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The sudden release of energy accumulated in
deformed rocks.
Seismology -the study of earthquakes.
What is an Earthquake?Danny M. Vaughn, Ph.D., CMS
Anatomy of an Earthquake
• Earthquake – a sudden release of strain energy stored along stressed boundaries. • All energy moves in the form of wavelengths. • Some elastic behavior in shallowdepths w/ cooler, more brittle rocks.• Failure occurs (yield point) w/ fracturing & faulting.• Plates move releasing energy as seismic (shaking) waves.
(Hypocenter)
Body waves• P-
waves• S-wavesSurface
waves• Compression forces contract & expand (dilate) the rocks.• First to arrive from the focus.• Crustal velocity: ~ 4 mi/sec. (6-7 km/sec.).• Wavelengths travel in all directions.
Seismic Body Waves
Primary (P) Waves
Seismic (Body) WavesSecondary (S) Waves
Body waves• P-waves• S-
wavesSurface
waves• Crustal velocity: ~ 2 mi/sec. (3.5 km/sec.).• Rocks move perpendicular (up & down) to the direction waves travel.• Vertical drag occurs as the wave passes, sheering rocks.
Shape is changed due to sheering.Volume remains unchanged.
Body waves• P-waves• S-wavesSurface
waves• Surface vibrations are initiated by body waves.• Crustal velocity: ~ 1.5 mi/sec (2.5 km/sec.).• Results in most damage to structures.
Seismic (Surface) Waves
Measuring Earthquakes
Mercalli Intensity Scale
• Intensity (amount of energy released), compared to destructiveness.• Fails to accurately locate an epicenter.• Inconsistent in comparing different rock types, building structure damage from poor construction. • Not useful in no population regions.
Measuring EarthquakesSeismographs & the Richter
Scale
Measures amplitude of largest peak.Logarithmic scale.Magnitude 1 to 2 = 10x increase in wave Amplitude.~33x increase in energy released.
e.g. 33x energy released from a 6 to 7;33x33 = 1,089x energy released from a6 to 8 magnitude earthquake.
therefore,
It would take >1,000 magnitude 6 quakesto release the accumulated energy of onemagnitude 8 quake.
Calibrated for California rocks, ineffective >7.0.
Measuring Earthquakes
Moment-magnitude scale
Seismic moment (total energy released) =(Total length of fault rupture) x
(Depth of fault rupture)x (Total amount of slip along rupture)
x (Strength of rock)• Longer fault; greater potential for a large quake.• Stronger rock, greater energy storage potential, larger quakes.• Present reporting system.• Chilean quake (1960) magnitude 9.5 (8.5 Richter).
Effects of Earthquakes
• Ground displacements
• Landslides• Liquefaction• Seiches• Tsunami• Fires
• What type of fault is this? • What is the direction of displacement?
(Strike-slip fault)
(Right lateral)
Effects of Earthquakes
• Ground displacements
• Landslides• Liquefact
ion• Seiches• Tsunami• Fires• Unconsolidated sediment.
• Silts & sands, some clay as best candidates.• Weakly cohesive (particle attractiveness).• Sheer strength (resistance to stress) weakened by vibration.• Water is disassociated from particles.• Cohesiveness & sheer strength are diminished.• Flowage ensues.
Effects of Earthquakes
• Ground displacements
• Landslides• Liquefacti
on• Seiches
(saych)• Tsunami• Fires
The rising and falling of water as it sloshes back and forth in an enclosed body of water, such as a lake or bay.
Standing waves w/ up & down movement.
Formation of Tsunami
Caused by –
• Vertical displacement on the seafloor due to faulting.• Submarine landslides.
Effects of Earthquakes
Tsunami
• Ground displacements
• Landslides
• Liquefaction
• Seiches• Tsunami• Fires
(December, 2004): Maximum wave height: ~90 ft.
Effects of Earthquakes
• Ground displacements
• Landslides
• Liquefaction
• Seiches• Tsunami• Fires
Homes surviving the 1906 San Francisco earthquake.
Now can I have that new car,dear?
Earthquakes and Plate Boundaries
• Earthquakes occur where energy builds up in cooler, brittle rocks.• Depth is variable dependent on plate boundary.• Major quakes occur along convergent (subduction) & transform boundaries.• Most stored energy.• Maximum depth ~450 miles. What controls the depth?
Intraplate EarthquakesNew Madrid, Missouri, 1811-12
Reelfoot Lake, Tennessee
Three major 8.5 shocks, 1,500 after shocks over 53 days.
Causes:• Unloading over old faults. • Isostatic rebound (ice removal).• Rainwater acting as a lubricant along faults.• Intraplate seismic activity. Rising softened mantle.• Plate motion creates frictional stress w/in the plate.
Coping with Earthquakes
Limit Quakes Caused by Humans
Relationship of injection of nerve gas waste to earthquake frequency.
Coping with EarthquakesAssessing Local Seismic History & Risk
What is not apparent about earthquakes on this map?
Coping with Earthquakes
Assessing Local Seismic History & Risk
What does the principle of cross-cuttingrelationships tell us about relative age?
Coping with Earthquakes
Careful Land-use Planning
Seismic waves shake more violently in unconsolidated deposits then in bedrock.
Coping with Earthquakes
Building Quake-resistant Structures
What questions need to be asked with respect to building structures in earthquake prone regions?
Coping with Earthquakes
Research on Earthquake Prediction
Long-range
forecastsSeismic gaps –
• Plates are temporarily locked.• Pressure builds.• Strain energy is released & activity along the fault dies down.
Anti-gap model –• Stress is not completelyreleased, rather increasesalong other faults.
Tectonic creep –• Gradual, continuous release of energy.
San Andreas Fault Observatory at Depth Project.
Understanding & Controlling Earthquakes
Releasing strain energy
Coping with Earthquakes
Research on Earthquake Prediction
Long-range forecasts:Seismic gaps
Regions overdue a major seismic event.
Coping with Earthquakes
Research on Earthquake Prediction
Short-range predictions
• Stress yields small cracks before complete failure.• Rocks expand in volume as cracks widen (dilate).• Dialancy produces side effects that aid in earthquake prediction.
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