Geoscience Reference
In-Depth Information
Figure 8.1.
Seismic velocity-depth models for the whole Earth. Since the early
determination by Jeffreys (1939), which was based on the Herglotz-Wiechert
inversion of the Jeffreys-Bullen compilation of travel-time and angular-distance
data, there have been many revisions, but the agreement among them is good. The
two regions where the models have been most revised and refined are the
low-velocity zone in the upper mantle (asthenosphere) and the
inner-core-outer-core transition zone. The Preliminary Reference Earth Model
(PREM) of Dziewonski and Anderson (1981) was determined by a joint inversion of
the free oscillation periods of the Earth, its mass and moment of inertia as well as
the travel-time-distance data. (After Bullen and Bolt (1985).)
discontinuity at 220 km, but the region above 220 km is sometimes referred to
as the
lid
. Standard velocity models (e.g., Fig. 8.1)vary in representation of the
uppermost mantle depending upon the data used and the assumptions made. A
low-velocity zone
for S-waves down to about 220 km is well established by the
surface-wave-dispersion data. The low-velocity zone for P-waves is based on a
shadow-zone effect for P-waves out to about 15
◦
(Fig. 8.2) and on a matching of
waveforms of P-wave arrivals with synthetic seismograms computed for possible
velocity structures and shows up in PREM. In contrast,
iasp91
has no low-velocity
zone for P- or S-waves. Beneath the low-velocity zone, P- and S-wave velocities
increase markedly until about 400 km depth. At depths of 400 and 670 km, there
are sharp changes in velocity; both P- and S-wave velocities increase by 5%-7%.
