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Lecture #11- Faults and Faulting

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Faults Bound the Major Plates

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Rock Deformation Rocks slowly deform as a result of Earth’s

convection and thermal cooling.

Where the rocks are cool and “brittle”, they can stick together along faults that fracture causing earthquakes.

Deep below Earth’s surface, where it is hot, the rocks bend and flow like taffy.

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Rock Deformation

The specific style of rock deformation depends on

• Composition• Temperature• Pressure• Strain Rate

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Rock Behavior

In general, rocks will either “flow” or fracture depending on their temperature. – However, even cold rocks can sustain some

strain before they break. – If the strain is released before the strength of

the rock is exceeded, the rock returns to its original shape.

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Rock Fractures (Faults & Joints)

A joint is a fracture across which the sides have not moved.

A fault is a crack across which the two sides have moved.

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Joints

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Joints and Weathering

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Fault Scarps

An exposed fault surface is called a scarp.

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Faults (Fairview Valley, NV)

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San Andreas (central CA)

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Fault Structure We don’t have many examples of what

faults look like far below Earth’s surface, but here’s an example.

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CaliforniaFaults &

Large Ruptures

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Earthquakes and Faults

Earthquakes occur on faults, but not all of the fault ruptures during each earthquake.

The hypocenter (or focus) is the place where the rupture begins, the epicenter is the place directly above the hypocenter.

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Hypocenter and Epicenter

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Rupture Size

Larger earthquakes rupture larger faults or larger fractions of faults.

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Fault Geometry Terminology

We need some definitions and concepts that we can use to discuss faults.

Important Terms: Hanging Wall / Foot Wall Strike Dip Slip

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Dip (& Hanging Wall/Foot Wall)

The orientation of the fault surface with respect to Earth’s surface is defined by the fault dip.

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Strike

Strike is an angle use to describe the orientation of the fault surface with respect to North.

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Slip Slip is the angle

used to describe the orientation of the movement of the hanging wall relative to the foot wall.

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Faulting Styles There are four basic styles of faulting

– Normal– Reverse– Strike-Slip– Oblique

The type of faulting depends on the slip direction (the movement of the hanging wall with respect to the foot wall).

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Slip Direction

If the slip is in the direction of the dip, we call it a dip-slip motion.

If the slip is in the direction of strike, we call it a strike-slip (or transform) motion.

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Normal Faulting

The hanging wall slides down the fault - as you would expect (that’s why it’s called “normal”).

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Faults (Fairview Valley, NV)

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Reverse Faulting

The hanging wall is pushed up the fault - not what you would expect (that’s why it’s called “reverse”).

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Strike-Slip Faulting The hanging wall

horizontally (no motion in the direction of fault dip).

There are 2 cases depending on how the rocks on the other side of the fault move - right lateral and left lateral.

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SF ‘06 - Left or Right Lateral?

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Oblique Faulting

A combination of dip-slip and strike-slip motion.

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Faulting Summary

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Stress Stress is a force per unit area. Examples:

Pressure Friction

Stress is an important parameter in faulting. A fault “fails” when the stress on the fault becomes larger than the frictional forces holding the fault together.

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Faulting and Stresses

The style of faulting (normal, reverse, etc.) also tells us about the stresses acting within Earth.

We describe the stresses by considering three stresses, two horizontal and the vertical.

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Normal Faulting Stresses

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Reverse Faulting Stresses

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Strike-Slip Faulting Stresses

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Stress & Faulting Summary

At any place there are three principal stresses acting on a fault.

If the vertical stress Largest - Normal Faulting Smallest - Reverse Faulting Intermediate - Strike-Slip Faulting

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Faulting and Seismograms

The nature of faulting affects the amplitudes and shapes of seismic waves (this allows us to use seismograms to study the faulting).

We call the variation in wave amplitude with direction the radiation pattern.

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Radiation Patterns

Radiation patterns are common in the study of sources of just about anything.

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Faulting & Shear Waves Faulting

generates large shear waves because earthquakes release shear strains stored in the rocks around the fault.

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Radiation Patterns in 3D

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Seismic “Beach Balls”

We use the radiation patterns of P-waves to construct a graphical representation of earthquake faulting geometry.

The symbols are called “Focal Mechanisms” or “Beach Balls”, and they contain information on the fault orientation and the direction of slip.

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Representing a Plane

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The Principal Mechanisms

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Faults and Plates

(1)The style of faulting tells us something about the forces acting in a particular part of Earth.

(2)Along plate boundaries, faulting reflects the motion of plates.– Divergent Boundary = Normal Faulting– Convergent Boundary = Reverse Faulting– Transform Boundary = Strike-Slip Faulting

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Example: East Africa

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Summary Faults are rock fractures across which the rocks

have moved.

Earthquakes occur along faults.

Faulting geometry (strike, dip, slip) is related to the stresses acting on the rocks.

The three faulting styles (normal, strike-slip, and reverse) reflect the deformation occurring within Earth and vary systematically with plate boundary style