Earthquakes

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Earthquakes
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The Day the World Series Died

• 504 PM, October 17th, 1989
• Game 3, World Series
• Warm-up before the first inning
• Bam! Candlestick Park started shaking
• Total 62 dead, 4000 wounded, 6 billion
• I-880

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How Do Earthquakes Work?

• 1906 San Francisco Quake A Mystery!
• Offset of fences, rocks, streams
• Cut rocks were of vastly different ages

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The Stress Builds
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Loma Prieta Magnitude 6.9 or 7.1?

• 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 ML6.5
• Moment Magnitude (Mw)
• Total energy expended during quake
• Measured from fault offset, area affected

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Earthquake Magnitude
avg. San Andreas quakes
Sumatran Tsunami quake
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Subduction and the Worlds Largest Quakes
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Sumatra Tsunami

• 759 AM, Dec. 26, 2004
• Mw9.0
• 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)

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Why There? Why So Big?

• Indian moving NE at 6 cm/yr
• Constant quakes

• Plate was stuck
• For likely 100s of years
• Burma Plate bends

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• Burma Plate flexes/moves
• 15 m in secs.
• Seafloor rises several m

• Causes sea level to rise/fall
• Big wave generated

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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

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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 gt30 minutes
• The benefit of a good education

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Notice bent waves
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Damage Control

• People outside are safer (even in tsunami)
• Biggest danger is overhead
• Neighboring buildings
• Unsupported buildings, glass, and just about
  anything not nailed down
• If caught inside
• Lie down next to something that wont flatten
  (not in doorways or under table)

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Volcanic Activity
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Volcanic Activity at Hot Spots and Mid-ocean
Ridges

• All Are Eruptions of Basalt

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Basalt

• High in Fe, Mg, Ca
• Low in silicon
• 1000-1200C
• Most common type of melt
• Very fluid up to 40 mph (ex Nyiragongo)
• Deposits generally thin (several meters)
• Very rarely explosive, mostly lava rivers

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Evidence of Low Viscosity
Hornito (formed from spatter)
Peles Tears
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Shield Volcano

• Gently sloped shield-shaped volcano several km
  high and 10s of km in diameter
• Built up of many overlapping runny lava flows
• Mauna Loa is taller than Mt. Everest (10 km) but
  most of it is under water
• Common above mantle plumes (hot spots)

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Fire-fountains
Eruption (p2fiss1.mov)

• Gas rises to top and explodes like soda pop
• Globs spatter to ground and cool
• Generally from linear fissures

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Pahoehoe (ropy) Lava

• Surface cools but internal parts stay molten
• Skin folds as material moves below

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Aa Lava

• Common expletive after walking on it
• Lost its gas, moves faster than pahoehoe
• Thick skin forms, breaks into jagged blocks which
  get pushed ahead like a plow

like so
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Lava Tubes and Lava Lakes

• Lavas flowing in channels
• Top slowly hardens over
• Up to 60 km

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Laze (lava haze)
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Homework

• Read Chapter 7, pgs 162-end of chapter
• Pay attention to earthquake prediction,
  forecasting, cost-benefit analysis
• Due Tuesday
• Review (pg 178) 9, 10
• Discussion (pgs 178-179) 6, 7