Earthquakes Faults Stress Quakes at plate boundaries Quakes within plates

1964 Alaska: M8.4

1988 Armenia: M6.9

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 Local rock strength Stress Earthquakes Time Stress gradually builds as tectonic forces deform rocks. When the stress exceeds the strength of the rocks… …the fault slips, causing an earthquake. The process repeats again and 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 1994 Magnitude 6.9 San Fernando 1971 Magnitude 6.7 Landers 1992 Magnitude 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, Alaska Mt. 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 trench deeper 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 Interval Large 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