EarthquakesEarthquakesChapter 12Section 2

100% Chance of an 100% Chance of an Earthquake Today!Earthquake Today!Somewhere today, an

earthquake will occur.

A major portion of the world’s quakes center around the edges of lithospheric plates.

Frequency of OccurrenceFrequency of OccurrenceDescriptor Magnitude Average Annually

Great 8 + 1

Major 7 – 7.9 18

Strong 6 – 6.9 120

Moderate 5 – 5.9 800

Light 4 – 4.9 6,200 (est.)

Minor 3 – 3.9 49,000 (est.)

Very minor Less than 3 Mag. 2-3 1,000/day

Mag. 1-2 8,000/day

Think - Pair - ShareThink - Pair - Share

What is an earthquake?

• Divergent boundaries are associated plates that move in opposite directions.

• This faulting creates a narrow band of numerous, shallow earthquakes.

Movement of the Plates

• Convergent boundaries have broad zones of earthquakes.

• Shallowest are near the surface, deepest under volcanic mountains.

• An earthquake is any seismic vibration of Earth caused by the rapid release of energy.

Causes of Earthquakes

• A strain is deformation in response to a stress.

• Deformation: A change in shape.

Deformation

• Stress is the force per unit area that acts on a material.

(1) compressive stress

(2) tension stress

(3) shear stress

Types of Stress

(4) torsion stress

• Elastic deformation: a material deforms as a stress is applied, but returns to its original shape when the stress is removed.

• Example = rubber band

Types of Deformation

• Plastic deformation: a material deforms, or changes shape, as a stress is applied and remains in the new shape when the stress is released.

• Example = modeling clay

Demonstrating Four Types of Demonstrating Four Types of StressStress1. Which type of stress did you

demonstrate?

• The point on Earth’s surface directly above the focus is the epicenter.

• Earthquake waves travel out in all directions from a point where strain energy is released. This point is the focus.

Earthquake Waves

WavesWavesBody Waves

P = Primary, particle movement is in the direction of travelS = Secondary, particle movement is perpendicular to direction of travel

Surface WavesL = Love, particle movement is side-to-sideR – Rayleigh, particle movement rolls like ocean waves

Surface Waves

Earthquake Measurement• The Modified Mercalli

scale ranks earthquakes in a range from I-XII.

XII being the worst and uses eyewitness observation and post-earthquake assessments to assign an intensity value.

Earthquake Measurement

• Richter magnitude is intended to give a measure of the energy released during the earthquake.

• The Richter magnitude scale uses the amplitude of thelargest earthquake wave.

Question 1

Which of the following is NOT a type of stress in rock?

A. compressionB. epicenterC. shearingD. tension

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Answer

The answer is B. The epicenter is the point on Earth’s surface located directly above the earthquake’s center.

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Question 2Where do P- and S-waves occur in relation to surface waves?

AnswerSeismic waves travel away from the epicenter in all directions. P-waves travel the fastest through rock material. S-waves move through the rock and cause particles to vibrate. Both P- and S-waves travel through the Earth’s interior while surface waves move along Earth’s surface.

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Question 3Why is it difficult to predict earthquakes?

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AnswerGeologists can monitor changes in Earth that are associated with earthquakes. Measuring devices have been developed to assess changes in groundwater level and rock layers; however, no single change in Earth occurs for all earthquakes.