E
EARTHQUAKES
Nina Kukowski
Institute of Geosciences, Friedrich-Schiller-University,
Jena, Germany
Synonyms
Seismic event
Definition
The term earthquake describes ground shaking and radiation of seismic energy, which is caused, e.g., by slip on a
fault, magmatic activity, or cavity collapse.
Introduction
Earthquakes are events of seismic ground shaking caused,
e.g., by the release of elastic stress accumulated along a
previously locked fault zone (Figure 1), by magmatic
activity, or by the collapse of caves. Further, human activity such as mining and hydrocarbon exploitation, underground nuclear explosions, geothermal fluid injection, or
rapid water level changes in dam lakes may
cause earthquakes. Mostly, stress release occurs suddenly
and unpredicted. In the case of large earthquakes, ground
shaking may be considerable and cause devastation and
fatalities.
By far the most earthquakes, about 90 % of all
natural seismic events, are of tectonic origin, and in
the following sections only these will be dealt with.
Earthquakes caused by magmatic or human activity have
characteristics different from those of tectonic events and
therefore in most cases can be identified by experienced
scientists.
Before instruments were developed to detect and record
ground motion related to earthquakes, knowledge about
earthquakes having occurred only came from human
observations and reporting. Therefore, historically only
earthquakes sufficiently large to be felt by humans were
identified. Instrumentally it is possible to detect very small
earthquakes, which remain unfelt. The basic principle of a
seismometer (from the Greek “seismos,” quake, and
“meter,” measure), as it was firstly invented by J. Milne
(1850–1913) in 1882, is a sensor that detects and
amplifies ground vibration, which may be recorded as a
seismogram either in a directly visible form or is
converted to an analogue or digital electronic signal before
display (Figure 1b).
Generation, travel, and recording of seismic waves
Faulting and earthquakes mechanisms
Tectonic earthquakes are related to motion along faults
occurring in either the three basic (Figure 1c) or mixed
types of faults. These reflect the variable orientations of
the principal axes of stress with respect to the horizontal
plane. When motion on the fault occurs as a pure up or
down slip in the slip direction of the fault plane, then it
is called a dip-slip fault. In case of upward motion this corresponds to a thrust or reverse fault, and when the motion
is downward to a normal fault. In case of horizontal
motion, parallel to the strike of the fault, this is called a
strike-slip fault. Thrust faulting, normal faulting, strikeslip faulting, or any combination of them will occur on a
fault, depending on whether the stress regime in the
corresponding region is compressional, extensional or in
a state of shear, and on how the usually preexisting fault
strike and dip are oriented with respect to the direction
of maximum compression and tension.
Faulting within the Earth occurs under high confining
pressure. At depth, the minimum tensional stress corresponds to the maximum compressional stress, and vice
versa.
Failure or faulting in case of brittle deformation, respectively, occurs on the plane of maximum shear stress. After
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016
EARTHQUAKES
Nina Kukowski
Institute of Geosciences, Friedrich-Schiller-University,
Jena, Germany
Synonyms
Seismic event
Definition
The term earthquake describes ground shaking and radiation of seismic energy, which is caused, e.g., by slip on a
fault, magmatic activity, or cavity collapse.
Introduction
Earthquakes are events of seismic ground shaking caused,
e.g., by the release of elastic stress accumulated along a
previously locked fault zone (Figure 1), by magmatic
activity, or by the collapse of caves. Further, human activity such as mining and hydrocarbon exploitation, underground nuclear explosions, geothermal fluid injection, or
rapid water level changes in dam lakes may
cause earthquakes. Mostly, stress release occurs suddenly
and unpredicted. In the case of large earthquakes, ground
shaking may be considerable and cause devastation and
fatalities.
By far the most earthquakes, about 90 % of all
natural seismic events, are of tectonic origin, and in
the following sections only these will be dealt with.
Earthquakes caused by magmatic or human activity have
characteristics different from those of tectonic events and
therefore in most cases can be identified by experienced
scientists.
Before instruments were developed to detect and record
ground motion related to earthquakes, knowledge about
earthquakes having occurred only came from human
observations and reporting. Therefore, historically only
earthquakes sufficiently large to be felt by humans were
identified. Instrumentally it is possible to detect very small
earthquakes, which remain unfelt. The basic principle of a
seismometer (from the Greek “seismos,” quake, and
“meter,” measure), as it was firstly invented by J. Milne
(1850–1913) in 1882, is a sensor that detects and
amplifies ground vibration, which may be recorded as a
seismogram either in a directly visible form or is
converted to an analogue or digital electronic signal before
display (Figure 1b).
Generation, travel, and recording of seismic waves
Faulting and earthquakes mechanisms
Tectonic earthquakes are related to motion along faults
occurring in either the three basic (Figure 1c) or mixed
types of faults. These reflect the variable orientations of
the principal axes of stress with respect to the horizontal
plane. When motion on the fault occurs as a pure up or
down slip in the slip direction of the fault plane, then it
is called a dip-slip fault. In case of upward motion this corresponds to a thrust or reverse fault, and when the motion
is downward to a normal fault. In case of horizontal
motion, parallel to the strike of the fault, this is called a
strike-slip fault. Thrust faulting, normal faulting, strikeslip faulting, or any combination of them will occur on a
fault, depending on whether the stress regime in the
corresponding region is compressional, extensional or in
a state of shear, and on how the usually preexisting fault
strike and dip are oriented with respect to the direction
of maximum compression and tension.
Faulting within the Earth occurs under high confining
pressure. At depth, the minimum tensional stress corresponds to the maximum compressional stress, and vice
versa.
Failure or faulting in case of brittle deformation, respectively, occurs on the plane of maximum shear stress. After
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016