Geoscience Reference
In-Depth Information
a temperature contrast between the mantle and core of 1000
500 K (Fig. 7.16).
In this sense the D
layer is similar to the lithosphere, the boundary layer at the
top of the mantle. The D
layer is a region of considerable lateral heterogeneity:
lateral variations of up to 4% are suggested by data on diffracted P- and S-waves.
Some variations can be explained by temperature anomalies of 200-300 K, others
require temperature anomalies coupled with local variation in the silicate/oxide
ratio.
±
Core
At the core-mantle boundary (CMB, also known as the
Gutenberg discontinuity
after its discoverer) the P-wave velocity drops sharply from about 13.7 to about
8.1 km s
−
1
, and the S-wave velocity drops from about 7.3 km s
−
1
to zero. This
structure is determined by the strong reflections PcP, ScS and so on. The P-wave
velocity increases slowly through the outer core until the boundary of the inner
core. This is determined mainly by the rays PKP and SKS. However, since PKP
rays do not sample the outermost core (Fig. 8.3), the velocities there are based
on SKS rays. The inner core was discovered in 1936 by Inge Lehmann, a Danish
seismologist (who died in 1993 aged 104), using seismograms from an earthquake
near Buller on the Southern Alpine Fault in New Zealand. She realized that, to
explain particular phases (observed at epicentral distances greater than 120
◦
with
travel times of 18-20 min), the core must contain a distinct inner region. The
phases she identified are then explained as being refractions through the higher-
velocity inner core (PKIKP in today's notation), which therefore arrive earlier
than does the PKP phase. The depth of the inner-core-outer-core transition can
be determined from the travel times of PKiKP (the reflection from this transition),
and the velocity increase/velocity gradient occurring controls the amplitude of
this reflected arrival. It has been suggested that the boundary between inner and
outer core should be termed the
Lehmann discontinuity
. The name has been used
for a discontinuity at
∼
220 km depth beneath North America, but it seems most
appropriate in the core.
A zero S-wave velocity for the outer core, which is consistent with its being
liquid, is in agreement with studies on the tides, which require a liquid core.
This conclusion is supported by all other seismological evidence and, indeed, is
essential if the Earth's magnetic field (and its secular variation) is to be accounted
for by convection currents in the outer core (Section 8.3.2). There is a transition
zone at the outer-core-inner-core boundary. The increase in velocity shown in
Fig. 8.1 is a feature both of PREM and of
iasp91
; the low-velocity zone of the
early Jeffreys-Bullen model is not required. The P-wave velocity increases from
10.36 to 11.03 km s
−
1
and the S-wave velocity from zero to 3.50 km s
−
1
at the
outer-core-inner-core transition.
The P- and S-wave velocities are both almost constant through the inner core.
The structure of the inner core is mainly determined by using the ray path PKIKP
(Fig. 8.4). The phases with an S-wave leg (J) through the inner core are very hard
