Earthquakes

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

The Stress Builds

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

Richter ScalePractical equivelents

avg. San Andreas quakes

Sumatran Tsunami quake

Earthquakes…globally

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

Video 1

Notice: bent Notice: bent waveswaves

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