Náttúrufræðingurinn - 1991, Page 75

SUMMARY
Seismic waves
by
Páll Einarsson
Science Institute
University of Iceland
Dunhaga 5
IS-107 REYKJAVÍK
Iceland
The main types of elastic waves in the
Earth are described. P- and S-waves are
body waves that travel through elastic ma-
terials along rays of minimum or maxi-
mum travel time. The body waves are re-
flected, refracted and converted at various
velocity discontinuities in the Earth. A
simple pulse at the source of a seismic
wave thus gives rise to a series of pulses
that travel along various paths through the
Earth. Guided waves also exist, e.g. Love
and Rayleigh waves that travel along
the Earth’s surface. Variations in the elas-
tic properties with depth lead to disper-
sion of the surface waves, i.e. waves of
different wave length travel at different
velocities.
Seismic waves carry with them a wealth
of information, both about the source that
produced them and the medium they
travel through. They can therefore be
used to study the source processes of
earthquakes as well as the interior of the
Earth. A large part of the knowledge of
the internal structure of the Earth is
derived from the study of the travel times
of body waves and the dispersion of sur-
face waves.
The crust and upper mantle in Iceland
have been studied extensively with seismic
waves. Early surface wave studies indicat-
ed that the velocity structure beneath Ice-
land differed from that of the surrounding
area. P-waves from explosions have
shown that the crust in Iceland is 8-15 km
thick. The crust in most parts of the coun-
try is underlain by a layer where the ratio
of P-velocity to S-velocity is abnormally
high and S-waves are attenuated. This lay-
er is inferred to be in a state of partial
melting. Delay of P-waves from distant
earthquakes indicate that the P-velocity in
the mantle is abnormally low to a depth of
about 300 km beneath Iceland, probably
an expression of a plume of upwelling
mantle material.
Tsunamis are catastrophic sea waves
produced by earthquakes and faulting at
the sea bottom. These waves are rare in
the Atlantic Ocean and only one example
is known from Iceland. An earthquake on
the Húsavík faults in 1872 produced a
small tsunami on the island of Flatey.
Earthquakes and submarine volcanic
eruption sometimes generate so called T-
waves, sound waves that are guided by a
low-velocity layer in the ocean. The T-
wave propagates with low attenuation
over distances of thousands of kilometers
and can couple into the crust in areas of
steep topography. One example of T-
waves is known from seismographs on
land in Iceland. Seismographs along the
south coast recorded T-waves from the
magnitude 7 Azores earthquake on Jan. 1,
1980.
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