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Earthquakes Faults Stress Quakes at plate boundaries Quakes within plates.
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Transcript of Earthquakes Faults Stress Quakes at plate boundaries Quakes within plates.
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Earthquakes
• Faults
• Stress
• Quakes at plate boundaries
• Quakes within plates
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1964 Alaska: M8.4
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1988 Armenia: M6.9
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Earthquakes occur on faults
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Earthquakes occur on faults
Active Fault
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Faults
Faults are fractures with movement
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Faults
Faults are fractures with movement
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Elastic Rebound Theory Explains Earthquakes
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STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH
Local rock strength
Stress
Earthquakes
Time
Stress gradually builds as tectonic forces deform rocks.
When the stress exceeds the strength of the rocks…
…the faultslips, causingan earthquake.
The process repeats againand again.
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Offset and Rupture• Offset is distance of
movement across fault
• Surface rupture length is total length of break
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Quakes are common along plate boundaries
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Southern California earthquakes(July 1970-June 1995)
Northridge 1994Magnitude 6.9
San Fernando 1971Magnitude 6.7
Landers 1992Magnitude 7.3
July 1970–June 1995
Key:
5+
<5
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Plate Boundaries and Faults• Strike-slip faults along transform boundaries• Reverse faults along convergent boundaries• Normal faults along divergent boundaries
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Strike-Slip Faults: Transform Boundary
• San Andreas Fault– Several magnitude 7
earthquakes recently
– usually occur in clusters
– arrows show motion/yr
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Faults not always parallel to boundary
Results in uplift of mountains
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Denali fault, AlaskaMt. McKinley
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Offset streams along the San Andreas Fault in the Carrizo Plain, CA
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Lithosphere
Asthenosphere
Deep-ocean trench (convergence)
Large shallow earthquakes occur mainly on thrust faults.
Further from trenchdeeper quakes
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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
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Long Recurrence IntervalLarge earthquakes occur in NW U.S.
every few hundred years
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Lithosphere
Asthenosphere
Transform fault(lateral shearing)
Rift valley(divergence)
Normal faulting
Mid-ocean ridge (divergence)
Shallow earthquakes coincide with normal faulting at divergent boundaries and with strike-slip faulting at transform boundaries.
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Rifting Also Occurs on Land
• Basin and Range of Nevada• Rio Grande Rift
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Basin and Range
Province
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New Mountains-Active Fault –Basin and Range Province
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Sandbox- Block rotation
Sand above plasticine
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Normal Fault Zone
Active Fault
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Intraplate Earthquakes
• New Madrid, Missouri: December 1811, January 1812, February 1812
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• Eastern U.S. earthquakes less frequent, but more widely felt
• Crust is older and transmits stress
Intraplate Earthquakes