Geoscience Reference
In-Depth Information
110
o
Figure 8.4.
Ray paths for
PKIKP, the direct P-wave
passing through the
mantle, outer core and
inner core. (Based on
Gutenberg (1959).)
Mantle
Outer
Core
0
o
180
o
Inner Core
to observe because they have a very low amplitude
2
and are easily masked by
interferences of other phases. The first clear identification of J phases, pPKJKP
and SKJKP, was made for a 1996 earthquake in the Flores Sea.
3
The S-wave
velocity of the inner core (3.50-3.67 km s
−
1
) can, however, be determined using
normal-mode data.
8.1.2 Density and elastic moduli for the whole Earth
The variation of seismic velocity with depth has been discussed in the previous
section.Tounderstand the internal structure of the Earth and its composition
further, it is also necessary to know how density and the elastic moduli vary with
depth.
We have already seen (Eqs. (4.4) and (4.5)) that the bulk or compressibility
modulus
K
, shear modulus
µ
and density
ρ
are related to the P- and S-wave
velocities by
K
+
4
3
µ
a
=
(8.1)
ρ
µ
ρ
β
=
(8.2)
Even if we know that
α
and
β
vary with depth in the Earth, these two equations
alone cannot tell us how
K
,
vary with depth because they contain three
unknowns. A third equation, which allows us to determine these three unknowns,
is the
Adams-Williamson
equation.
µ
and
ρ
2
There is very poor conversion of P- to S-waves at the outer-core-inner-core boundary: the amplitude
of an inner-core S-wave will be some five times smaller than that of a comparable inner-core P-wave.
In addition, the attenuation of short-period waves in the inner core is high.
3
The Flores Sea earthquake occurred on 17 June 1996 (depth 584 km,
M
w
7.9). The identification of
inner-core shear phases has been achieved for observations of this earthquake (Deuss
et al
. 2000).
