Cascadia subduction zone and Mexico City

Cascade 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

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 1700 Cascadia earthquake 19 times in the past 10,000 years

Organic layers are overlain by tsunami deposits 7 cycles the last 3500 years 1700 Cascadia earthquake

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 KM Equal to M w 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 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 zonezone

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 zonezone 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 zonezone

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.