Part 8: Fold Types

Tensional Stress Compressive Stress Shear Stress Orientation of stress leads to different folds

Ductile Strain

Earthquakes Stanford, 1906

Tensional Stress Compressive 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 Earthquakes Time

STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time Earthquakes are the result of stress that builds up over time.

STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time Earthquakes are the result of stress that builds up over time. Stress gradually builds as tectonic forces deform rocks.

STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time 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…

STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time 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.

STRESS BUILDS UNTIL IT EXCEEDS ROCK STRENGTH Local rock strength Stress Earthquakes Time 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. The process repeats again and again.

Focus 0 Seconds Rupture expands circularly on fault plane, sending out seismic waves in all directions. 5 Seconds Rupture continues to expand as a crack along the fault plane. Rocks at the surface begin to rebound from their deformed state. 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. Fault cracks at surface Fault crack extends

World seismicity from 1976 to 2002 EUROPE AFRICA ASIA INDIAN OCEAN AUSTRALIA SOUTH AMERICA NORTH AMERICA ANTARCTIC OCEAN PACIFIC OCEAN ATLANTIC OCEAN ≤ 50 km deep (shallow focus) 50–300 km deep ≥ 300 km deep (shallow focus)

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 faultSan Gabriel Mountains North American Plate North American Plate Pacific Plate Pacific Plate Los Angeles Motion of Pacific Plate relative to motion of North American Plate

Southern California fault traces San Andreas faultSan Gabriel Mountains North American Plate North American Plate Pacific Plate Pacific Plate Los Angeles Motion of Pacific Plate relative to motion of North American Plate Here, the San Andreas fault is parallel to plate motion, and the faulting is right-lateral strike slip.

Southern California fault traces San Andreas faultSan Gabriel Mountains North American Plate North American Plate Pacific 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. Here, the San Andreas fault is parallel to plate motion, and the faulting is right-lateral strike slip.

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

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

Lithosphere Asthenosphere 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

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