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Cascadia subduction zone and Mexico City

Cascade Subduction Zone

• Similar size to the amount of crust that moved during the Sumatra earthquake.

• Dark red area indicates cooler crust storing energy

• Lighter red zone could possibly rupture

• Areas of silent earthquakes?

1700 Cascadia earthquake


• Drop in elevation

Vegetation exposed to sea water and dies

10:57

• Layers of sediment are shaken loose of the continental shelf

• Deposited on the ocean floor


19 times in the past 10,000 years

• Organic layers are overlain by tsunami deposits

• 7 cycles the last 3500 years


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

Silent earthquakes

• Movement is detected using Global Positioning Satellite technology

• Change in movement from compression to extension

• Significance– Understand how much silent earthquakes reduce overall

energy– Large earthquakes every 200-700 years

Silent earthquakes

Graph showing slip associated with silent earthquakes

Mexico

Silent earthquakes

• Yellow: GPS data– Slow slip or silent

earthquakes– Early- 2002, mid-2006

• Red/Green: seismic stations– Circled area,

earthquakes

Silent earthquakes: indicative of earthquakes

• Shallow and then becomes more steep under Mexico City

Mexican subduction zone

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

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

• Earthquakes are more shallow than other subduction zones

Mexican subduction zone

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

Common Building Failures

• Top floors fail-resonance• T-shaped structures• Flexible structures

between stiff structures

Building Failures

• Hammering

Soft story collapse

Not all subduction zones are created equally but are capable of producing

large earthquakes.