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Part 8: Fold Types
Compressive
Stress
Tensional
Stress
Orientation of stress leads to different folds
Shear
Stress
Ductile Strain
Earthquakes
Stanford, 1906
Compressive
Stress
Tensional
Stress
Shear
Stress
Earthquakes occur on faults
Active Fault
Faults
Faults are fractures with movement
Faults
Faults are fractures with movement
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Local
rock
strength
Stress
Time
Earthquakes
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Local
rock
strength
Stress
Time
Earthquakes
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Stress gradually builds
as tectonic forces
deform rocks.
Local
rock
strength
Stress
Time
Earthquakes
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Stress gradually builds
as tectonic forces
deform rocks.
Local
rock
strength
Stress
Time
Earthquakes
When the stress
exceeds the strength
of the rocks…
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Stress gradually builds
as tectonic forces
deform rocks.
When the stress
exceeds the strength
of the rocks…
…the fault
slips, causing
an earthquake.
Local
rock
strength
Stress
Time
Earthquakes
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Stress gradually builds
as tectonic forces
deform rocks.
When the stress
exceeds the strength
of the rocks…
…the fault
slips, causing
an earthquake.
Local
rock
strength
The process
repeats again
and again.
Stress
Time
Earthquakes
Focus
0 Seconds
Rupture expands circularly on
fault plane, sending out seismic
waves in all directions.
Fault cracks
at surface
5 Seconds
Rupture continues to expand
as a crack along the fault plane.
Rocks at the surface begin to
rebound from their deformed
state.
Fault crack
extends
10 Seconds
The rupture front progresses
down the fault plane, reducing
the stress.
20 Seconds
Rupture has progressed along
the entire length of the fault.
The earthquake stops.
World seismicity from 1976 to 2002
ASIA
EUROPE
PACIFIC OCEAN
NORTH
AMERICA
ATLANTIC
OCEAN
AFRICA
SOUTH
AMERICA
INDIAN OCEAN
AUSTRALIA
≤ 50 km deep (shallow focus)
50–300 km deep
≥ 300 km deep (shallow focus)
ANTARCTIC OCEAN
Plate Boundaries and Faults
• Strike-slip faults along transform boundaries
• Reverse faults along convergent boundaries
• Normal faults along divergent boundaries
Strike-Slip Faults: Transform Boundary
• San Andreas Fault
– Several magnitude 7
earthquakes recently
– usually occur in clusters
– arrows show motion/yr
Faults not always parallel to boundary
Results in uplift of mountains
Denali fault, Alaska
Mt. McKinley
Offset streams along the San Andreas
Fault in the Carrizo Plain, CA
Southern California fault traces
San Andreas fault
San Gabriel Mountains
North American
Plate
Pacific
Plate
Los Angeles
Motion of Pacific Plate
relative to motion of
North American Plate
Southern California fault traces
Here, the San Andreas fault is
parallel to plate motion, and the
faulting is right-lateral strike slip.
San Andreas fault
San Gabriel Mountains
North American
Plate
Pacific
Plate
Los Angeles
Motion of Pacific Plate
relative to motion of
North American Plate
Southern California fault traces
Here, the San Andreas fault is
parallel to plate motion, and the
faulting is right-lateral strike slip.
San Andreas fault
San Gabriel Mountains
North American
Plate
Pacific
Plate
Los Angeles
Motion of Pacific Plate
relative to motion of
North American Plate
The “Big Bend” causes the Pacific
Plate to compress against the
North American Plate, causing
thrust faulting.
Southern California earthquakes
(July 1970-June 1995)
Northridge 1994
Magnitude 6.9
Key:
5+
<5
July 1970–June 1995
San Fernando 1971
Magnitude 6.7
Landers 1992
Magnitude 7.3
Subduction = biggest quakes
–
–
–
–
–
–
–
1960 Chile, magnitude 9.5
2004 Sumatra, magnitude 9.2
1964 Alaska, magnitude 9.2
1868 Peru, magnitude 9.0
2001 Peru, magnitude 8.4
2007 Sumatra, magnitude 8.4
2007 Peru, magnitude 8.0
Deep-ocean trench (convergence)
Large shallow
earthquakes occur
mainly on thrust faults.
Further from trench
deeper quakes
Long Recurrence Interval
Large earthquakes occur in NW U.S.
every few hundred years
Mid-ocean ridge (divergence)
Normal faulting
Transform fault
(lateral shearing)
Rift valley
(divergence)
Shallow earthquakes coincide with normal
faulting at divergent boundaries and with
strike-slip faulting at transform boundaries.
Rifting Also Occurs on Land
• Basin and Range of Nevada
• Rio Grande Rift
Basin and
Range
Province
New Mountains-Active Fault –Basin
and Range Province
Sandbox- Block rotation
Sand above plasticine
Normal Fault Zone
Active Fault