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7.1

What Are Earthquakes?

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Describe what happens during an earthquake.

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Earthquakes

•The movements or shaking of the ground

•Mostly take place at plate boundaries

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Earthquakes

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Earthquakes

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Joke of the day

What do you call a cow during an earthquake?

A milkshake.

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Earthquakes at Divergent Boundaries

•Two tectonic plates pull away from one another

•Tension

•Normal fault

•Happens at mid-ocean ridge

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Earthquakes at Convergent Boundaries

•Two tectonic plates collide.

•This causes a mountain or subduction (one plate moves over another).

•Compression

•Reverse fault

•Plates thrust over one another

•Happens at subduction zones

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Earthquakes at Transform Boundaries

•Two plates move past one another horizontally

•Shearing

•Strike-slip fault

•Happens mostly between plates made of oceanic lithosphere, but does happen between continental lithosphere.

•Occur at shallow depths

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Why do earthquakes happen at transform boundaries?

At transform boundaries, shear stress causes the rock to break into a series of blocks. The

blocks form a series of strike-slip faults. Earthquakes happen along the strike-slip faults

when stress becomes too great.

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Boundaries

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Fault Zones

•Places along plate boundaries where large numbers of interconnected faults are located

•Example: San Andreas fault zone

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Joke Of the Day

Here in California, when a bridge falls down, who’s fault is it?

San Andrea’s fault

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Fault Zones

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Why Earthquakes Happen

•Plastic deformation – deform like clay being molded

•Elastic deformation – rock is stretched like a rubber band

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Elastic rebound

(when elastic deformation returns to its original shape)

•Causes fault to jerk and break

•Leads to earthquakes

•Energy is released

• seismic waves- energy travels through rock

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How does elastic rebound cause earthquakes?

Tectonic forces push rock on either side of a fault in opposite directions. Because the rock is locked together, it does not move and deforms in an elastic manner. When

enough stress builds up, the rock along the fault fails, the blocks of rock on either side of the fault move, and energy is released as an

earthquake.

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Earthquake Waves

Body Waves (Seismic waves) – waves that travel through Earth’s interior•P Waves

•Pressure waves or primary waves

•First waves to be detected

•Travel through solids, liquids, and gases

•S Waves

•Shear waves or secondary waves

•Move side to side

• Cannot travel through liquids

•Slower than P waves

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How do P waves and S waves differ in the way that they transfer energy?

P waves transfer energy by squeezing and stretching rock. S waves transfer

energy by shearing rock horizontally form side to side.

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P Waves

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S Waves

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Surface Waves

•Move only along the Earth’s surface.

•Cause the most damage

•Travels slower than body waves

•One type is rolling, up and down motion

•Another type is back and forth motion

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Explain why surface waves are the most destructive type of seismic waves.

The energy of surface waves is concentrated at the surface of Earth.

Therefore, surface waves tend to cause a lot of damage.

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Seismic Waves

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Review Questions

1. Describe the three types of plate motion and the faults that are characteristic of each motion.

Normal faults are characteristic of divergent motion, where plates pull apart. Reverse faults are characteristic of convergent motion, where plates collide. Strike-slip faults are characteristic of transform motion, where plates slide past each other horizontally.

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2. How do the locations of earthquakes support the theory of plate tectonics?

Most earthquakes occur at tectonic plate boundaries rather than in the interior of tectonic plates. This indicates that most active faults occur at tectonic plate boundaries as a result of plate motion.

3. Why are surface waves more destructive to buildings than P waves or S waves are?

Surface waves are more destructive to buildings than P waves and S waves are because the energy of surface waves is concentrated at Earth’s surface.

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4. Imagine that you are splashing water in a pool. Explain how you could model each type of seismic wave. Then, try to explain why S waves do not travel through liquids.

You could model P waves in a swimming pool by pushing water out in front and away from you. You could not model S waves in a swimming pool, and S waves do not move through liquid, because you cannot create a shearing motion by pushing liquid back and forth. You could model surface waves in a swimming pool by bobbin up and down in the pool, which would create an up and down motion in the water.

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Review 7.1•Earthquake•Body waves•P wave•S wave•Surface waves•Seismic waves•Elastic deformation•Elastic rebound•Normal fault•Reverse fault•Strike-slip fault•Stress on Earth because of plate movement