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Significance of seismic tomography within the wider
geoscience
communityComputational seismology works on improving imaging
methods. Other geoscientists care mainly about the images
themselves.
Hopefully, tomography results make sense in light of surface
studies (geology, tectonics) and mantle convection simulations
(geodynamics).
Solutions are non-unique -- but how bad is it? And how do I
convey that to a non-specialist?
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Example study: the mantle under North America -- window into a
distant pastSubduction on the west coast: an oceanic plate gets
submerged beneath the continent
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A simple tectonic history(?)
Ren et al 2007, after Engebretson 1985
A single large plate has been subducting beneath the west coast
for 180 million years. No significant interference from other
plates.
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Geologists puzzle about episodes of rapid change in the past
5-10 million years later: the sea is gone; 4-km-high mountains
have risen.
75 million years ago: A shallow inland sea covers the Rocky
Mountain area
Western U.S., 75 Myr ago 65 Myr ago
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Geometry of the body-wave tomography experiment
Signal sources are P-waves generated by large but distant
earthquakes
637 earthquake sources
1125 broadband receivers (seismometers)
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Sensitivity kernels (shaded red) map the areas sampled by the
body waves used.
Nissen-Meyer, 2007
How body waves sample the earth
time in sec
synthetic seismogram
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North America
To solve the inverse problem, expand the sensitivity kernels on
a global grid
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Inverse problem (linear)
KT
reg
dT
reg
dln(vp )*=
data physical model (rows = kernels)
3-D earth structure (unknowns)
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Result: a 3-D model of seismic P-wave velocities under North
America
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The subducted slab (blue=fast=cold) in the mantle down to 1800
km depth
B49N
42N
A
CD
PA
F
42NS2S2 F2F2
F1F1S1S1CC
dVp /Vp in %
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Image of the subducted Farallon slab in the mantle
Seismically fast material is contoured (fast means cold).
Color signifies depth. We can confidently image ~1500 km
deep.
Crust and lithosphere not rendered.d
e
p
t
h
/
k
m
Sigloch, McQuarrie, Nolet 2008, Nature Geo
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The current subduction system
d
e
p
t
h
i
n
k
m
S1
S1N1
N2S2
S2
W
N1
SGMe
Birds-eye view from eastern Canada on the currently active
western subduction system.
Notice the large vertical offsets of some adjacent
fragments.
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Resolution tests
Assume some hypothetical earth structure
Compute synthetic data from it
Invert the data. Is the input structure recovered?
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Questions
New model is surprising but plausible (because it explains more
geological observations that earlier models).
How different can other plausible models be that also fit the
data? Possible to generate such models a priori?
Alternatively, at what level of confidence can we say that
certain interesting features are real? (Example: tears in the
plate, which are geodynamically important.)
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Inverse problem
KT 0
KA KQ
regreg
dT
dA
reg
dln(vp )
dln(Q-1)*
=
data physical model (rows = kernels)
3-D earth structure (unknowns)
Significance of seismic tomography within the wider geoscience
communityExample study: the mantle under North America -- window
into a distant pastA simple tectonic history(?)Geologists puzzle
about episodes of rapid change in the pastGeometry of the body-wave
tomography experimentHow body waves sample the earthTo solve the
inverse problem, expand the sensitivity kernels on a global
gridInverse problem (linear)Result: a 3-D model of seismic P-wave
velocities under North AmericaThe subducted slab (blue=fast=cold)
in the mantle down to 1800 km depthImage of the subducted Farallon
slab in the mantle The current subduction system Resolution
testsQuestionsInverse problem