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Stick-Slip Activity Intro

Exploring an earthquake model

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What is Stick-Slip?

• The jerking motion that often occurs when two surfaces are sliding over each other.– The surfaces alternate between sticking and

slipping– Common examples

• Music from a violin (bow across string)• Sound of noisy brakes on a car• Screeching car tires• Glass Harp (wet finger around the rim of a wine glass)

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What is Required for Stick-Slip?• The force required to “un-stick” the surfaces is greater

than the force required to keep the surfaces moving.

– The static coefficient of friction must be higher than the kinetic coefficient of friction

• A force that varies with displacement is also required.– An elastic (“springy”) material stores elastic energy as it

deforms and could supply such a force.

Rigid materials are elastic to some degree – even rock!

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The Stick-Slip Cycle

One cycle of stick-slip:1. Surfaces are stuck2. An external shear force is applied (continuously)3. Elastic material deforms, storing elastic energy4. Surfaces remain stuck, since the force delivered to the

contact area is less than the force needed to overcome static friction

5. Eventually the force at the contact area overcomes static friction – the surfaces begin to slide

6. Stored energy in the elastic material is released, causing the surfaces to accelerate as they slide past each other

7. As the elastic material returns to its previous shape (elastic rebound) the force delivered to the surfaces decreases until it can no longer overcome kinetic friction and the surfaces stop moving

8. The surfaces are stuck again

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“Stick”

“Slip”

Stick-Slip on an Earthquake Fault

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Friction along fault segment

Deformed rocks store elastic potential energy Elastic energy

released!

EARTHQUAKE!

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Earthquake Slip Activity

• Demo

• Workshop Activity

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Vocabulary• Creep: Slow, steady fault movement.

• Deformation: The change in shape of an object as a result of the application of stress to it.

• Elasticity: The property of returning to an initial form following deformation.

• Elastic rebound: As plates move, elastic strain energy builds along edges, until rocks rupture and return to their undeformed shapes.

• Strain: deformation of the earth.

• Static friction: The force between two objects that are not moving relative to each other. Friction that keeps things from moving.

• Stress: A force applied over an area.

• Stick-slip: Adjacent surfaces alternating between sticking to each other and sliding past each other.