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Chapter 11: Earthquakes

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Introduction Earthquake: Vibration of the Earth produced by rapid release of energy Most often caused by slippage along a fault Energy is in the form of waves Focus: AKA Hypocenter Source of earthquake from which energy is released in all directions Epicenter: Surface location directly above the focus Seismometer: Instrument sensitive to vibrations Located worldwide & used to measure earthquakes Earthquake: Vibration of the Earth produced by rapid release of energy Most often caused by slippage along a fault Energy is in the form of waves Focus: AKA Hypocenter Source of earthquake from which energy is released in all directions Epicenter: Surface location directly above the focus Seismometer: Instrument sensitive to vibrations Located worldwide & used to measure earthquakes

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Focus & Epicenter

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Elastic Rebound Stress applied along pre-existent fault Stress builds up, straining the rock Eventually, frictional resistance is overcome Slippage occurs, releasing the built up strain Vibrations (earthquake) occur as the rock elastically returns to original shape Stress applied along pre-existent fault Stress builds up, straining the rock Eventually, frictional resistance is overcome Slippage occurs, releasing the built up strain Vibrations (earthquake) occur as the rock elastically returns to original shape

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Foreshocks & Aftershocks Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks

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San Andreas Fault Best studied fault system Fault creep: Slow, gradual displacement Some segments slip at ~regular intervals Results in small to moderate quakes Stick-slip motion: Storing elastic energy for hundreds of years before rupturing in great earthquakes Quakes every 50 to 200 years Best studied fault system Fault creep: Slow, gradual displacement Some segments slip at ~regular intervals Results in small to moderate quakes Stick-slip motion: Storing elastic energy for hundreds of years before rupturing in great earthquakes Quakes every 50 to 200 years

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Seismology Study of earthquake waves Dates back 2000 years to the Chinese Seismographs: Instruments which record earth’s movement relative to stationary mass Stationary due to high inertia Study of earthquake waves Dates back 2000 years to the Chinese Seismographs: Instruments which record earth’s movement relative to stationary mass Stationary due to high inertia

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Seismographs Vertical motion Horizontal motion

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Parts of Waves Frequency = 1/T Types of seismic waves: P-waves S-waves Surface waves Frequency = 1/T Types of seismic waves: P-waves S-waves Surface waves T

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P-waves P rimary waves Com p ressional waves ( AKA P ush-pull waves) Travel through liquids & solids P rimary waves Com p ressional waves ( AKA P ush-pull waves) Travel through liquids & solids

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S-waves Secondary waves Shake waves Travel only through solids Secondary waves Shake waves Travel only through solids

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Surface Waves Travel along Earth’s surface Greatest amplitude, slowest velocity Cause greatest destruction AKA Long waves or L-waves Travel along Earth’s surface Greatest amplitude, slowest velocity Cause greatest destruction AKA Long waves or L-waves

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Locating the Source Epicenter located from difference in arrival time of P-wave and S-wave Requires seismograms from three locations AKA Triangulation Epicenter located from difference in arrival time of P-wave and S-wave Requires seismograms from three locations AKA Triangulation

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Triangulation

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Practice Triangulation LocationTime Interval Distance (km) Distance (cm) Cape Town 13 min 30 sec Rio de Janeiro 10 min 10 sec Luanda, Angola 7 min 45 sec

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Practice Triangulation LocationTime Interval Distance (km) Distance (cm) Cape Town 13 min 30 sec 4800 Rio de Janeiro 10 min 10 sec Luanda, Angola 7 min 45 sec

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Practice Triangulation LocationTime Interval Distance (km) Distance (cm) Cape Town 13 min 30 sec 4800 Rio de Janeiro 10 min 10 sec 3700 Luanda, Angola 7 min 45 sec

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Practice Triangulation LocationTime Interval Distance (km) Distance (cm) Cape Town 13 min 30 sec 4800 Rio de Janeiro 10 min 10 sec 3700 Luanda, Angola 7 min 45 sec 2900

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Practice Triangulation LocationTime Interval Distance (km) Distance (cm) Cape Town 13 min 30 sec 4800 Rio de Janeiro 10 min 10 sec 3700 Luanda, Angola 7 min 45 sec 2900 1.2 cm = 1050 km 1.2 cm = x 1050 km distance Luanda

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Practice Triangulation LocationTime Interval Distance (km) Radius (cm) Cape Town 13 min 30 sec 4800 5.5 Rio de Janeiro 10 min 10 sec 3700 Luanda, Angola 7 min 45 sec 2900 1.2 cm = 1050 km 1.2 cm = x 1050 km distance Luanda

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Practice Triangulation LocationTime Interval Distance (km) Radius (cm) Cape Town 13 min 30 sec 4800 5.5 Rio de Janeiro 10 min 10 sec 3700 4.2 Luanda, Angola 7 min 45 sec 2900 1.2 cm = 1050 km 1.2 cm = x 1050 km distance Luanda

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Practice Triangulation LocationTime Interval Distance (km) Radius (cm) Cape Town 13 min 30 sec 4800 5.5 Rio de Janeiro 10 min 10 sec 3700 4.2 Luanda, Angola 7 min 45 sec 2900 3.3 1.2 cm = 1050 km 1.2 cm = x 1050 km distance Luanda

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Measuring Earthquake Sizes Intensity Scale Mercalli Intensity Scale Based on destruction of buildings Magnitude Scales Richter Magnitude (logarithmic) Based on the Amplitude of the largest seismic wave recorded on seismogram Intensity Scale Mercalli Intensity Scale Based on destruction of buildings Magnitude Scales Richter Magnitude (logarithmic) Based on the Amplitude of the largest seismic wave recorded on seismogram

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Mercalli Intensity Scale

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Richter Magnitude Scale

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Earthquake Destruction Seismic Vibrations Tsunami Landslides & Ground Subsidence Fire Seismic Vibrations Tsunami Landslides & Ground Subsidence Fire

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Formation of Tsunamis

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Global Distribution of Earthquakes

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Evidence for Plate Tectonics

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