Download 04.Earthquakes

Earthquakes
• Faults
• Stress
• Quakes at plate boundaries
• Quakes within plates
1964 Alaska: M8.4
1988 Armenia: M6.9
Earthquakes occur on faults
Earthquakes occur on faults
Active Fault
Faults
Faults are fractures with movement
Faults
Faults are fractures with movement
Elastic Rebound Theory Explains Earthquakes
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
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
Offset and Rupture
• Offset is distance of
movement across fault
• Surface rupture
length is total length
of break
Quakes are common along plate boundaries
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
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
Deep-ocean trench (convergence)
Large shallow
earthquakes occur
mainly on thrust faults.
Further from trench
deeper quakes
Subduction = biggest quakes
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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
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
Intraplate Earthquakes
• New Madrid,
Missouri:
December 1811,
January 1812,
February 1812
Intraplate Earthquakes
• Eastern U.S.
earthquakes less
frequent, but more
widely felt
• Crust is older and
transmits stress