Cascadia subduction zone and Mexico City

1
Cascadia subduction zone and Mexico City
2
Cascade Subduction Zone

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

3
1700 Cascadia earthquake

• Dark red area indicates cooler crust storing
  energy
• Lighter red zone could possibly rupture
• Areas of silent earthquakes?

4
1700 Cascadia earthquake

• Drop in elevation

1057
Vegetation exposed to sea water and dies
5
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
6
1700 Cascadia earthquake

• Organic layers are overlain by tsunami deposits
• 7 cycles the last 3500 years

7
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

8
Silent earthquakes

• Movement is detected using Global Positioning
  Satellite technology
• Change in movement from compression to extension

9
Silent earthquakes

• Significance
• Understand how much silent earthquakes reduce
  overall energy
• Large earthquakes every 200-700 years

Graph showing slip associated with silent
earthquakes
10
Mexico
11
Silent earthquakes
12
Silent earthquakes indicative of earthquakes

• Yellow GPS data
• Slow slip or silent earthquakes
• Early- 2002, mid-2006
• Red/Green seismic stations
• Circled area, earthquakes

13
Mexican subduction zone

• Shallow and then becomes more steep under Mexico
  City

14
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

15
(No Transcript)
16
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

17
Mexican subduction zone

• Earthquakes are more shallow than other
  subduction zones

18
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

19
Common Building Failures

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

20
Building Failures

• Hammering

Soft story collapse
21
Not all subduction zones are created equally but
are capable of producing large earthquakes.