Page 24 - Global Tectonics

THE INTERIOR OF THE EARTH 11

to oscillate at right angles to the direction of propaga- ities, or accelerations associated with the passage of
tion. The velocity of an S wave V s is given by: seismic waves. Since 1961 there has been an extensive
and standardized global network of seismograph
stations to monitor earthquake activity. The original
μ
V s = World-Wide Standardized Seismograph Network
ρ (WWSSN), based on analogue instruments, has gradu-
ally been superseded since 1986 by the Global (Digital)
Because the rigidity of a fluid is zero, S waves cannot Seismograph Network (GSN). By 2004 there were 136

be transmitted by such a medium. well-distributed GSN stations worldwide, including one

A consequence of the velocity equations for P and S on the sea floor between Hawaii and California. It is

waves is that the P velocity is about 1.7 times greater hoped that this will be the first of several in oceanic
than the S velocity in the same medium. Consequently, areas devoid of oceanic islands for land-based stations.
for an identical travel path, P waves arrive before S Digital equipment greatly facilitates processing of the
waves. This was recognized early in the history of seis- data and also has the advantage that it records over a
mology, and is reflected in the names of the body waves much greater dynamic range and frequency bandwidth

(P is derived from primus and S from secundus). The than the earlier paper and optical recording. This is
passage of body waves through the Earth conforms to achieved by a combination of high frequency, low gain
the laws of geometric optics in that they can be both and very broadband seismometers (Butler et al., 2004).
refracted and refl ected at velocity discontinuities. Most countries have at least one GSN station and many
Seismic waves whose travel paths are restricted to countries also have national seismometer arrays.
the vicinity of a free surface, such as the Earth’s surface, Together these stations not only provide the raw data
are known as surface waves. Rayleigh waves cause the for all global and regional seismological studies but also
particles of the transmitting medium to describe an serve an important function in relation to monitoring
ellipse in a vertical plane containing the direction of the nuclear test ban treaty, and volcano and tsunami
propagation. They can be transmitted in the surface of warning systems.
a uniform half space or a medium in which velocity Earthquakes occurring at large, or teleseismic, dis-
changes with depth. Love waves are transmitted when- tances from a seismograph are located by the identifi ca-
ever the S wave velocity of the surface layer is lower tion of various phases, or seismic arrivals, on the
than that of the underlying layer. Love waves are essen- seismograph records. Since, for example, the direct P
tially horizontally polarized shear waves, and propagate and S waves travel at different velocities, the time sepa-
by multiple reflection within this low velocity layer, ration between the arrival of the P phase and the S

which acts as a wave guide. phase becomes progressively longer as the length of the
Surface waves travel at lower velocities than body travel path increases. By making use of a standard

waves in the same medium. Unlike body waves, surface model for the velocity stratification of the Earth, and
waves are dispersive, that is, their different wavelength employing many seismic phases corresponding to dif-
components travel at different velocities. Dispersion ferent travel paths along which the seismic waves are

arises because of the velocity stratification of the Earth’s refracted or reflected at velocity discontinuities, it is

interior, longer wavelengths penetrating to greater possible to translate the differences in their travel times
depths and hence sampling higher velocities. As a result, into the distance of the earthquake from the observa-
surface wave dispersion studies provide an important tory. Triangulation using distances computed in this
method of determining the velocity structure and way from many observatories then allows the location
seismic attenuation characteristics of the upper 600 km of the epicenter to be determined.
of the Earth. The focal depths of teleseismic events are deter-
mined by measuring the arrival time difference between
the direct phase P and the phase pP (Båth, 1979). The
pP phase is a short path multiple event which follows a
2.1.4 Earthquake location similar path to P after first undergoing a refl ection at

the surface of the Earth above the focus, and so the
Earthquakes are detected by seismographs, instruments P–pP time difference is a measure of focal depth. This
that respond to very small ground displacements, veloc- method is least accurate for foci at depths of less than