F
FAULT-PLANE SOLUTIONS OF EARTHQUAKES
Wolfgang Frisch
Department of Geosciences, University of Tübingen,
Tübingen, Germany
Definition
Earthquakes are caused by ruptures (faults) in solid rock
with instantaneous movement, during which seismic body
waves are emanated by elastic rebound. By evaluation of
the seismic waves, the focus (location) of the earthquake,
the spatial orientation of the fault plane, and the mode of
movement can be determined. The result is called “faultplane solution.”
Fault-plane solutions
The orientation of planes of movement at plate boundaries
or within any block of solid rock can be deduced from
earthquake data. In the case of an earthquake triggered at
a fault plane, the two blocks move by creating an instantaneous offset up to several meters. This results in the generation of the two types of seismic body waves. Primary (P-)
waves oscillate in the longitudinal direction of propagation. They are faster than secondary (S-) waves that oscillate transversally. If all of the seismic data from a given
earthquake collected around the Earth are put into
a diagram, four quadrants and the two separating planes
(Figure 1) can be determined with their spatial orientation.
In the two quadrants that are in the direction of movement
of each block, the first motion of the primary waves is
away from the earthquake focus, and an observer on the
Earth’s surface first receives a push; the wave starts with
a compressive movement (compressive first motion
shown as black quadrants in Figure 1). First motion in
the other two quadrants, shown in white, is in the opposite
direction; it starts with a tension and is dilatational. Each
of these motions is registered by seismographs.
One of the separating planes represents the slip plane
generated by the earthquake; the other one is an aiding
plane that has no use in nature. However, initially it is
not possible to decide which one of these two planes was
the slip plane (Figure 2). Commonly, this can be deduced
from geological observations if the approximate orientation of a fracture zone is known. On the other hand, careful
analysis of seismic data generated by the aftershock activity following every large earthquake provides the opportunity to identify the slip plane because of the shift of the
seismic centers. If the slip plane is known, the sense of
movement is easily detected (Figure 2). Direction of
movement in the slip plane is orthogonal to the aiding
plane. A process similar to that used for analyzing
P-waves can also be used for the analysis of S-waves.
Conclusions
Using this method, which is called fault-plane solution,
the orientation of a slip plane and the sense of movement
can be determined with high accuracy. Fault-plane solutions allow for a reconstruction of plate boundaries and
their movement patterns. The analysis of earthquake first
motion data impressively confirmed the concept of three
different types of plate boundaries.
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016
FAULT-PLANE SOLUTIONS OF EARTHQUAKES
Wolfgang Frisch
Department of Geosciences, University of Tübingen,
Tübingen, Germany
Definition
Earthquakes are caused by ruptures (faults) in solid rock
with instantaneous movement, during which seismic body
waves are emanated by elastic rebound. By evaluation of
the seismic waves, the focus (location) of the earthquake,
the spatial orientation of the fault plane, and the mode of
movement can be determined. The result is called “faultplane solution.”
Fault-plane solutions
The orientation of planes of movement at plate boundaries
or within any block of solid rock can be deduced from
earthquake data. In the case of an earthquake triggered at
a fault plane, the two blocks move by creating an instantaneous offset up to several meters. This results in the generation of the two types of seismic body waves. Primary (P-)
waves oscillate in the longitudinal direction of propagation. They are faster than secondary (S-) waves that oscillate transversally. If all of the seismic data from a given
earthquake collected around the Earth are put into
a diagram, four quadrants and the two separating planes
(Figure 1) can be determined with their spatial orientation.
In the two quadrants that are in the direction of movement
of each block, the first motion of the primary waves is
away from the earthquake focus, and an observer on the
Earth’s surface first receives a push; the wave starts with
a compressive movement (compressive first motion
shown as black quadrants in Figure 1). First motion in
the other two quadrants, shown in white, is in the opposite
direction; it starts with a tension and is dilatational. Each
of these motions is registered by seismographs.
One of the separating planes represents the slip plane
generated by the earthquake; the other one is an aiding
plane that has no use in nature. However, initially it is
not possible to decide which one of these two planes was
the slip plane (Figure 2). Commonly, this can be deduced
from geological observations if the approximate orientation of a fracture zone is known. On the other hand, careful
analysis of seismic data generated by the aftershock activity following every large earthquake provides the opportunity to identify the slip plane because of the shift of the
seismic centers. If the slip plane is known, the sense of
movement is easily detected (Figure 2). Direction of
movement in the slip plane is orthogonal to the aiding
plane. A process similar to that used for analyzing
P-waves can also be used for the analysis of S-waves.
Conclusions
Using this method, which is called fault-plane solution,
the orientation of a slip plane and the sense of movement
can be determined with high accuracy. Fault-plane solutions allow for a reconstruction of plate boundaries and
their movement patterns. The analysis of earthquake first
motion data impressively confirmed the concept of three
different types of plate boundaries.
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016