Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 187

A P -wave that reflects from the outer core boundary is
termed a PcP phase. Should the P -wave penetrate the liq-
uid outer core (Section 4.17.4) then it is designated a K
phase; if it transits the inner core then it is given an I code.
The overall code for a P -wave that has passed from the
mantle to traverse just the outer core is thus PKP . The
passage into inner core means it has two K transits and two
mantle transits and overall it is termed PKIKP . Should the
outer core wave reflect from the inner core back through
the outer core and the mantle, then it is given the notation
PKiKP . Reflected waves from the surface have the
repeated suffix, like PP or SS for two loops and PPP or SSS
for three loops and so on. Should conversion have taken
place upon reflection of refraction then the original code is
followed by the transformed code, like PS, SP , and the like.

of the largely fluid nature of the outer core. The interface
is the site of generation of the Earth's magnetic field and is
the ultimate site of submerged lithospheric plate, the
so-called slab graveyard of the D seismic layer.
In 1909 Mohorovicic used P -wave travel time data from
Balkan earthquakes and the concept of a critical reflection
angle (see Fig. 4.135) to determine that a fundamental and
sharp change in velocity delimited a step-change in rock
density at about 50 km depth. This is now known as the
famous Moho interface between mantle and crust,
detectable by either refracted or reflected seismic waves
(Fig. 4.134), the wave speeds increasing by 25 percent
across it. This variation of transmissibility matches that pre-
dicted between dense ( 3,300 kgm 3 ) silicate upper

mantle rich in minerals like olivine and spinel and less dense
(

m

2,800 kgm 3 ) silicate lower crustal rocks of general
granitic composition rich in feldspar and quartz. The Moho
depth is now known to be typically about 30 km, though
thickened crust occurs in many mountain belts to the max-
imum of 50 km as determined by Mohorovicic in the
Balkans. To their great delight, geologists can directly rec-
ognize the Moho (confirming it as a very sharp interface)
within mountain ranges where gigantic faults have thrust it
up toward the surface during past tectonic plate collisions.
Fragments of mantle may also turn up in volcanic vents.
In 1936 Lehmann recognized P -wave arrivals (termed by
her as P 1

c

4.17.4 Internal structure of Earth from seismic waves

We demonstrated in Section 1.4 that values computed for
Earth mass require a very dense interior and intimated in
Section 1.5 and earlier in this chapter that the planetary
interior was made up of well-defined concentric shells or
layers. It was seismology that revealed the existence of a
layered Earth. We briefly highlight the major develop-
ments in historical order.
We have already seen how Oldham's travel time data
(Fig. 4.130) led him to recognize in 1906 that a central
core existed with a sharp and distinct physical character
that caused it to slow down P - and S -waves entering from
arc distances of greater than 105

) at large arc separations (Fig. 4.139) that
were refracted on arriving at the mantle/core boundary
and also subsequently upon leaving it. More significantly
there were clear P -wave records within Oldham's shadow
zone of reflections (105-142

and P 2

. We know now that the
core-blocking shadow between 105

arc separation) that could
only have come as reflected phases from the outer surface
of a solid inner core. Such P 3

slows all
refracted P -waves and excludes all direct S -waves because

and 142

-waves (now called PKKP)

Assume P rays are straight and with constant mantle (10 km s -1 ) and
core (8 km s -1 ) velocities.
Rays 1 and 2 are entirely within the mantle (these we now call P waves).
Rays 3 and 7 are diffracted at the core : mantle boundary and focused
toward the antipodes (these we call PKP waves).
Rays 4 and 6 are nearly normally incident on the core : mantle boundary
(these we now call PKIKP waves).
The zone between ray 2 and 3 is the conventional “shadow zone ,
between about 105 º and 142 º arc distance.
Within the “shadow zone” arrivals like 5 (termed P´ originally by Lehmann)
but now known as PKiKP were interpreted to have reflected from a solid
inner core of radius about 0.2 whole Earth radii.

1

2

5

3
4

N
Z

6
7

Lehmann

Z

The critical evidence came from seismic records (seismograms) like this one at Sverdlovsk Observatory
located 135 o arc distance from the focus of the New Zealand earthquake, that is, within the general P - and
S -wave shadow zone. Lehmann, s P ´ waves are near first arrivals at these localities (arrowed).

E

Fig. 4.139 The discovery of the solid inner core - Lehmann's 1936 discovery, logic, and analysis.