Answer: A fault is a fracture or
zone of fractures between two blocks of rock. Faults allow
the blocks to move relative to each other. This movement
may occur rapidly, in the form of an earthquake - or may
occur slowly, in the form of creep. Faults may range in
length from a few millimeters to thousands of kilometers.
Most faults produce repeated displacements over geologic
time. During an earthquake, the rock on one side of the
fault suddenly slips with respect to the other. The fault
surface can be horizontal or vertical or some arbitrary
angle in between.
Earth scientists use the angle
of the fault with respect to the surface (known as the
dip) and the direction of slip along the fault to
classify faults. Faults which move along the direction of
the dip plane are dip-slip faults and described as either
normal or reverse, depending on their motion. Faults which
move horizontally are known as strike-slip faults and are
classified as either right-lateral or left-lateral. Faults
which show both dip-slip and strike-slip motion are known
as oblique-slip faults.
The following definitions are adapted
from The Earth by Press and Siever.
normal
fault - a dip-slip fault in which the block above the
fault has moved downward relative to the block below. This
type of faulting occurs in response to extension and is
often observed in the Western United States Basin and Range
Province and along oceanic ridge systems.
thrust fault - a dip-slip
fault in which the upper block, above the fault plane, moves
up and over the lower block. This type of faulting is common
in areas of compression, such as regions where one plate
is being subducted under another as in Japan. When the dip
angle is shallow, a reverse fault is often described as
a thrust fault.
strike-slip fault - a fault
on which the two blocks slide past one another. The San
Andreas Fault is an example of a right lateral fault.
A left-lateral strike-slip fault
is one on which the displacement of the far block is to
the left when viewed from either side.
A right-lateral strike-slip
fault is one on which the displacement of the far
block is to the right when viewed from either side.
(UC Berkeley)
For animations of each type of fault,
see:
IRIS
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