DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Fata Morgana A complex mirage made up
of multiple images as of cliffs and buildings, distorted
and magnified, which occasionally gives
the impression of elaborate castles floating in
the air. A Fata Morgana can only occur where
there are alternating warm and cold layers of air
near the ground or surface of water. Named after
the effect seen in the Straits of Medina, and
attributed to the sorcery of King Arthur’s halfsister,
Morgan Le Fay.
fathom A nautical unit of depth, equal to
6 ft. It is also sometimes used for horizontal
measure; a related unit is the cable, equal to 100
fathoms. Also, a nautical mile is approximately
1000 fathoms. See nautical mile.
fault A seismic fracture across which lateral
motion occurs. In some cases a fracture is simply
a crack in rocks where slip has occurred.
Major faults may be broad zones of granulated
rock (fault gouge) accommodating lateral motions
of 1000 km or more. Faults can be classified
as thrust faults, normal faults, or strike-slip
faults.
fault constitutive law A representation
of fault mechanical properties using relations
among stress, strain rate, and displacement on
fault planes. A variety of constitutive laws
have been proposed such as the slip weakening
model which represents the relation between
shear stress and slip, and the rate- and statedependent
friction law which represents the relation
between friction and slip accompanied by
changes in slip rate. These are empirical laws
derived from laboratory experiments of rock
friction. There are two modes for friction: stable
sliding and unstable sliding. It is thought that
aseismic slips such as fault creep and slow earthquakes
correspond to the former, while usual
earthquakes correspond to the latter. Numerical
simulations are being carried out to tackle
problems of earthquake cycle and seismic nucleation
process of large earthquakes, using the
fault constitutive laws.
fault gouge The granulated material on a
fault that has been generated during the many
earthquakes that have occurred on the fault.
© 2001 by CRC Press LLC
fault scarp
fault parameter Parameter that characterizes
faulting. As a geometrical quantity, there
is the strike of a fault plane, dip angle and slip
direction, while as quantities which represent
fault size, there is fault area (length times width)
and the amount of slip. Furthermore, rupture
velocity, rise time, and slip rate are physical
parameters which represent source processes.
From these fault parameters, seismic moment
and stress drop can be calculated. These fault
parameters can be obtained from radiation patterns
of seismic waves, waveforms, and aftershock
distributions.
fault plane solution An earthquake is generated
by the relative motion of rocks across a
fault. The movement has to be parallel to the
fault plane, i.e., a vector representing the motion
of one side with respect to the other will lie
on the fault plane, and therefore there will be
a plane normal to this vector (termed the “auxiliary
plane”) that will be perpendicular to the
fault plane. For the simplest type of earthquake
— simple movement along a flat fault plane,
known as a “double-couple” earthquake — the
pattern of radiation is divided into four lobes
separated by the fault and auxiliary planes. The
reason for this is that these planes divide the rock
into areas in which the released stress is compressive
or extensional, and thereby directions
in which the first motions of the propagated radiation
are compressions or rarefactions. With
sufficient seismic data from around the world, it
is possible to reconstruct the pattern of radiation
for these first motions, and so to pick out the fault
and auxiliary planes. This is known as the fault
plane solution. Without other information it is
difficult to tell which plane is the fault plane and
which is the auxiliary plane, but local observations
of the fault where the earthquake occurred
or knowledge of the tectonic setting can help
to determine this. The fault plane solution is
invaluable in discriminating between different
types of fault (e.g., thrust faults and transcurrent
faults).
fault scarp When there is vertical movement
on a fault during an earthquake, one side of the
fault is elevated relative to the other. This is a
fault scarp.
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