180
6 Dec 1999 23:12
120
240
240
120
0.2
150
ing is PKKP (figure 5 and figure 6a). Several
scattering geometries for PKKP exist but only
scattering from the CMB will be discussed here.
As for PKP, the special ray geometry of PKKP
allows the analysis of the scattered energy in the
precursory PKKP wavefield (figure 5 and figure
6). PKKP is especially suited to study the CMB
region because the scattered energy arrives in a
time window which is free from any interfering
body waves (figure 5b). The best observation
window for PKKP scattering ranges from 0 to
approximately 60° epicentral distance from the
earthquake. In this distance window the PKKP
scattered energy is visible in array recordings
210
180
28 Jan 1999 08:10
0.0
150
as increased seismic energy in an otherwise
relatively quiet time window starting at about
1720 s after the earthquake (figure 6a). The lack
of interfering major arrivals will allow detecting
the low-energy scattered arrivals in the seismic
wavefield and separating them from ambient
noise. Nonetheless, to separate the coherent
scattered wavefield from the incoherent seismic
noise, array analysis is necessary (figure 6b).
Seismic arrays
Seismic arrays were developed in the 1950s and
1960s mainly to monitor underground nuclear
explosions. Their concept is similar to arrays
A&G • April 2010 • Vol. 51
Rost: Bullerwell lecture
−2.5
−1.0
0.0
$VS (%)
1.0
2.5
7: Map of located scatterers from data recorded at arrays in India (orange circles) and Canada (yellow
circles) (after Rost and Earle 2010). Array locations are marked by triangles. Each circle represents
one distinct scatterer location determined using the slowness and backazimuth information of
PK•KP. Background model shows S-wave velocity variations from a tomographic model (Ritsema
and van Heijst 2002). Scattered energy arrives dominantly from the edge of the LLSVP beneath
southern Africa and the subduction zone area beneath central and southern America.
in radio-astronomy. A seismic array is able to
extract directivity information from the seismic
wavefield as well as increasing the ratio of the
amplitude of a coherent signal relative to the
amplitude of incoherent noise. The analysis of
PKKP will exploit both of these characteristics.
Several methods have been developed to that end
(for reviews see Rost and Thomas 2002, 2009
and Schweitzer et al. 2002) which follow the
same mathematical principles as those applied
to arrays in other disciplines (e.g. sonar, radioastronomy, radar, optics, infrasound). The
PKKP scattered energy (PK•KP) is detected by
time-series stacking over many slownesses (i.e.
vertical incidence angle of a plane wavefront)
and back-azimuths (horizontal angle against
north of the wavefront as measured at the sta­
tion). PK•KP shows a characteristic peak of
energy travelling off great circle path (i.e. not
along the shortest path between source and
receiver) and arriving with a slowness indicative
of energy produced at the CMB (figure 6b).
Figure 7 shows scattering areas identified
using two small-scale arrays located in Canada
(Yellowknife Array – YKA) and India (Gau­
ribidanur Array – GBA) (Rost and Earle 2010).
Both arrays are small-aperture arrays with sta­
tions deployed along two approximately 20 km
long legs. The inter-station spacing is about
2.5 km and the legs are roughly perpendicu­
lar, in a cross shape for YKA, and L-shaped
for GBA. The array elements are equipped with
short-period vertical seismometers that are
well suited to detect PK•KP, which is strong­
A&G • April 2010 • Vol. 51 est between 0.9 Hz and 2.1 Hz. For each array
approximately 650 earthquakes were analysed.
Only a small percentage (about 10% for YKA
and 5% for GBA) of the analysed earthquakes
show evidence for PK•KP. The array analysis
determines both the vertical (slowness) and
horizontal (backazimuth) incidence angle for
PK•KP. Using this information a back-tracing
of the scattered energy to its origin using a 1D
velocity model of the Earth using ray tracing
is possible. The detected scatterers cluster in
specific regions mainly beneath southern Africa
and Southern and Central America. Detailed
analysis (Rost and Earle 2010) shows that these
patches indeed are regions of increased smallscale structure and not regions of the high­
est resolution of PK•KP or dependent on the
source–receiver combinations.
A comparison with previous geophysical
results of the structure of the deep Earth can
reveal potential origins for the small-scale
heterogeneities detected by PK•KP. The com­
parison with tomographic images of the velocity
structure at the CMB shows that the scattering
cluster beneath southern Africa is located in a
region of reduced seismic velocities, while the
Southern and Central American clusters are in
good agreement with high seismic velocities.
Due to the correlation of high seismic velocities
at the CMB with the surface location of sub­
duction zones, these are generally interpreted
as material from subducted slabs residing at
the CMB. Tomography images sheets of high
seismic velocities from the subduction zone
to the CMB in these areas (van der Hilst et al.
1997), which have been modelled to reach the
bottom of the mantle (Lithgow-Bertelloni and
Richards 1998). The interpretation of low seis­
mic velocities as dense thermo-chemical piles
has been discussed earlier. This indicates several
mechanisms active in the lower mantle produc­
ing small-scale heterogeneities (figure 8).
Scattering beneath southern Africa might be
related to the LLSVP in this region. Indeed, pre­
vious studies of PKP scattering in this region
show increased scattering from a region that has
been interpreted as the source for the Comoros
hot spot (Wen 2000) and similar observations
have been made elsewhere (Thomas et al. 1999).
The scattering region is located at the edge of
the tomographic LLSVP which, as discussed,
shows the highest temperature of the dense
thermo-chemical piles. The scattering therefore
might be related to the existence of partially
molten material in this region and ultimately to
ULVZ structure.
Subduction is the dominant process introduc­
ing chemical heterogeneities into the mantle by
recycling of crustal and upper mantle material.
The existence of small-scale heterogeneities
related to long-lasting subduction might indi­
cate the final resting place of the slab material
that is only incompletely mixed back into the
mantle material (Becker et al. 1999, Albarede
2005), which is in agreement with previous work
(Krüger et al. 1995, Miller and Niu 2008).
The application of a new probe to sample
small-scale structure in the deep Earth shows
that the scattered wavefield contains valuable
information on the interior of the Earth. This
information can often be used to understand
larger scale dynamical structure such as ULVZs,
LLSVP and the subduction process. Using the
scattered wavefield to extract material prop­
erties for the CMB region is part of ongoing
efforts to better understand the dynamics and
evolution of the Earth’s interior.
Conclusions
In this article I have summarized some recent
attempts to use seismic array data to resolve
the small-scale structure of the CMB region
about 3000 km beneath the surface. The highresolution images indicate that the interior of
the Earth holds a multitude of structures. We
can use information from the high-resolution
seismic studies to understand large-scale
dynamical processes acting in the Earth from
the existence of dense thermo-chemical piles to
the way subducted crustal material is recycled
or stored at the CMB.
Ultra-low velocity zones at the CMB are
intermittent features resolved by seismologi­
cal data that show a preference for the edges
of large-low shear velocity provinces. Using
information from seismology, mineral physics
and geodynamics, we will be able to understand
2.31
Rost: Bullerwell lecture
Sebastian Rost, the 2009 Bullerwell Lecturer, is
Lecturer in the School of Earth and Environment
at the University of Leeds.
Acknowledgments. The work presented here
was possible only through collaboration with
colleagues from the USA and UK. I would
like to thank Ed Garnero, Justin Revenaugh,
Quentin Williams, Paul Earle, Allen McNamara,
Michael Thorne and Tine Thomas for ideas
and contributions. Part of this work has been
supported by a NERC grant (NE/F000898/1).
Comments by Hannah Bentham and Jon Mound
improved the manuscript. I thank the British
Geophysical Association for selecting me as
Bullerwell Lecturer 2009.
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