Geoscience Reference
In-Depth Information
Chapter 4
Seismology Measuring
the interior
4.1 Waves through the Earth
4.1.1 Introduction
Seismology
is the study of the passage of elastic waves through the Earth. It is
arguably the most powerful method available for studying the structure of the
interior of the Earth, especially the crust and mantle. There are various other geo-
physical techniques, including the study of gravity, magnetism and the electrical
properties of the Earth, which can be applied on scales ranging from the planet as
awhole to large regions or small areas or even individual rock samples (Telford
et al
. 1990; Dobrin and Savit 1988); but seismology is probably the most widely
used and the most informative. This chapter discusses the methods by which we
obtain information about the interior of the planet from the study of elastic waves
passing through the Earth.
Earthquake seismology is perhaps the best technique for investigating the
Earth's interior. The study of earthquakes was of major significance in giving us
our understanding of plate tectonics: earthquake foci have delineated the bound-
aries of the tectonic plates very accurately. It has also helped us to map the internal
structure of our planet. The distribution of earthquakes shows us where the Earth
is active (mostly near the surface), and the passage of seismic waves through the
Earth allows us, as it were, to CAT-scan its interior.
When an earthquake or an explosion occurs within the Earth, part of the energy
released takes the form of elastic waves that are transmitted through the Earth.
These waves can be detected by an instrument called a
seismograph
,
1
consisting
of a
seismometer
,which measures and amplifies the motion of the ground on
which is rests, and a recorder, which transfers the data onto paper, magnetic tape
or disc. The speed with which these elastic waves travel depends on the density
and elastic moduli of the rocks through which the waves pass. There are two types
of elastic waves:
body waves
and
surface waves
.
1
The first seismograph capable of detecting distant events was developed by John Milne (1850-1913)
in the 1890s.
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