What is an earthquake?

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What is an earthquake?
1906 San Francisco earthquake

• Shaking or vibration of the ground
• rocks undergoing deformation break suddenly along
  a fault

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Oblique view of the San Andreas fault and San
Francisco
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Where are earthquakes found?

• The Earths surface is composed of a number of
  mobile tectonic plates which are in constant
  motion
• Most earthquakes are found at plate margins

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Plate tectonics

• The constant movement of the plates is referred
  to as plate tectonics
• There are three main types of plate boundaries
• divergent
• convergent
• transform

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Divergent margins

• Here two tectonic plates are in the process of
  being created
• Magma is injected into a crack, then cools and
  becomes new crust

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An example of a wide, mature divergent margin

• The middle of the Atlantic Ocean is a divergent
  margin which is being torn, or rifted, apartthe
  two plates are separating continuously at a rate
  of several cm/yr

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An immature divergent plate margin

• The Red Sea represents a young rift which is just
  beginning to separate Arabia from Africa
• Here, too, volcanism is evident, as a result of
  rifting

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Volcanism in the Afar triangle

• Erta Ale, a volcano slightly west of the Red
  Sea, represents the splitting apart and thinning
  of the African continent

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Convergent margins I

• Instead of two plates being created, they are
  being consumed
• Here an oceanic plate slides beneath a
  continental plate, since the former is denser
• geologists refer to this process as subduction
• Large, destructive earthquakes occur here

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Convergent margins II

• If two continental plates collide, they do not
  subduct, because they are too buoyant
• Instead, intense compression with crustal
  shortening and thickening occur
• Large, destructive earthquakes also are generated
  in this situation

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Transform margins

• The third type of plate margin is called a
  transform boundary
• Here, plates are neither created nor destroyed
• they simply slide by one another

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So heres the big picture of what were living on
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Where are the worlds earthquakes in terms of
plate tectonics?

• The great majority of earthquakes are located at
  plate margins
• This where magmatism, friction, faulting, etc.,
  are most intense
• Earthquakes in plate interiors are comparatively
  rare

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The Pacific Rim of Fire

• This notorious zone is characterized by
  subduction zones
• Earthquakes and volcanoes here are particularly
  violent
• friction from subduction produces large
  destructive quakes

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North American seismic hazards
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Canadian seismic hazards
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Seismic hazard in eastern Canada
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Faults associated with earthquakes

• Faults are planes of weakness along which the
  Earth has been broken
• Movements on a fault can be either slow (ductile
  deformation) or fast (brittle fracture)
• When a fault behaves in a brittle manner and
  breaks, earthquakes are generated

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Three types of dominantly vertical faults

• A normal fault is the result of tensional forces
  (e.g., rifting)
• Reverse and thrust faults are the result of
  horizontal compression

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Faults whose movement is dominantly horizontal

• These faults are termed strike-slip faults
• They are a small-scale version of transform plate
  tectonic margins
• They are termed left-lateral (sinistral) or
  right-lateral (dextral) according to their
  movement

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Earthquake generation along a fault

• The earthquake focus is its point of origin along
  a fault plane
• Its epicenter is the vertical projection of the
  focus to the surface

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Elastic rebound theory

• Before fault rupture, rock deforms
• after rupture, rocks return to their original
  shape
• ...maybe1
• 1Pallett Creek shows similar slip amounts after
  different periods of time possibly not resetting
  to zero? See Sieh and Levay, 1998, p. 90

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Richter magnitudes

• The Richter magnitude measures the maximum
  amplitude of ground shaking
• It is a logarithmic scale
• 1 Richter unit difference is x 10 for ground
  motion and x 33 for energy

• Globally, small earthquakes are more frequent
  than large
• 800,000/yr for events of magnitude 2.0-3.4
• while an event of magnitude 8 occurs once every
  5-10 years

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Richter magnitudes
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Destructiveness of an earthquake

• Earthquake magnitude
• Distance to epicenter
• Depth
• Strength of building
• Nature of soil or bedrock on which foundations
  are built
• Other local conditions

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A challenge

• You yourself can calculate Richter magnitudes and
  epicenters from seismogram data. Go to
• http//vcourseware.sonoma.edu/VirtualEarthquake/
• Not only will you understand the science behind
  earthquake determinations, there are also
  material rewards...

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Diplomas !
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The San Andreas fault

• Along much of the west coast, the plate boundary
  is a transform margin

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San Andreas fault

• Although some people think San Francisco is
  falling into the Pacific Ocean, part of the
  city is actually already part of the Pacific
  plate
• The San Andreas is a right-lateral strike-slip or
  transform fault

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San Andreas fault
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Right-lateral motion
Photos from Shelton, 1966
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Right-lateral motion
Photo, diagram from Sieh and LeVay, 1998
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Some history

• The strike-slip nature of the San Andreas was not
  widely appreciated for up to 50 years after the
  1906 San Francisco earthquake

• Yet rocks on either side of the fault are
  different
• The older the rocks, the greater the displacement
• Eocene-age rocks (37-58 Ma) show offsets up to
  300 km

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San Francisco, 18 April 1906

• Magnitude 7.8, epicenter near San Francisco
• 400 million US in damage
• this is 1906 dollars equivalent to hundreds of
  billions of dollars today
• 700 people reported killed
• this is probably a 3-4 times underestimate thus
  2,000-3,000 dead, mostly in San Francisco

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1906 - location and seismic trace
Seismic trace of 1906 quake from a seismic
station 15,000 miles away in Gottingen, Germany
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1906 - comparative magnitude

• This event is northern Californias most powerful
  event in recorded history

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1906 - extent and slip
The northernmost 430 km of the San Andreas
ruptured, with horizontal slippage up to 8-9
meters
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1906 - slip

• This photograph shows a fence near Bolinas offset
  2.5 meters

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1906 - intensity and shaking

• Maximum Mercalli values were VII to IX, which
  represent severe damage
• Shaking lasted 45-60 seconds (for Loma Prieta
  1989 and Northridge 1994, shaking lasted 5-10 s)
• Shaking intensity correlated with geology, e.g.,
  bedrock vs. landfill

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1906 - earthquake damage in San Francisco
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1906 - earthquake damage in San Francisco
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1906 - earthquake damage in San Francisco
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1906 - some lessons learned

• Big quakes can be followed by decades of seismic
  quiet
• Quakes the size of the 1906 event appear to occur
  every several hundred (200?) years

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1906 - some lessons learned (ctd.)

• In the short term, San Francisco and environs are
  most at risk from an event of magnitude 6-7

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1906 - some lessons not learned
A topographic map of San Francisco from 1950...
and a 1980 version of the same map
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Future quakes in the San Francisco Bay area
Note the high probability of an earthquake of
M gt 6.6 occurring before 2030 in this area
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Cascadia

• In the Pacific Northwest, the tectonic regime is
  subduction-related, rather than transform as we
  have seen in California

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Cascadia
Here, there is evidence for very large
earthquakes over the last several thousand
yearsthe most recent is 300 years ago
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Quebec

• The St. Lawrence region has high levels of
  seismicity for a zone in the interior of a
  tectonic plate
• This seismicity may be related to old, aborted
  rifts about 200 Ma ago

Map from Lamontagne (1999)
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Quebec - Montreal region

• Ottawa River axis
• more active Montreal-Maniwoki axis
• M 5.8, 1732, Montreal
• M 6.2, 1935, Temiscamingue
• M 5.6, 1944, Cornwall-Massena, NY

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Quebec - Charlevoix region

• Events 1638 M7 1663 M6 1791 M6.5 1870 M6.2
  1925 ( 2 million in damage at the time)
• fracturing and high pore fluid pressures
• old rift faults serving as conduits for
  pressurized crustal fluids, which trigger quakes

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Charlevoix
Charlevoix also has evidence for a meteorite
impact crater, which served to fragment and
fracture rocks (from Lamontagne, 1999)
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Effects of earthquakes aftershocks

• Aftershocks normally occur after a major
  earthquake
• There may be many thousands of aftershock events
  over the space of months or even years
• Although their magnitudes generally decrease with
  time, aftershocks have potential to cause
  significant damage to already weakened materials
  (e.g., rocks, soils, buildings, power and gas
  lines)

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Effects liquefaction
Liquefaction hazard in the San Francisco Bay area

• Wet, unsolidated soils and sediments are highly
  vulnerable
• Under shaking, the ground simply flows
• Landfills, harbours, and the like are at risk

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Effects landslides

• The ground vibrations and severe shaking
  associated with an earthquake can induce
  landslides in mountainous areas
• This example in the Santa Susana Mtns. was caused
  by the 1994 Northridge event near Los Angeles

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Effects tsunamis

• Tsunamis are ocean waves caused by displacements
  from earthquakes, landslides, etc.
• They can be devastating at great distances from
  the epicenter

Tsunami damage in Hilo, Hawaii, as a result of
the 22 May 1960 Chile earthquake
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Effects building destruction

• Buildings are damaged or destroyed by ground
  vibrations and shaking
• The magnitude and duration of shaking are
  important factors in the extent of damage
• Liquefaction and aftershocks increase the damage

Building damage near the epicenter of the 1989
Loma Prieta earthquake
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Effects on building materials

• Masonry is not capable of withstanding
  significant bending stresses
• Wood is more resistant because it is more
  yielding
• But wood is vulnerable to fires...

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Effects fires

• The ground shaking will rupture power and gas
  lines
• and damage to water mains prevents or hinders
  fire fighting efforts
• the photo shows a broken gas line from the 1994
  Northridge earthquake

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Devastating fires in San Francisco after the 1906
earthquake
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Effects personal loss

• We are examining earthquakes from a scientific
  perspective
• but we must not forget the human element and the
  pathos conveyed by this photograph from the 1994
  Northridge earthquake

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Mitigating earthquakes

• Seismic hazard maps and risk maps help to
  properly site and construct buildings

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Where to build your dream or trophy house - and
where not to build

• Avoid unstable soils and unconsolidated
  materials...
• avoid mountainous terrain prone to landslides
• and above all, avoid active faults !

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Appropriate building codes which can withstand
earthquake damage

• Bedrock foundations best
• Avoid asymmetrical buildings
• Bolt house firmly to foundations
• Appliances firmly bolted down
• Gas lines flexible
• Cupboards, shelving attached to walls
• Heavy objects at low levels anchor heavy
  furniture
• Beds away from windows to avoid broken glass

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Warning and prediction

• Precursory seismicity
• Precursory deformation
• Changes in physical properties of rocks near a
  fault
• Changes in water levels, soil gases
• Unusual behaviour of animals

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Earthquake prediction

• Important concepts
• earthquake recurrence intervalseismic gap
• role of paleoseismology

• Yet our predictive ability is rudimentary, so we
  use probabilities
• e.g., 86 probability that a destructive quake of
  Mgt7 will hit southern California in the next 30
  years (1994 estimate)

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Earthquakes - reading

• U.S. Geological Survey, 1999. Major quake likely
  to strike between 2000 and 2030. U.S. Geological
  Survey Fact Sheet 152-99, 4 pp.
  (http//pubs.usgs.gov/fs/1999/fs152-99/)
• Pelman, D., 2000. Tiny movements ease fault risk
  in East Bay pressure builds up less in northern
  Hayward segment. San Francisco Chronicle, 18
  August 2000. (http//www.sfgate.com/)
• Eastern Canadian seismicity
• http//earthquakescanada.nrcan.gc.ca/historic_eq/2
  0th/e_damaging_e.php

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Earthquakes - web

• Canadian seismicity
• http//www.pgc.nrcan.gc.ca
• US seismicity
• http//earthquake.usgs.gov/
• San Francisco Bay area
• http//www.abag.ca.gov/bayarea/eqmaps