2. WHAT IS A FAULT?
• 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.
3. • 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.
4.
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7. WHAT IS A FAULT LINE?
• A fault line is a geological fracture where the movement of
masses of rock have displaced parts of the earth's crust
• A fault plane is the plane that represents the fracture
surface of a fault. A fault trace or fault line is the
intersection of a fault plane with the ground surface. A
fault trace is also the line commonly plotted on geologic
maps to represent a fault.
9. THINK OF FAULTS LIKE TAKING A THICK MAT AND
SNAPPING IT:
• If it splits with an incline, it’s a dip-slip type of fault
• If it doesn’t have an incline, it’s strike-slip
• Now, you have 2 mats. Put them on water so they float
against each other. Here’s what could happen with their
relative movements:
10. 1. NORMAL FAULT
• The upper block moves downward relative to the lower block. (dip-slip)
13. NOW, YOU CAN SUBSTITUTE THESE TERMS:
• Fault = Fracture in two mats with relative movement
• Dip-slip = Incline split with vertical movement (upwards or
downwards)
• Strike-slip = Straight split with horizontal movement (right or left
lateral)
• Lithosphere = Floating mat
• Asthenosphere = Water
14. WHAT IS THE SPEED OF FAULT MOVEMENT?
• At faults, there is an enormous amount of friction between
plates. Like a stretched spring, it stores tremendous
potential energy near the fault.
• CREEP: If movement is slow at the fault, it’s called “creep”.
By definition, “creep” means the fault is always absent of
sudden movements that could create an earthquake.
15. • EARTHQUAKE: But if the two plates have a sudden jerky
movement, this generates enough force to produce an
“earthquake”. At this point, elastic waves shoot outwards
which is the force one would feel from an earthquake.
16. WHAT IS THE DIFFERENCE BETWEEN ACTIVE AND
INACTIVE FAULTS?
• An active fault is a fault that has displayed recent seismic
activity, while an inactive fault has not displayed recent
seismic activity.
• Do not be fooled by the word "recent," however, as we are
talking about "recent" from a geological perspective,
which is much different from a non-geologic perspective.
17. • Because of the fickle nature of plate tectonics, an active
fault could have earthquakes as often as once every few
years or once every one thousand years.
• Conversely, it's very hard to call a fault inactive if we don't
know it's quake history, and for some faults, geologists
will wait ten thousand years in between quakes to call
them inactive
18. • There are a variety of techniques that geologists can use to help
them determine the frequency of earthquakes among faults,
however. If a history of quakes coming from the fault are available,
scientists can look at the average period of time in between quakes
to determine whether a fault is presently "active" or "inactive."
Scientists can also measure creep among fault lines to check for
seismic activity.
19. • There really is no way to concretely define a fault as
"active" or "inactive" (especially because inactive faults can
suddenly become active again), but it's more or less safe
to say that if a fault hasn't shown tectonic activity for
about 5,600 years, it's probably inactive.
20. • Just to add to that explanation, if movement occurs on
long-quiescent fault in a new phase of tectonic activity,
the fault is described as 're-activated', and the new
movement can be the opposite to the original.
