Powerpoint Presentation Physical Geology, 10e

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More U.S. and Canadian Earthquakes Natural
Disasters, 6th edition, Chapter 7
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U.S. and Canadian Earthquakes

• Earthquakes occur throughout North America, not
  just in California

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U.S. and Canadian Earthquakes

• U.S. earthquakes above magnitude 3.5 (1974-2003)
  were mostly in Alaska, California, Hawaii and
  Nevada
• Look at large U.S. and Canada earthquakes not on
  tectonic boundary

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Western North America Plate Tectonic-Related
Earthquakes

• North American plate moves southwest at 2.5 cm/yr
• Pacific plate moves northwest 8 cm/yr

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Western North America Plate Tectonic-Related
Earthquakes

• Much of Farallon plate has subducted under North
  America
• Western United States uplifted, creating Rocky
  Mountains, Sierra Nevada, Colorado Plateau
• Earthquakes throughout western United States

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Pacific Northwest Oregon, Washington, and
British Columbia
Subduction of the Juan de Fuca plate caused
earthquakes deep below surface (54 and 60 km).
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Pacific Northwest Oregon, Washington, and
British Columbia

• Puget Sound, Washington, 1949 and 1965 magnitudes
  7.1 and 6.5
• Puget Sound, Washington, 2001
• Earthquake near epicenter of 1949 earthquake,
  magnitude 6.8 and 52 km deep
• Did not result in any deaths, although 400
  injured and 2 billion in damage
• Damage would have been far worse but for improved
  building codes after 1965 earthquake

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Pacific Northwest Oregon, Washington, and
British Columbia

• Future Earthquakes in the Washington Region
• 1949, 1965 and 2001 earthquakes on subducting
  Juan de Fuca plate at depth under North America
• Much energy lost before reaching surface
• Earthquakes on near-surface faults also possible
  in region
• Seattle (Tacoma) fault zone
• 4 to 6 km zone with three or more south-dipping
  reverse faults
• Last major fault movement 1,100 years ago
• Uplift of shoreline in magnitude 7 earthquake
• Large landslides
• Tsunami deposits six major rock avalanches in
  Olympic Mountains
• Coarse sediment layers at bottom of Lake
  Washington
• Seattle sits on basin of soft sediments that
  amplify shaking

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Western North America Plate Tectonic-Related
Earthquakes
Western Great Basin Eastern California, Western
Nevada

• Owens Valley, California, 1872
• Fault runs along Interstate 395, on east side of
  Sierra Nevada
• Third longest fault rupture in California
  history, after 1906 San Francisco and 1857 Fort
  Tejon
• Earthquake with estimated magnitude 7.4

Figure 7.4
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Western North America Plate Tectonic-Related
Earthquakes

• Western Great Basin Eastern California, Western
  Nevada
• The Western Great Basin Seismic Trend
• Earthquake belt through eastern California and
  western Nevada
• Nevada averages one magnitude 6 earthquake per
  decade and one magnitude 7 earthquake per 27 years

Figure 7.5
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Western North America Plate Tectonic-Related
Earthquakes

• The Western Great Basin Seismic Trend
• Area between Sierra Nevada in California and
  Wasatch Range in Utah is expanding in east-west
  direction, opening up by several 100 km (Basin
  and Range or Great Basin)

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Western North America Plate Tectonic-Related
Earthquakes

• Intermountain Belt Utah, Idaho, Wyoming, Montana
• North-trending curve 1,500 km long and 100-200 km
  wide separating Basin and Range from Colorado
  Plateau and Rocky Mountains
• Faults on east side are down-to-the-west and on
  the west side are down-to-the east center is
  down-dropped

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Intermountain Belt Utah, Idaho, Wyoming, Montana

• Hebgen Lake, Montana, 1959
• Two faults moved within five seconds of each
  other with magnitude 6.3 and 7.5 earthquakes
• Created landslide that dammed canyon and formed
  Earthquake Lake
• Dropped north end of Hebgen Lake 7-8 m, creating
  seiche that sloshed back and forth for almost 12
  hours

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Western North America Plate Tectonic-Related
Earthquakes
Intermountain Belt Utah, Idaho, Wyoming, Montana

• Borah Peak, Idaho, 1983
• Lost River fault ruptured in 7.3 earthquake
• Borah Peak (Idahos highest point) 0.3 m higher,
  Thousand Springs Valley few meters lower
• Ground shaking caused fountains of groundwater

Figure 7.11
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Intermountain Belt Utah, Idaho, Wyoming, Montana

• The Wasatch Fault
• More than 80 of Utah population within sight of
  Wasatch Fault, which separates Wasatch Mountains
  from Great Basin
• Capable of generating magnitude 6-7 earthquakes
  every 350 years
• No major earthquakes since arrival of Brigham
  Young (Brigham City next likely candidate)

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Western North America Plate Tectonic-Related
Earthquakes

• Rio Grande Rift New Mexico, Colorado,
  Westernmost Texas, Mexico
• Major continental rift series of interconnected
  fault-block valleys for more than 1,000 km
• Continental crust is being heated and thinned by
  normal faulting
• In last 26 million years, 8 km of extension

• Historic earthquakes have been small to moderate
  but potential for large earthquake exists

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Intraplate Earthquakes Stable Central United
States

• Clusters of earthquakes at few locations
• Away from active plate edges
• Fewer earthquakes, but can be just as large

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Intraplate Earthquakes Stable Central United
States

• New Madrid, Missouri, 1811-1812
• Series of earthquakes, with four very large
  events
• Eight considered violent, ten very severe
• Total of 1,874 events
• Hypocenters beneath thick sediments of
  Mississippi and Ohio Rivers at Mississippi River
  embayment, near town of New Madrid (called
  Gateway to the West before destruction by
  earthquakes)
• Long-lasting effects on topography
• Two new lakes
• Low cliffs and domes formed
• Waterfalls in streams

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New Madrid, Missouri, 1811-1812

• Felt Area
• Felt area was largest for any U.S. earthquake
• Must consider difference in wave propagation in
  eastern vs. western North America

• Young, tectonically fractured rocks of west coast
  impede wave propagation and cause wave energy to
  die out faster than older, more homogeneous rocks
  of central U.S.

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Intraplate Earthquakes Stable Central United
States

• Magnitudes - New Madrid, Missouri, 1811-1812
• Using felt area to estimate magnitudes ? 8 to 8.3
• Studies of small earthquakes occurring today
  (aftershocks of 1811-1812 events) in this area
  can map out the faults
• First earthquake on Cottonwood Grove fault,
  triggered two of following earthquakes on
  Reelfoot blind thrust
• Remaining earthquake difficult to locate

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New Madrid, Missouri, 1811-1812

• Magnitudes
• Using fault-rupture length estimates from
  aftershock locations gives smaller moment
  magnitudes ? 7.3 to 7.7
• Soft, water-saturated sediment of ground
  amplifies shaking accounting for amplification
  gives magnitudes 7.0 to 7.5

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New Madrid, Missouri, 1811-1812

• The Future
• 1811-1812 New Madrid earthquakes did not cause
  great damage because population of area at time
  was so low
• Future earthquakes will affect population of St.
  Louis, Memphis
• Buildings not designed for earthquake shaking
• Soft sediments will amplify ground shaking
• Very large area will be affected

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New Madrid, Missouri, 1811-1812

• The Future
• Neotectonic analyses show earthquakes in the area
  around 500, 900, 1300 and 1600
• Magnitude 7 or higher earthquakes occur here
  about every 500 years
• U.S. Geological Survey forecasts 90 probability
  of magnitude 6-7 earthquake in next 50 years
• Why do earthquakes occur here in middle of
  continent? ? Reelfoot Rift

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Intraplate Earthquakes Stable Central United
States

• Reelfoot Rift Missouri, Arkansas, Tennessee,
  Kentucky, Illinois
• Why do earthquakes follow same linear pattern as
  deposition of sediments by Mississippi River
  system?

• Linear structural depression underlying New
  Madrid region ? ancient rift valley, Reelfoot
  Rift
• Formed 550 million years ago and since filled
  with sedimentary rocks and covered with younger
  sediments

Figure 7.20
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Intraplate Earthquakes Stable Central United
States

• Ancient Rifts in the Central United States
• As Pangaea tore apart around 200 million years
  ago, many rifts formed
• Some separated the landmass and formed Atlantic
  Ocean
• Others were failed rifts, left behind weakened
  zones in continent
• Can be reactivated by plate-tectonic stresses

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Intraplate Earthquakes Stable Central United
States

• Ancient Rifts in the Central United States
• Failed rifts correlate to active faults at the
  surface
• St. Louis arm ? St. Genevieve fault zone
• Rough Creek rift ? Rough Creek fault zone
• Southern Oklahoma rift ? Wichita Mountains
  frontal-fault system, Meers fault across Oklahoma
• Southern Indiana arm ? Wabash Valley fault zone

Figure 7.22
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Intraplate Earthquakes Eastern United States

• New England Earthquakes
• Offshore from Cape Ann, Massachusetts 1638
  earthquake with estimated 5.5 magnitude
• Newbury, Massachusetts 1727 earthquake caused
  damage, quicksand conditions

Figure 7.23

• Offshore from Cape Ann, Massachusetts 1755
  earthquake felt from Nova Scotia to South
  Carolina with estimated magnitude of 6.3
• May be related to reactivated faults bounding
  former rift valleys, with potential for future
  earthquakes, with proportionally more damage
  across densely populated East Coast

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Intraplate Earthquakes Eastern United States

• St. Lawrence River Valley Earthquakes
• River follows path of 500-600 million year old
  rift valley, coinciding with most of significant
  earthquakes in southeastern Canada up to
  magnitude 7

Figure 7.23

• Most active area is Charlevoix earthquakes
  occurred in 1534, 1663, 1791, 1860, 1870, 1925
• Site of large meteorite impact 350 million years
  ago, fracturing the area into faults that are
  being reactivated

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Fracture-Zone Hypothesis of Major Earthquakes

• Transform faults offset mid-Atlantic spreading
  center continue east and west as fracture zones
• Fracture zones may continue under North America
  and form zones of weakness that give way in
  earthquakes

• Some locations of east coast earthquakes seem to
  line up with continuation of fracture zones in
  Atlantic

Figure 7.24
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Fracture-Zone Hypothesis of Major Earthquakes

• Charleston, South Carolina, 1886
• Ruined by Civil War, devastated by 1885 hurricane
• Massive earthquake in 1886 destroyed 90 of
  buildings
• No surface faulting from earthquake
• Below surface studies indicate at least five
  similar earthquakes in last 3,000 years
  recurrence interval of about 600 years

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Earthquakes and Volcanism in Hawaii

• Movement of magma can cause earthquakes
• Expansion forces fracturing of surrounding
  brittle rock
• Magma movement generates continuous swarm of
  small earthquakes ? harmonic tremors
• Movement can also cause up to magnitude 6 and 7
  events
• 1975 magnitude 7.2 earthquake was caused by
  movement on normal fault of block into sea
  caused tsunami up to 12 m high

Figure 7.27
Figure 7.28
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End of Chapter 6