Ground Deformation: Faulting
and Folding
Earthquakes and MountainBuilding
This is referred to as stress.
Earthquake Terms
• Movement of rock bodies past other rock bodies is
known as an earthquake.
• The locus of earthquake movement is called a fault
• Faults come in all scales; millimeters to meters of
separation of lithospheric plates.
• Initial point of rupture or source is known as the
focus
• The point on the Earth’s surface directly above the
focus is known as the epicenter.
An Earthquake is a rapid
vibration of the Earth’s surface
created by a sudden movement of
a part of a plate along a fault.
Energy released radiates in all
directions from its source, the focus
Energy propagates in the form of
seismic waves
Three types of seismic waves
http://physicsquest.homestead.com/quest15eq.html
Types of seismic waves
• Body waves
– Travel through Earth’s interior
– Two types based on mode of travel
– Primary (P) waves
» Push-pull motion (compressional)
» Travel thru solids, liquids & gases
– Secondary (S) waves
» Moves at right angles to their direction of travel
(shear or zig/zag)
» Travels only through solids
• Surface waves
–
–
–
–
Complex motion, great destruction
High amplitude and low velocity
Longest periods (interval between crests)
Termed long or L waves
P and S waves
Smaller amplitude than surface (L) waves, but faster, P arrives first
L-wave
Seismographs are sensitive
instruments around the world that
record the events (Earthquakes)
Seismograph
What is a Fault or Fault line?
A Fault is a fracture in rock along
which displacement has taken
place- associated with a plate
boundary.
Faults can be active or inactive, and
can be associated with either
current or old plate boundaries.
Types of Faults
Normal
Reverse
Strike-slip
Where do Earthquakes occur?
What causes an Earthquake??
So, how does energy released by
slippage at a fault travel through
the ground?
Energy from
an Earthquake
travels in
seismic
waves.
Detecting and Locating
Earthquakes
Seismograph: A device that records
earthquake waves.
Seismogram: The “picture” drawn by a
seismograph.
How far is the epicenter of an earthquake
from a seismic station, if the difference
between the arrival time of the P and the S
wave is 5 minutes?
The P-Wave Shadow Zone
P-waves travel through
the liquid outer core
bend, leaving a low
intensity shadow zone
103 to 143 degrees away
from the source, here
shown as the north pole
HOWEVER, P-waves
traveling straight
through the center
continue, and because
speeds in the solid inner
core are faster, they
arrive sooner than
expected if the core was
all liquid.
Behavior of waves through center reveal Earth’s Interior
The S-Wave Shadow Zone
Since Shear (S) waves
cannot travel through
liquids, the liquid outer
core casts a larger
shadow for S waves
covering everything
past 103 degrees away
from the source.
Folded and Faulted
Mountains
Folding creates non-volcanic
mountains
• Folding occurs at convergent boundaries, more specifically,
a collision boundary (two continents colliding).
• Mountains form when compressional stress is applied
slowly, then solid rock will start to display plastic
properties and fold.
• Anticline is a fold that is convex-up and usually has
oldest rock layer (bed) of rock closer to the center.
• Syncline is a fold that is convex-down and usually has
the youngest rock layer (bed) of rock closer to the center.
Faulted Non-volcanic Mountains
Faulted mountains form when stress at plate boundaries is
applied more quickly causing rock to fracture. These types
of mountains are referred to fault block mountains.
• The fractures cause rock to slide either up or down
forming mountains with features referred to as horst
and graben.
• Horst occurs at collision boundaries
• Graben occurs at divergent boundaries
Fault-Block Mountains
Dome Mountains
Dome mountains form from a stress that is pushing upward.
Collision of continental plates cause the dome to form or
possible rising magma exerts enough pressure to cause over
laying rock layer to fold into a dome structure.
Domed Mountains
Normal
Reverse