3 Seismology and Earth Structure
Ordinary language undergoes modification to a high pressure form when applied to the interior of the earth; a few examples of equivalents follow:
Ordinary meaning:
High pressure form:
dubious
certain
perhaps
undoubtedly
vague suggestion
positive proof
trivial objection
unanswerable argument
uncertain mixture of all the elements
pure iron
Francis Birch, 1952
3.1 Introduction
A major application of seismology is the determination of the
distribution of seismic velocities, and hence elastic properties,
within the earth. This distribution, known as earth structure,
gives the basic constraint on the mineralogical, chemical, and
thermal state of the earth’s interior. Seismological data are
important for this purpose because their resolving power is
generally superior to that of other geophysical methods. For
example, although gravity and magnetic data indicate the presence of a dense fluid core at depth, they provide only relatively
weak constraints on its density and size. By contrast, seismological data indicate the depth of the core–mantle boundary
and the sharp change in properties that occurs there. Above the
boundary, both P and S waves propagate in the solid mantle,
whereas in the liquid outer core no S waves propagate and the
P-wave velocity drops sharply. The observed velocities are the
primary basis for our models of the physical properties and
chemical composition of the material on either side of this
boundary. Similarly, the distinction between the crust and the
mantle and many inferences about their structure and composition come from seismological observations. More generally,
by establishing the essentially layered structure of the earth,
seismology provides the primary evidence for the process
of differentiation whereby material within planets became
compositionally segregated during their evolution. As a result,
many crucial issues about the other terrestrial planets could be
resolved if seismological data were available.
Constraints from seismology are crucial for other disciplines
of the earth sciences, and vice versa. Seismology gives earth
models describing the distribution of P- and S-wave velocities
and density. Going from an earth model to a description of
the chemical, mineralogical, thermal, and rheological state of
the earth’s interior requires additional information. There are
thus two types of uncertainty in our knowledge of the earth’s
interior. In some cases, such as the structure of the inner core,
the seismological results are still under discussion. In others —
for example, the nature of the 660 km discontinuity in the
mantle — the basic seismological results are generally accepted,
but their mineralogic and petrologic interpretations remain
under investigation. Given our scope here, we only summarize
the implications of seismological data for models of the earth’s
interior.
The fundamental data for seismological studies of the
earth’s interior are the travel times of seismic waves. The measurements available are the arrival times of seismic waves at
receivers. To convert these to travel times, the origin time and
location of the source must be known. These parameters,
which are known for artificial sources, must be estimated from
the observations for earthquake sources. Hence travel time
data include information about both the source and the properties of the medium, and separating the two is a challenge
in many seismological studies.
The travel times are used to learn about the velocity structure
between the source and the receiver. As we saw in the last
chapter, waves follow paths that depend on the velocity
Ordinary language undergoes modification to a high pressure form when applied to the interior of the earth; a few examples of equivalents follow:
Ordinary meaning:
High pressure form:
dubious
certain
perhaps
undoubtedly
vague suggestion
positive proof
trivial objection
unanswerable argument
uncertain mixture of all the elements
pure iron
Francis Birch, 1952
3.1 Introduction
A major application of seismology is the determination of the
distribution of seismic velocities, and hence elastic properties,
within the earth. This distribution, known as earth structure,
gives the basic constraint on the mineralogical, chemical, and
thermal state of the earth’s interior. Seismological data are
important for this purpose because their resolving power is
generally superior to that of other geophysical methods. For
example, although gravity and magnetic data indicate the presence of a dense fluid core at depth, they provide only relatively
weak constraints on its density and size. By contrast, seismological data indicate the depth of the core–mantle boundary
and the sharp change in properties that occurs there. Above the
boundary, both P and S waves propagate in the solid mantle,
whereas in the liquid outer core no S waves propagate and the
P-wave velocity drops sharply. The observed velocities are the
primary basis for our models of the physical properties and
chemical composition of the material on either side of this
boundary. Similarly, the distinction between the crust and the
mantle and many inferences about their structure and composition come from seismological observations. More generally,
by establishing the essentially layered structure of the earth,
seismology provides the primary evidence for the process
of differentiation whereby material within planets became
compositionally segregated during their evolution. As a result,
many crucial issues about the other terrestrial planets could be
resolved if seismological data were available.
Constraints from seismology are crucial for other disciplines
of the earth sciences, and vice versa. Seismology gives earth
models describing the distribution of P- and S-wave velocities
and density. Going from an earth model to a description of
the chemical, mineralogical, thermal, and rheological state of
the earth’s interior requires additional information. There are
thus two types of uncertainty in our knowledge of the earth’s
interior. In some cases, such as the structure of the inner core,
the seismological results are still under discussion. In others —
for example, the nature of the 660 km discontinuity in the
mantle — the basic seismological results are generally accepted,
but their mineralogic and petrologic interpretations remain
under investigation. Given our scope here, we only summarize
the implications of seismological data for models of the earth’s
interior.
The fundamental data for seismological studies of the
earth’s interior are the travel times of seismic waves. The measurements available are the arrival times of seismic waves at
receivers. To convert these to travel times, the origin time and
location of the source must be known. These parameters,
which are known for artificial sources, must be estimated from
the observations for earthquake sources. Hence travel time
data include information about both the source and the properties of the medium, and separating the two is a challenge
in many seismological studies.
The travel times are used to learn about the velocity structure
between the source and the receiver. As we saw in the last
chapter, waves follow paths that depend on the velocity