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Chapter 19 Earth quakes

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I. Forces within Earth

A. Stress and Strain

1. Most earthquakes occur when rocks fracture, or break, deep within Earth.

2. Fractures form when stress, forces per unit area acting on a material exceeds the strength of the rocks involved.

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3. Compression is stress that decreases the volume of a material.

4. Tension is stress that pulls a material apart.

5. Shear is stress that causes a material to twist.

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Compression Strain

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Tensional strain

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Shear Strain

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B. Faults

1. A fracture along which movement occurs is called a fault.

2. There are three basic types of faults, reverse, normal and strike-slip fault.

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3. Reverse faults are fractures that form as a result of horizontal compression.

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4. Normal faults are fractures caused by horizontal tension.

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Strike-slip faults are fractures caused by horizontal shear.

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Top-reverse/ Center –normal/ Bottom- Strike Slip

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C. Earthquake waves

1. Primary waves or P-waves squeeze and pull rocks in the same direction along which the waves are traveling.

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2. Secondary waves, or S-waves cause rocks to move at right angles in relation to the direction of the waves.

3. Surface waves are a third type of seismic wave that move in two directions as they pass through rock.

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4. The point where an earthquake originates is the focus of the earthquake.

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II. Seismic Waves and Earth’s Interior

A. Seismometers and Seismograms

1. Seismometer is an instrument that detect and record vibrations.

2. Seismogram- the record produced by a seismometer.

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19.3 Measuring and Locating Earthquakes

A. Earthquake Magnitude and Intensity

1. The amount of energy released during and earthquake is measured by its magnitude.

2. An earthquake’s rating on the Richter scale is based on the size of the largest seismic waves generated by the quake.

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3. The moment magnitude scale takes into account the size of the fault rupture, the amount of movement along the fault, and the rock’s stiffness.

4. Modified Mercalli scale rates the types of damage and other effects of an earthquake as noted by observes during and after its occurrence.

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B. Locating an earthquake

1. The exact location of an earthquake’s epicenter and the time of the quake’s occurrence are initially unknown.

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2. All epicenter locations, as well as times of occurrence, can be determined using seismograms.

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C. Seismic Belts

1. Seismologists have collected and plotted the locations of numerous earthquake epicenters.

2. Almost 80% of all earthquakes occur in the Circum-Pacific Belt.

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19. 4 Earthquakes and Society

A. Some earthquake hazards

1. The damage produced by an earthquake is directly related to the strength or quality of the structures involved.

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2. Most severe damage occurs to unreinforced buildings made of stone, concrete, or other brittle material.

3. Wooden structures are strong.

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4. Usually the supporting walls of the ground floor fail and cause the upper falls to fall and collapse.

5. In addition to their effects on structures made by humans, earthquakes can destroy Earth itself-landslides.

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6. Fault Scarps are fault movement associated with earthquakes can produce areas of great vertical offset where the fault intersects the ground surface.

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7. A tsunami is a large ocean wave generated by vertical motions of the seafloor during an earthquake.

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Tsunami before and after

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7. The probability of future quakes is much greater in the belts than elsewhere around the globe.

8. Seismic gaps are sections of active faults that haven’t experienced significant earthquakes for a long period of time.

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THE END