ESYS 150 LECTURE 8 EARTHQUAKES IN WESTERN N AMERICA

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ESYS 150LECTURE 8EARTHQUAKES IN WESTERN N
AMERICA

• North America History 85 Ma to Present,
• Subduction Zone Earthquakes
• Alaskan Trench
• Middle America Trench
• Cascadia Subduction Zone
• Spreading Center Earthquakes

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NORTH AMERICA 85 MA TO PRESENTAge determined
from magnetic anomaliesOverlap isochrons to
reconstruct position of plates
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NORTH AMERICA 85 MA TO PRESENTPACIFIC
PLATE/NORTH AMERICA PLATE
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NORTH AMERICA 38MA - PRESENTDEVELOPMENT OF SAN
ANDREAS FAULT
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NORTH AMERICA 20MA - PRESENTDEVELOPMENT OF
TRANVERSE RANGES
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SUBDUCTION ZONE EARTHQUAKESLARGEST
EARTHQUAKESThrust faults

• .
• Many in Alaskan Trench.
• Also in Middle America
• Trench but not so large

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SUBDUCTION ZONE EARTHQUAKESALASKA, MIDDLE
AMERICA, CASCADIAThree major areas of subduction
zone earthquakes (red)

• Alaska Trench, Middle America Trench, Cascadian
  Subduction Zone
• Ocean/Ocean, Ocean/Continent, Ocean/Continent
  collisions

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SUBDUCTION ZONE EARTHQUAKES1964 ALASKA
EARTHQUAKEPacific plate subducted beneath North
American Plate.

• 1000 km by 400 km slab moved
• 19.5 m with 3-4 min shaking.
• Produced uplift and subsidence
• Damage mostly due to tsunami
• Critical earthquake for under
• standing thrusting in subduction
• zones

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SUBDUCTION ZONE EARTHQUAKE1985 MEXICO CITY
Earthquake in Middle America Trench

• Earthquake in middle America trench but damage
  mostly in Mexico City.
• Cocos plate moved 2.3 m at 18o over 200 by 200 km
  area
• Damage because soft lake sediments in Mexico City
  resonated with 1 - 2 sec
• frequency of surface waves.

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SUBDUCTION ZONE EARTHQUAKES1985 MEXICO CITY
East West Accelerations at stations around Mexico
City.

• UNAM on hard ground small accelerations. SCT on
  soft lake sediments lots
• of acceleration due to amplification effect of
  resonance of soft lake sediments

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SUBDUCTION ZONE EARTHQUAKES1985 MEXICO
CITYBuilding response problems during the
earthquake.

• 6 - 16 storey high building
• worst hit.
• Buildings of different
• heights banged into each
• other.
• Buildings with different heights
• tore apart.

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SUBDUCTION ZONEEARTHQUAKE GAPImportant concept
in predicting where next major quake

• West coast of Mexico showing dates of earthquakes
  and the fault areas.
• Note that the Michoacan gap was filled by 1985
  earthquake.
• Geurrero gap is overdue for a major movement.

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SUBDUCTION ZONE EARTHQUAKESPACIFIC
NORTHWESTJuan de Fuca plate subducted beneath
North America

• Volcanoes in Pacific Northwest
• indicate Subduction is currently active.
• Youthful ocean floor and strong
• coupling with overriding plate make
• this area particularly dangerous.
• Had 6.8 earthquake in 2001
• Suggested had 9.0 earthquake in
• 1700

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SUBDUCTION ZONE EARTHQUAKESPACIFIC NORTHWEST6.8
2001020228 Nisqually earthquake
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SUBDUCTION ZONE EARTHQUAKEPACIFIC NORTHWEST2001
Nisqually earthquake

• Map of seismicity in the
• Pacific Northwest.
• Earthquake occurred deep
• under Tacoma
• Because it was so deep
• caused remarkably little
• damage at the surface even
• though 6.8 magnitude.

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SUBDUCTION ZONE EARTHQUAKESPACIFIC
NORTHWEST2001 Nisqually earthquake

• Normal fault solution at the bend in
• the plate as it descends into the
• mantle
• Below the zone where the most
• devastating thrust earthquakes
• occur.
• Normal fault and depth are reason
• why damage was so small.

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SUBDUCTION ZONESPACIFIC NORTHWEST26 January
1700 9.0 Cascadia earthquake

• Annual growth rings in drowned trees
• along coast from BC to Oregon show
• all died at the same time in Jan 1700.
• Explanation was the ground dropped
• and sea water got to the roots.
• Had to have been a huge earthquake
• to cause such an extensive drop.

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SUBDUCTION ZONESPACIFIC NORTHWESTEvidence for a
major thrust zone from margin sediments

• Diagram shows 1960 Chile Earth
• quake sequence on Cascadia
• subduction zone.
• Much seismic energy is locked up in
• the cold surface area of the fault.
• Scientists argues that if the whole fault
• released it could create a huge
• earthquake that would rotate the entire
• land area and create a huge tsunami.
• Both would kill the trees.

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SUBDUCTION ZONESPACIFIC NORTHWESTJapanese
records show evidence of a tsunami January 26
1700 that was not from a local earthquake but
probably from Pacific Northwest.
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SPREADING CENTER QUAKESSALTON SEA Very active,
many normal fault earthquakes

• Map of northwestern Gulf of
• California and the Salton Sea.
• Note the two buried spreading centers
• between the San Andreas Fault and
• the northern gulf.
• Many earthquakes but sparsely
• populated and little damage.
• Thermal power station at Cerro Prieto.