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Seismic tomography
The four maps show the
variations in S-wave
speeds at depths of 70
km, 200 km, 500 km, and
2 800 km in the mantle
derived from seismic
tomography. Regions
with faster S-wave speeds
indicate relatively colder,
denser rock. Regions with
slower S-wave speeds
(red and yellow) indicate
relatively hotter, less
dense rock. (S-wave
speeds: G. Ekström and
A. Dziewonski, Harvard
University; maps: L. Chen
and T. Jordan, University
of Southern California)
S
70 km. Near Earth’s surface, hot rocks in the asthenosphere
slow S waves, as revealed by the warm colors (red and yellow)
along oceanic spreading centers.
200 km. Moving deeper, we see the cold lithosphere of continental cratons (blue and purple regions) and warmer asthenosphere beneath ocean basins (red regions).
500 km. Deeper in the mantle, the features no longer match the
continental positions.
2 800 km. Near the mantle-core boundary, the S-wave patterns
reveal colder regions around the Pacific that may be the «graveyards» of sinking lithospheric slabs.
Fantini, Monesi, Piazzini - Elementi
eismic tomography is an adaptation of a method now commonly used in medicine to map
the human body, called computerized axial tomography (CAT). CAT scanners construct threedimensional images of organs by measuring small
differences in X rays that sweep the body in many
directions. Similarly, geologists use the seismic
waves from earthquakes recorded on thousands of
seismographs all over the world to sweep Earth’s
interior in many different directions and construct a three-dimensional image of what’s inside.
The method enables them to find places where
seismic waves speed up or slow down. They make
the reasonable assumption, consistent with laboratory experiments, that regions where seismic
waves speed up are composed of relatively cool,
dense rock (for example, subducted ocean plates),
whereas regions where seismic waves slow down
in­d icate relatively hot, buoyant matter (for example, rising convection plumes).
Seismic tomography has revealed features in
the mantle clearly associated with mantle convection. Figure 2 pre­sents a tomographic model of Swave speed variations in the mantle constructed
by researchers at Harvard University. The model
is displayed as a series of global maps at depths
ranging from just below the crust down to the
core-mantle boundary. Near the surface, you can
clearly see the structure of plate tectonics. The
low S-wave speeds caused by the upwelling of
hot asthenosphere along the mid-ocean ridges are
shown in warm colors; the high S-wave speeds
from cold lithosphere in the old ocean basins and
beneath the continental cratons are shown in cool
colors. At greater depths, the features become
more variable and less coherent with the surface
plates, reflecting what geologists infer to be a complex pattern of mantle convection. Some largescale features stand out, however. For example,
you will notice that, just above the core-mantle
boundary, there is a red region of relatively low
S-wave speeds beneath the central Pacific Ocean,
surrounded by a broad blue ring of higher S-wave
speeds. Seismologists have speculated that the
high speeds represent a «graveyard» of oceanic
lithosphere subducted beneath the Pacific’s volcanic arc, the Ring of Fire, during the last 100 million years or so.
di Scienze della Terra • Italo Bovolenta editore - 2013
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