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What are Earthquakes?
• Systemic Waves:The shaking or trembling caused by the sudden release of energy
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• Usually associated with faulting or breaking of rocks
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• Continuing adjustment of position results in aftershocks
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
Body Waves: P and S waves
• Body 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
Origins of Earthquakes
• Focus:Exact point in the earth where rock fracture happens (varies in depth. no
deeper than 700 KM)
• Epicenter:Surface of the earth directly above the focus point.
• Focal Depth:Distance between the focus and the epicenter (varies greatly from one
quake to another)
Origins of Earthquakes Cont...
Causes of Earthquakes:
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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Causes of Earthquakes:
The Focus and Epicenter of an Earthquake
Causes of Earthquakes
• Underground movement of magma (in active Volcano’s can cause an earthquake)
• Isostatic readjustment:Land masses sink or rise due to adjustment in their weight.
Examples: Ice or large dam reservoir could put enough pressure on the land below
which can trigger an earthquake.
• Subsidence: removal of deep pockets of groundwater (also causing earth tremors)
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Causes of Earthquakes Continued...
• Look at Fig. 8.2 pg.15 (Txt Book)
• 1.) Fault is locked: rock under stress=elastic deformation
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• 1.) Fault is locked: rock under stress=elastic deformation
• 2.) Stress+ Elastic= deformation continues
• 3.) Fault Slips= pressure released (EARTHQUAKE OCCURS!!!!)
Where Do Earthquakes Occur and How Often?
Where do Earthquakes Occur?
• Plate Boundaries:
• Along these cracks in the Lithosphere one plate grinds against another plate. (Not
always smooth and stress can build up)
• Each time sudden movement happens, it can cause a strong earthquake
Where do Earthquakes Occur?
Seismologists in Action
• Seismographs: Instruments that records earthquakes.
• Richter Magnitude Scale: Strength of earthquake. Scale is from 1 (weakest) to 8.9
(strongest). Each # on the Richter scale represents a magnitude 10 times larger
than the previous number.
• Ex: Earthquake of 7 is 10X larger than a 6 and 100X larger than a 5.
• Tremors that are less than 3 won’t be felt. Magnitude 5-5.9 causes damage but
usually doesn't collapse building. Magnitude 6.5 and more will cause a lot of
damage.
How are the Size and Strength of an Earthquake Measured?
• Magnitude
– Richter scale measures total amount of energy released by an earthquake;
independent of intensity
– Amplitude of the largest wave produced by an event is corrected for distance and
assigned a value on an open-ended logarithmic scale
Shock waves generated by Earthquakes
• Right after an earthquake, shock waves race outward.
• 3 Kinds of Waves:
• a.) Compression Waves
• b.) Shear Waves
• c.) Surface Waves
#1 Compression Wave
• Primary wave that travels by compressing and expanding the ground it travels
through.
• It moves the fastest.
• 1st wave to reach earthquake recording stations
#2 Shear Wave
• Secondary wave that’s generated by an earthquake.
• Slower than the 1st wave (Compression) and this wave travels side-to-side motion.
#3 Surface Waves: R and L waves
• Surface Waves
– Travel just below or along the ground’s surface
– Slower than body waves; rolling and side-to-side movement
– Especially damaging to buildings
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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be used to calculate the distance from the seismograph to the epicenter.
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
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
Can Earthquakes be Predicted?
Earthquake Prediction Programs
– include laboratory and field studies of rocks before, during, and after earthquakes
– monitor activity along major faults
– produce risk assessments
Most Common Earthquake Hazards
• Ground Motion: The stronger the earthquake, the greater the ground shaking.
• Fire: Often the #1 cause of destruction.
• Landslides: In hilly region landslides are expected when earthquakes happen.
• Liquefaction: When sandy soil is shaken up during an earthquake, the ground
becomes unstable like quick sand potentially causing buildings sink or collapse. This
can happen in areas like river deltas and dry lake beds.
• Tsunamis: Huge waves can happen when a sudden ground slippage occurs on the
sea floor and causes an earthquake.
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