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If the Earth’s a rockin’….
• Deformation– Elastic deformation (rubber band)—object
being deformed stretches building up potential energy
• Remove stress energy released object returns to normal
– Everything has limits• Stretching a rubber band until it breaks
– Rocks behave similarly
Where do earthquakes occur
• Tend to form along plate boundaries
• Earth’s surface is divided into ~16 plates
• Plates interact by ramming into and sliding past each other
Earthquake Strength• Old system—Mercalli scale
– Based on building damage
– inaccurate
• Richter Scale (ML)
– Magnitude 5.0 has amplitude 10x that of 4.0
– Based on height of largest peak on seismograph
– Richter less accurate above ML=6.5
• Moment Magnitude (Mw)
– Total energy expended during quake
– Measured from fault offset, area affected
– Each increment is 30 times stronger than the one previous to it
Transform Boundary EQs
• Transform margin– Plates grind past each
other—strike-slip fault– One large fault—
numerous smaller ones
• San Andreas Fault Zone– 10,000 Eqs/year– Periodic large quakes
Great San Francisco Quake of 1909
• Mw 7.8• City of San Fran
completely destroyed• Fires caused by
ruptured gas mains caused 99% of the damage—rebuilding costs comparable to Hurricane Katrina
• 3000 people died
Strike-slip motion
• 3 ways that the NAM plate slides past the Pac plate– Fault creep—smooth– Hopping—movement
occurs as a series of jumps—series of small Eqs
– Catastrophic breaks—fault sticks and releases generating one big Eq
Convergent Plate Eqs
• Benioff zone—area along top of subducting plate frequented by Eqs
• Tend to be large and devastating– Coastal population threat
• Tsunamis often generated– “harbor wave”– Huge waves caused by displacement of water
Sumatra Tsunami
• 7:59 AM, Dec. 26, 2004
• Mw=9.0
• Subduction Zone– Indian + Burma Plate– 15 m total movement,
seafloor rises sev. m– 700 kph (420 mph)– Total deaths: 283,000
(~2.5x pop of McKinney)
Why There? Why So Big?
• Indian moving NE at 6 cm/yr
• Plate was stuck– For likely 100s of
years– Burma Plate
bends
Burma Plate
Indo-Australian Plate
• Burma Plate flexes/moves
• 1200 km length of subduction zone displaced vertically 15 m
• Causes sea level to rise/fall
• Big wave generated
Tsunami Like an Ocean Wave
• A particle on the surface moves in a circular fashion– Effects of waves disappear at
about ½ wavelength– Tsunami wavelength ~ 180 km
(much deeper than oceans)– Tsunami drags on ocean floor
Sumatra Tsunami, 2004
• Waves were ~10m (32’) and 24m locally
• In Sumatra and Thailand – people live on coast– Homes are usually less than 15’– Energy carries waves inland, maybe 1-8 km– And then wave goes back out– Time between waves can be >30 minutes– The benefit of a good education
Damage Control
• People outside are safer (even in tsunami)People outside are safer (even in tsunami)
• Biggest danger is overheadBiggest danger is overhead– Neighboring buildingsNeighboring buildings– Unsupported buildings, glass, and just about Unsupported buildings, glass, and just about
anything not nailed down anything not nailed down
• If caught inside:If caught inside:– Lie down next to something that won’t flatten Lie down next to something that won’t flatten
(not in doorways or under table)(not in doorways or under table)
Politics and Predictions
• Hysteria vs. withholding information
• Predictions and chance
• The need for short-term predictionsThe need for short-term predictions
• Uncertainty!Uncertainty!
Tangshan 1976Tangshan 1976242,769 deaths242,769 deathsNo warningNo warning
Problems and Precursors
• Large earthquakes and changing stress– Large earthquakes can put stress on
neighboring faults
• Foreshocks
• Radon Gas??– Fracturing of rock allows radon gas to seep
into foundations
• Earthquake frequency changes, clustering
Long-Term Forecasts
• If you knew decades in advance:– Construction projects, insurance
• More reliable than short-term predictions
• Recurrence interval of past events
• Risk Maps