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 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.

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 1994Magnitude 6.9

San Fernando 1971Magnitude 6.7

Landers 1992Magnitude 7.3

July 1970–June 1995

Key:

5+

<5

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, AlaskaMt. McKinley

Offset streams along the San Andreas Fault in the Carrizo Plain, CA

Lithosphere

Asthenosphere

Deep-ocean trench (convergence)

Large shallow earthquakes occur mainly on thrust faults.

Further from trenchdeeper quakes

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

Long Recurrence IntervalLarge earthquakes occur in NW U.S.

every few hundred years

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.

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

• Eastern U.S. earthquakes less frequent, but more widely felt

• Crust is older and transmits stress

Intraplate Earthquakes