Page 32 - Global Tectonics

THE INTERIOR OF THE EARTH 19

using local arrays of recorders, while the deeper layers
have been investigated using global networks to detect
seismic signals that have traversed the interior of the
Earth.
The continental crust was discovered by Andrija
Mohorovicˇic´ from studies of the seismic waves gener-
ated by the Croatia earthquake of 1909 (Fig. 2.14).
Figure 2.13 Great circle paths from two earthquakes Within a range of about 200 km from the epicenter, the

(stars) to recording stations (dots) (after Thurber & Aki, first seismic arrivals were P waves that traveled directly
1987). from the focus to the recorders with a velocity of
−1
5.6 km s . This seismic phase was termed P g . At greater
ranges, however, P waves with the much higher velocity
−1
of 7.9 km s became the fi rst arrivals, termed the P n
procedure is then similar to that for teleseisms. One of
phase. These data were interpreted by the standard
the uses of the resulting three-dimensional velocity dis-
tributions is to improve focal depth determinations. techniques of refraction seismology, with P n represent-
ing seismic waves that had been critically refracted at a
Global methods commonly make use of both surface
and body waves with long travel paths. If the Earth velocity discontinuity at a depth of some 54 km. This
discontinuity was subsequently named the Mohorovicˇic´
were spherically symmetrical, these surface waves
would follow great circle routes. However, again making discontinuity, or Moho, and it marks the boundary
between the crust and mantle. Subsequent work has
use of Fermat’s Principle, it is assumed that ray paths
in a heterogeneous Earth are similarly great circles, demonstrated that the Moho is universally present
beneath continents and marks an abrupt increase in
with anomalous travel times resulting from the hetero- −1

geneity. In the single-station configuration, the surface seismic velocity to about 8 km s . Its geometry and
refl ective character are highly diverse and may include
wave dispersion is measured for the rays traveling
directly from earthquake to receiver. Information from one or more sub-horizontal or dipping refl ectors (Cook,
2002). Continental crust is, on average, some 40 km
only moderate-size events can be utilized, but the
source parameters have to be well known. The great thick, but thins to less than 20 km beneath some tec-
tonically active rifts (e.g. Sections 7.3, 7.8.1) and thick-
circle method uses multiple circuit waves, that is, waves
that have traveled directly from source to receiver and ens to up to 80 km beneath young orogenic belts (e.g.
Sections 10.2.4, 10.4.5) (Christensen & Mooney, 1995;
have then circumnavigated the Earth to be recorded
again (Fig. 2.13). Here the differential dispersion Mooney et al., 1998).
A discontinuity within the continental crust was
between the fi rst and second passes is measured, elimi-
nating any undesirable source effects. This method is discovered by Conrad in 1925, using similar methods.
As well as the phases P g and P n he noted the presence
appropriate to global modeling, but can only use those
large magnitude events that give observable multiple of an additional phase P* (Fig. 2.15) which he inter-
preted as the critically refracted arrival from an inter-
circuits.
face where the velocity increased from about 5.6 to
−1
6.3 km s . This interface was subsequently named the
Conrad discontinuity. Conrad’s model was readily
2.2 VELOCITY adopted by early petrologists who believed that two
layers were necessarily present in the continental
STRUCTURE OF crust. The upper layer, rich in silicon and aluminum,
was called the SIAL and was believed to be the source
THE EARTH of granitic magmas, while the lower, silicon- and mag-
nesium-rich layer or SIMA was believed to be the
source of basaltic magmas. It is now known, however,
that the upper crust has a composition more mafi c
Knowledge of the internal layering of the Earth has than granite (Section 2.4.1), and that the majority of
been largely derived using the techniques of earthquake basaltic magmas originate in the mantle. Consequently,
seismology. The shallower layers have been studied the petrological necessity of a two-layered crust no