Encyclopedia of the Solar System 2nd ed

202 Encyclopedia of the Solar System

Rayliegh

Rayliegh

SS

SSSS5

SKS

SKKS

Love

Sdiff

Pdiff

Radial

Transverse

PKKP

SP

Vertical

PP

5

0

− 5

×

10+4

5

0

0

− 5

− 5

− 10

− 10

×

10+4

×

10+4

× 10+2

5

10

10 15 20 25 30 35 40 45

FIGURE 12 Three component

recording at a GSN digital, high

dynamic range station. Note

identification of various phases.

The dispersed Rayleigh waves are

seen on the radial and vertical

component and Love waves are

seen on the transverse

component.

distribution and thus provide additional constraints on the
mass distribution other than the average density and mo-
ment of inertia. Figure 16 shows an example of a spectrum
of a vertical component recording of a very large deep earth-
quake under Bolivia; the lowest frequency mode shown has
a period of about 40 minutes.

FIGURE 13 Observed travel times from a Bulletin of
International Seismological Centre are compared with
predictions for model IASP91. There are additional observed
branches, such as PPP and SSS, for which travel times have not
been computed.

Sometime in the 1970s it became clear that further

refinements in one-dimensional Earth models cannot be

achieved, and perhaps do not make much sense, without

considering the three dimensionality of the Earth’s struc-

ture. All three types of data described earlier are sensitive to

the lateral heterogeneity. Travel times will be perturbed by

slight variations of the structure along a particular ray path,

compared to the prediction by a one-dimensional model. All

FIGURE 14 Comparison of a velocity model by Jeffreys (ca,

1937) with model IASP91. Notice that for the most part changes

have been minor, except for the discontinuities in the transition

zone, solidity of the inner core and structure just above inner

core boundary.