earthquakes lecture
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Earthquakes
Chapter 19
Earthquakes
Natural vibrations of ground caused by movement along fractures in Earth’s crust or by volcanic eruptions
Stress and Strain
Stress- forces per unit area exceed the strength of the rocks involvedCompression-Decreases VolumeTension- Pulls material apartShear- Causes Twisting
Strain- Deformation of materials caused by stress
Faults- fracture of Earth’s crust
Reverse- Horizontal compression, Causes shortening, Foot wall goes down
Faults- Con’t
Normal- Horizontal tension, Causes extension, Foot wall goes up
Faults- Con’t
Strike-Slip- Horizontal Shear, causes offset
Waves
Primary- P- Pressure Wave Secondary- S- Side-to-side wave Surface Wave- L-Wave
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Location of Earthquakes
Focus- point where an earthquake originates
Epicenter- Place on Earth's surface where EQ occurred
Seismic Waves
Seismology- Study of Earthquake waves Seismograph- instrument for recording
earthquake waves
Waves
P wave- arrives 1st
S wave- arrives 2nd L wave- last and largest
Clues to Earth’s Interior
Seismic waves change speed and direction when they encounter different materialsP and S waves travel fairly straight through
the mantleP waves strike the core and are bent (11,000
Km from focus) Re-emerge at 16,000 km from focus, after shadow
Clues to Earth’s Interior- Con’t
S-waves cannot travel through liquids so they can’t pass through the core (never beyond shadow)
P waves speed up through solids
Measuring and Locating EQ
Magnitude- amount of energy releasedRichter Scale- size of largest seismic waves
released by quake Each # increase is a 10 fold increase in energy Each increase in amplitude (height of wave) is a 32
time increase in energy
Measuring and Locating EQ- Con’t
Moment Magnitude Scale- uses size of fault rupture, amount of movement and rock’s stiffness
Modified Mercalli Scale- uses damage to buildings at location of worst damage Uses I-XII Roman Numerals- Called Intensity Measures Surface Waves
Depth of Focus- Deeper EQ usually less intense
Measuring and Locating EQ-Con’t
Locating EQ- time and location unknownDistance determined by time between P wave
and S wave--- AT 3 STATIONSTime calculated using same P-S interval
Seismic Belts
Earthquakes not randomRing of Fire- Edge of Pacific Plate- 80% of all
EarthquakesMediterranean- Asian Belt- 15% of EQ
Earthquakes and Society
Earthquake Hazards Structural failure
Ground floor fails → Upper Floors Fail → Pan caking Height- 5-15 stories causes vibrations which are similar to
natural sway; Shorter and taller buildings are different Land/Soil Fracture
Landslides- Common in saturated areas Soft ground conducts waves well → more damage
Fault Scarps- vertical offsets → waterfalls, road problems
Earthquakes and Society- Con’t
Tsunami- ocean waves created by EQTravels fast and are low (less than 4 feet
high) in open ocean, but pile up hundreds of feet tall near shore
Ocean recedes dramatically before tsunami
Earthquake Risk
Here we have a 3% risk over 50 years Hot Spot in US:
CARocky MountainsWA, ORMO