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 ITEXCEEDS ROCK STRENGTH

Local rock strength

Stress

Earthquakes

Time

STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH

Local rock strength

Stress

Earthquakes

Time

Earthquakes arethe result ofstress that buildsup over time.

STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH

Local rock strength

Stress

Earthquakes

Time

Earthquakes arethe result ofstress that buildsup over time.

Stress gradually builds as tectonic forces deform rocks.

STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH

Local rock strength

Stress

Earthquakes

Time

Earthquakes arethe result ofstress that buildsup over time.

Stress gradually builds as tectonic forces deform rocks.

When the stress exceeds the strength of the rocks…

STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH

Local rock strength

Stress

Earthquakes

Time

Earthquakes arethe result ofstress that buildsup over time.

Stress gradually builds as tectonic forces deform rocks.

When the stress exceeds the strength of the rocks…

…the faultslips, causingan earthquake.

STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH

Local rock strength

Stress

Earthquakes

Time

Earthquakes arethe result ofstress that buildsup over 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.

Focus0 SecondsRupture expands circularly on fault plane, sending out seismic waves in all directions.

5 SecondsRupture continues to expand as a crack along the fault plane. Rocks at the surface begin to rebound from their deformed state.

10 SecondsThe rupture front progresses down the fault plane, reducing the stress.

20 SecondsRupture has progressed alongthe entire length of the fault.The earthquake stops.

Fault cracksat surface

Fault crackextends

World seismicity from 1976 to 2002

EUROPE

AFRICA

ASIA

INDIAN OCEAN

AUSTRALIA

SOUTHAMERICA

NORTHAMERICA

ANTARCTIC OCEAN

PACIFIC OCEAN ATLANTICOCEAN

≤ 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, AlaskaMt. McKinley

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

Southern California fault traces

San Andreas fault San Gabriel Mountains

North AmericanPlate

PacificPlate

Los AngelesMotion of Pacific Platerelative to motion of North American Plate

Southern California fault traces

San Andreas fault San Gabriel Mountains

North AmericanPlate

PacificPlate

Los AngelesMotion of Pacific Platerelative 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 fault San Gabriel Mountains

North AmericanPlate

PacificPlate

Los AngelesMotion of Pacific Platerelative 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 1994Magnitude 6.9

San Fernando 1971Magnitude 6.7

Landers 1992Magnitude 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 trenchdeeper quakes

Long Recurrence IntervalLarge earthquakes occur in NW U.S. every few hundred years

Lithosphere

Asthenosphere

Transform fault(lateral shearing)

Rift valley(divergence)

Normal faultingMid-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