Earth as a Planet: Surface and Interior 203
FIGURE 15 Mantle waves observed on multiple orbits
around the Earth. The symbol “R” designates Rayleigh waves
and “G” Love waves. Odd numbered (1,3) arrivals correspond
to minor arc arrivals plus an integer number of complete paths
around the Earth. Even numbered wavegroups correspond to
initial propagation in the major arc direction. The signal
between arrivals of the fundamental mode wavegroups
represents contribution of overtones. Top traces are observed
seismograms, bottom traces are synthetic seismogram
computed for 3-D model of upper mantle M84C; if 1-D model
(PREM) was used, there would be significant differences
between observed and computed traces.we need is many observations of travel times along criss-
crossing paths. Many millions of such data are available
from the routine process of earthquake location; they are
assembled from some 6000 stations around the world by
the National Earthquake Information Center in Golden,
Colorado, and by the International Seismological Centre
in England (see Figure 13). Surface waves, mantle waves,
and periods of free oscillations in a three-dimensional Earth
also depend on the location of the source and the receiver.
Progress during the last decade in global seismographic
instrumentation, in terms of the quality and distribution
of the observatories and exchange and accessibility of the
data, makes the required observations much more readily
available.
5. Seismic Sources
Even though the field of seismology can be divided into
studies of seismic sources (earthquakes, explosions) and of
the Earth’s structure, they are not fully separable. To ob-
tain information on an earthquake, we must know what
happened to the waves along the path between the source
and receiver, and this requires the knowledge of the elastic
and anelastic Earth structure. The reverse is also true: in
studying the Earth structure, we need information about
the earthquake; at least its location in space and time, but
sometimes also the model of forces acting at the epicenter.
Most of the earthquakes can be described as a process
of release of shear stress on a fault plane. Sometimes the
stress release can take place on a curved surface or involve
multiple fault planes: the radiation of seismic waves is more
complex in these cases. Also, explosions, such as those asso-
ciated with nuclear tests, have a distinctly different mecha-
nism and generate P and S waves in different proportions,
which is the basis for distinguishing them from earthquakes.
Figure 17 shows three principal types of stress release,
sometimes also called the earthquake mechanism. The top
part of Figure 17a is a view in the horizontal plane of two
blocks sliding with respect to each other in the direction
shown by the arrows. Such a mechanism is called strike
slip, and the sense of motion is left-lateral; there is also anFIGURE 16 Amplitude spectrum of a vertical
component seismogram of the great deep Bolivia
earthquake of 1994. The peaks in the spectrum
correspond to periods of free oscillations
(vibrations) of the Earth. The symbols designate
the specific normal modes. Some of them appear in
groups which indicate a possibility of coupling
between modes close in frequency. Usually the
fundamental modes (pre-subscript “0”) are excited
most strongly.