The study of the propagation of seismic waves
through the Earth, including analysis of earthquake sources,
mechanisms, and the determination of the structure of the
Earth through variations in the properties of seismic waves.
Measuring the Crust
A Yugoslavian seismologist named Andrija Mohorovicˇic´
(1857–1936) noticed slow and fast arrivals of seismic waves
in seismic experiments he completed in Europe and proposed
that there must be a seismic discontinuity at around 22 miles
(35 km) beneath the surface of continents to explain his
observations. This discontinuity is now known as the Moho
and it is interpreted as the base of the crust. We now use this
bounding reflection surface as a measure of the thickness of
the crust.
Seismology is also used to determine the detailed structure
of shallow levels in the Earth’s crust. Seismic reflection
techniques are widely used by energy companies in their
search for oil and gas trapped in geological structures and
formations. Very subtle structures can now be found and
mapped in three dimensions using sophisticated seismic surveys
and computers. Some techniques involve moving the
seismic source (an explosion, sound, thump, etc.) from place
to place on the surface and noticing the difference in the
receiving functions at different receiving stations. Other techniques
involve moving the seismic sources and receivers up
and down drilled boreholes and determining the geologic
structure and layers between the two boreholes.
Seismologists really are measuring the speed of travel of
seismic waves through different rock types. They deduce
what the rock types are either by measuring seismic velocities
between stations and then correlating this with laboratory
studies, or by drilling samples of the area and correlating the
samples with places of specific seismic velocity. By putting
many observations of this type together, geologists and seismologists
are able to obtain a good understanding of the
overall structure of the Earth.
Determination of the structure of the deep parts of the
Earth can only be achieved by remote geophysical methods
such as seismology. There are seismographs stationed all over
the world, and by studying the propagation of seismic waves
from natural and artificial source earthquake and seismic
events, we can calculate changes in the properties of the
Earth in different places. If the Earth had a uniform composition,
seismic wave velocity would increase smoothly with
depth, because increased density is equated with higher seismic
velocities. However, by plotting observed arrival time of
seismic waves, seismologists have found that the velocity does
not increase steadily with depth, but that several dramatic
changes occur at discrete boundaries and in transition zones
deep within the Earth.
We can calculate the positions and changes across these
zones by noting several different properties of seismic waves.
Some wave energy is reflected off interfaces, whereas other
wave energy is refracted, changing the ray’s velocity and
path. These reflection and refraction events happen at specific
sites in the Earth, and the positions of the boundaries are calculated
using wave velocities. The core-mantle boundary at
1,802 miles (2,900 km) depth in the Earth strongly influences
both P and S-waves. It refracts P waves, causing a P-wave
shadow, and because liquids cannot transmit S waves, none
get through causing a huge S-wave shadow. These contrasting
properties of P and S-waves can be used to accurately map
the position of the core-mantle boundary.
There are several other main properties of the deep Earth
illustrated by variations in the propagation of seismic waves.
Velocity gradually increases with depth, to about 62 miles
(100 km), where the velocity drops a little at 62–124 miles
(100–200 km) depth, in a region known as the low velocity
zone. The reason for this drop in velocity is thought to be
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