Seismic tomography

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Seismic tomography
Tomography attempts to determine anomalous
structures within the Earth as revealed by
deviations from average seismic properties at
depth. Average is usually determined by one of
the simple radial structural models of the
Earth. PREM (Anderson and Dziewonski, 1981) is
the most commonly used reference Earth model.
Thanks and/or apologies to Barbara Romanowicz, UC
Berketey whose slides have been used liberally in
this presentation.
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PREM
Anderson and Dziewonski (1981) determined a
spherical shell model of the Earth that was most
consistent with the observed travel times from
seismic sources to seismic stations that had been
accumulated in the previous 80 years of
seismology. Note that the model is layered
and laterally averaged over the whole Earth
within layers and so no lateral variations in
structure are modelled.
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P-wave velocity
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S-wave velocity
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Preliminary Reference Earth Model
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What are we missing?
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What are we seeking?
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Crust and Mantle Structures
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What we would like to see...
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Body and surface waves
Seismic waves integrate the seismic velocities
experienced point-by-point along their paths
T(?) ?x/v(x) dx along path
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Seismic wave paths
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A tomographic slice
Over a diametrical slice through the Earth, we
look for regions that are anomalously slow or
fast compared to the PREM average for that depth
within the Earth.
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Basic concept
Each of these 3 paths is the same
distance. S1-A no variation S2-B encounters a
fast region S1-C encounters a slow region
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Earthquakes and seismographs
Earthquakes and seismographs are not uniformly or
even with uniform randomness distributed over the
world. We only have biased data sets.
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Sample bias -- 1
Density of paths transiting a region of between
660 and 870km
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Sample bias -- 2
Density of paths transiting a region of between
2670km and CMB
Vasco and Johnson, 1998
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Example results
Van der Hilst, et al., 1998
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The Scripps SB4L18 model
Seismic tomography is very fashionable, now, and
most major seismic laboratories are presenting
mantle velocity anomaly models. This one, the
Scripps SB4L18 model, presents results as depth
layers over the Earth. What is plotted is slow
S-wave and fast S-wave velocities.
Laske, Masters and Reif, AGU 2001
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Berkeley vs. CalTech
Laske, Masters and Reif, AGU 2001