Geoscience Reference
In-Depth Information
103
o
C
B
P
A
E 143
o
Mantle
F
PKP
P
P
D
Inner
Core
Outer
Core
o
C 188
Figure 8.3.
Ray paths for PKP, the direct P-wave passing through the mantle and
outer core. The mantle P-wave (C) which has grazing incidence on the core has an
epicentral distance of 103
◦
. Beyond this distance, there can be no direct mantle
P-waves, although PP and PPP can be recorded at greater distances, and a weak
diffracted P can be recorded out to about 120
◦
. Because there is a sharp decrease in
velocity for P-waves refracted into the core, rays bend towards the normal at the
mantle-core boundary and give rise to a
shadow zone
for P-waves. The PKP ray with
the shallowest angle of incidence on the outer core (C') is refracted and finally
emerges at an epicentral distance of 188
◦
. With increasing angle of incidence (C', D,
E, F), PKP rays emerge at epicentral distances decreasing to 143
◦
and then increasing
to about 155
◦
. Each ray penetrates deeper into the outer core than does its
predecessor. Thus, no direct P-waves are recorded at epicentral distances between
103
◦
and 143
◦
; this is the shadow zone. At 143
◦
there is a caustic; the amplitude of
PKP is large (this shows clearly in Fig. 4.18). (Based on Gutenberg (1959).)
lower mantle at depths down to 2700 km is referred to as the D
shell.
1
The lower-
most 150-200 km of the mantle (depth
2700-2900 km) is referred to as the D
shell. Velocities increase slowly with depth through the lower mantle. The direct
P-wave through the mantle can be observed out to 103
◦
.Atepicentral distances
between 103
◦
and 120
◦
,aweak P-wave is diffracted (Section 4.4.4)atthe core-
mantle boundary into what is called the
shadow zone
(Fig. 8.3). There is evidence
that velocity gradients are much reduced in the D
shell. This could be due to
chemical heterogeneity and interaction between the core and mantle and/or to a
thermal boundary layer that would conduct, not convect, heat (refer to Sections
8.2 and 8.3): if the temperature at the base of the mantle is about 3000 K, there is
∼
1
The names D
and D
are the sole survivors of a labelling of the internal layering of the Earth from
AtoGintroduced by Bullen in 1947. A was the crust; B the mantle down to 400 km depth; C the
mantle from 400 to 1000 km depth; D the mantle from 1000 km depth to the core; E and F the outer
core; and G the inner core.
