Cascadia subduction zone and Mexico City
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Transcript of Cascadia subduction zone and Mexico City
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Cascadia subduction zone and Mexico City
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Cascade Subduction Zone
• Similar size to the amount of crust that moved during the Sumatra earthquake.
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• Dark red area indicates cooler crust storing energy
• Lighter red zone could possibly rupture
• Areas of silent earthquakes?
1700 Cascadia earthquake
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1700 Cascadia earthquake
• Drop in elevation
Vegetation exposed to sea water and dies
10:57
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• Layers of sediment are shaken loose of the continental shelf
• Deposited on the ocean floor
1700 Cascadia earthquake
19 times in the past 10,000 years
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• Organic layers are overlain by tsunami deposits
• 7 cycles the last 3500 years
1700 Cascadia earthquake
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Silent earthquakes
• Creep events that release large amounts of strain energy without detectable seismic shaking
• Last 2-3 weeks, 14.5 month recurrence• A few centimeters at depths 30-50 KM• Equal to Mw 6-7
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Silent earthquakes
• Movement is detected using Global Positioning Satellite technology
• Change in movement from compression to extension
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• Significance– Understand how much silent earthquakes reduce overall
energy– Large earthquakes every 200-700 years
Silent earthquakes
Graph showing slip associated with silent earthquakes
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Mexico
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Silent earthquakes
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• Yellow: GPS data– Slow slip or silent
earthquakes– Early- 2002, mid-2006
• Red/Green: seismic stations– Circled area,
earthquakes
Silent earthquakes: indicative of earthquakes
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• Shallow and then becomes more steep under Mexico City
Mexican subduction zone
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Mexico City Earthquake
• 50 x 170 kilometers of displacement along the subduction zone
• M 8.1 • Mexico city is 400 kilometers away• City was built on the sediments of Lake
Texcoco
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Mexican subduction zone• Cocos tectonic plate is
subducting under the North American Plate
• Two plates lock• Stress builds and energy is
stored• Stress exceeds frictional
force• Release of energy in terms
of an earthquake
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• Earthquakes are more shallow than other subduction zones
Mexican subduction zone
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Mexico City
• Drained Lake Texcoco• Clay sedimentary layers• Low frequency surface
waves amplified• 1-2 second frequencies• Matched the periods of
buildings 6-16 stories
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Common Building Failures
• Top floors fail-resonance• T-shaped structures• Flexible structures
between stiff structures
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Building Failures
• Hammering
Soft story collapse
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Not all subduction zones are created equally but are capable of producing
large earthquakes.