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Published byHenry Todd Modified over 6 years ago
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What are Earthquakes? The shaking or trembling caused by the sudden release of energy Usually associated with faulting or breaking of rocks Continuing adjustment of position results in aftershocks Aftershock: small earthquakes following a large earthquake
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What is the Elastic Rebound Theory?
Explains how energy is stored in rocks Rocks bend until the strength of the rock is exceeded Rupture occurs and the rocks quickly rebound to an undeformed shape Energy is released in waves that radiate outward from the fault
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What is the Elastic Rebound Theory?
Due to motion along a fault Fault is a broken rock formation Opposite sides stretch Force applied to the rocks eventually exceeds the strength Force is called stress The rock moves Strain is the movement of the rocks Strain occurs along plate boundaries Caused by explosions or volcanic eruptions System returns to unstrained conditions (Rebound)
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The Focus and Epicenter of an Earthquake
The first rupture occurs at the focus…deep in the Earth The point of the Earth’s surface above this rupture is called the epicenter Deep focuses cause little damage Beneath 700 km the rock is too hot and plastic to break….it flows
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Seismographs record earthquake events
Seismograms are the records that are produced
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Types of Faults: Dip-slip (vertical motion)
Normal fault Hanging wall moves down compared to footwall Reverse Fault Hanging wall moves up compared to footwall
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Types of Faults: Strike-slip (horizontal motion)
Lateral right Motion is to the right Lateral left Motion is to the left
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What are Seismic Waves? Response of material to the arrival of energy fronts released by rupture Two types: Body waves P and S Surface waves R and L
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Body Waves: P and S waves
P or primary waves fastest waves travel through solids, liquids, or gases compressional wave, material movement is in the same direction as wave movement S or secondary waves slower than P waves travel through solids only shear waves - move material perpendicular to wave movement
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Surface Waves: R and L waves
Travel just below or along the ground’s surface Slower than body waves; rolling and side-to-side movement Especially damaging to buildings
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How is an Earthquake’s Epicenter Located?
Seismic wave behavior P waves arrive first, then S waves, then L and R Average speeds for all these waves is known After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.
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How is an Earthquake’s Epicenter Located?
Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves
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How is an Earthquake’s Epicenter Located?
Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter
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Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in the circum-Pacific belt most of these result from convergent margin activity ~15% occur in the Mediterranean-Asiatic belt remaining 5% occur in the interiors of plates and on spreading ridge centers more than 150,000 quakes strong enough to be felt are recorded each year
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How are the Size and Strength of an Earthquake Measured?
Intensity subjective measure of the kind of damage done and people’s reactions to it Based on geology, construction, and perception Modified Mercalli Intensity Map 1994 Northridge, CA earthquake, magnitude 6.7
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Mercalli Scale
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How are the Size and Strength of an Earthquake Measured?
Magnitude Richter scale measures total amount of energy released by an earthquake; Maximum amplitude Correct to a standard seisometer Correct for distance from epicenter Logarithmic in microns
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Richter Scale
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What are the Destructive Effects of Earthquakes?
Ground Shaking amplitude, duration, and damage increases in poorly consolidated rocks
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Can Earthquakes be Predicted?
Earthquake Precursors changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the ground
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Can Earthquakes be Predicted?
Earthquake Prediction Programs (Statistical models) Based on uniformitarianism Recurrence levels Fault history
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Can Earthquakes be Predicted?
Earthquake Prediction Programs (Geophysical models) Ground tilt Magnetism Resistivity Seismic velocity Radon emission Foreshocks Stress changes
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Can Earthquakes be Predicted?
Earthquake Prediction Programs Seismic Gaps: active faults that are not broken Animal Behavior Predictability based on magnitude, occurrence, rupture length and displacement
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Can Earthquakes be Controlled?
Graph showing the relationship between the amount of waste injected into wells per month and the average number of Denver earthquakes per month Some have suggested that pumping fluids into seismic gaps will cause small earthquakes while preventing large ones
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Earthquake Hazards Ground shaking Surface ruptures
Changes in ground level Landslides Liquefaction Fires Tsunamis
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Ground Shaking What most people think about when you say earthquake
Lessens with distance
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Surface Ruptures Very rare Occurs in the movies
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Changes in Ground Level
Large areas of land moved up or down
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Landslides Unstable ground loosens and moves
Can include snow, rock, debris, and ice Depends on the slope, vegetation, and rainfall Can be very deadly
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Liquefaction Saturated ground “liquefies” and can no longer hold the weight Buildings sink
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Fires Indirect damage of an earthquake Ruptured gas lines
Downed power lines Ruptured water lines All problems created by civilization
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Tsunamis Vertical displacement of a large amount of water
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The Economics and Societal Impacts of EQs
Damage in Oakland, CA, 1989 Building collapse Fire Tsunami Ground failure
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