Cascadia subduction zone and
Mexico City
Cascade Subduction Zone
• Similar size to the
amount of crust that
moved during the
Sumatra earthquake.
1700 Cascadia 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
10:57
Vegetation exposed to sea water
and dies
1700 Cascadia earthquake
• Layers of sediment are
shaken loose of the
continental shelf
• Deposited on the ocean
floor
19 times in
the past
10,000 years
1700 Cascadia earthquake
• 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
Silent earthquakes
• Significance
– Understand how much silent earthquakes reduce overall
energy
– Large earthquakes every 200-700 years
Graph
showing slip
associated
with silent
earthquakes
Mexico
Silent earthquakes
Silent earthquakes: indicative of
earthquakes
• Yellow: GPS data
– Slow slip or silent
earthquakes
– Early- 2002, mid-2006
• Red/Green: seismic
stations
– Circled area,
earthquakes
Mexican subduction zone
• Shallow and then
becomes more steep
under Mexico City
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
Mexican subduction zone
• Earthquakes are more
shallow than other
subduction zones
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.