EARTHQUAKES

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EARTHQUAKES
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• An earthquake is the vibration, sometimes
  violent, of the Earth's surface that follows a
  sudden release of stored energy when a fault
  ruptures.
• This energy can be generated by a sudden
  dislocation of segments of the crust, by a
  volcanic eruption, or by manmade explosions.

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Global Distribution of Earthquakes
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Plate Tectonics
USGS Earthquake Hazards Program
Lisa Wald USGS Pasadena
U.S. Department of the Interior U.S. Geological
Survey
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• Faults form by brittle failure.
• Most earthquakes occur along plate boundaries,
  where plates bump against each other due to
  mantle convection.

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FAULTS

• A fault is a fracture in the Earth's crust along
  which two blocks of the crust have slipped with
  respect to each other.
• Faults are divided into three main groups,
  depending on how they move. Normal faults occur
  in response to pulling or tension the overlying
  block moves down the dip of the fault plane.
  Thrust (reverse) faults occur in response to
  squeezing or compression the overlying block
  moves up the dip of the fault plane. Strike-slip
  (lateral) faults occur in response to either type
  of stress the blocks move horizontally past one
  another.
• Most faulting along spreading zones is normal,
  along subduction zones is thrust, and along
  transform faults is strike-slip.

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EARTHQUAKE LOCATIONS

• The focus is the location of an earthquake
  Longitude, Latitude, and depth.
• The epicenter of an earthquake is the point on
  the Earth's surface directly above the focus.

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About 800,000 quakes (events), occur per year.
about 4,000/yr are felt, and there are about 1-2
great (Mb gt 7.5) events per year
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Earthquake Distribution and Plate Tectonics

• Most of the great earthquakes (Mgt7.5) occur at
  active plate boundaries
• 1) active convergent plate boundary (c.p.b.)
  85
• 2) active transform plate boundary (t.p.b.)
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• 3) recently active c.p.b. 4
• (active less than 20 million yrs ago)
• 4) recently active t.p.b. 2
• 5) active d.p.b. lt 1

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Seismic Velocities

• VELOCITIES Vp gt Vs gt VL
• VELOCITY increases with depth in earth
• P S L
• top of crust 5.5 3.5 2 km/sec
• 30 km 7.5 5.5 4 km/sec

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Seismic Velocities

• ___________
• Vp-wave v (4/3m k)/r
• _____
• Vs-wave v (m/r)
• m shear modulus
• k rigidity
• r density in g/cc

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EARTHQUAKE SIZE

• Richter or Magnitude Scale
• Measures actual energy release - developed in
  1930's at Cal Tech
• Scale -2 to 12.
• -CALIBRATION if A is 1 micron at instrument 100
  km from event, then the Magnitude -2
• - there is an increase of 32x as much energy from
  one integer to the next. (Therefore, a magnitude
  4.1 is about 3x the size of a 4.0!)

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EARTHQUAKE SIZE

• Modified Mercalli intensity scale (MMI)
• Measures damage to human (man made) structures.
• Intensity depends on reporting accuracy,
  population, development, building codes, and
  enforcement.
• Intensity Scale is I - XII.
• Useful for all pre-instrumental events. The few
  seismographs operating in the early part of the
  last century were isolated and uncalibrated.
  Calibration with explosions occurred from the
  1930s to present.
• The newer Mw scale gives more accurate magnitudes
  for very large events.
• To assess seismic risk and locations of possible
  future events, we need to look at
  pre-instrumental records.

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• TO CALCULATE ENERGY RELEASE from MAGNITUDE
• E (25.1kJ)(31.6) M
• 1 Mton of TNT 4.181015 Joules, or about a 7.8

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• DIFFERENT MAGNITUDE TYPES
• log10 A of P wave Mb scale -good for 0 - 7
• log10 A of S wave Ms scale -good for 3 - 7
• log10 A of L wave ML scale -surface waves 3 - 7
• For all very large earthquakes occurring since
  1973, seismologists currently integrate the area
  under curve of all waves on the seismogram. This
  results in the Mw scale, which is more accurate
  for very large (7.5 and up) events

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• Great Chilean Earthquake in May of 1960 is the
  largest earthquake ever recorded. Its Mw 9.5
  Rupture length 1000 km. Here the Nazca Plate
  under the ocean collides with the South American
  Plate at 9.7 cm/yr. Currently the worlds fastest
  rate.

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• The largest recorded event in North America was
  the 1964 Alaska Earthquake,which has a revised
  magnitude, Mw of 9.2. It ruptured 2000 km of the
  Alaska coast. The Pacific Plate is colliding with
  the North American Plate at the coast at about 7
  cm/yr.

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SEISMIC MOMENT

• Seismic Moment, M0 kDavA
• K rigidity modulus
• Dav average displacement (slip) of one side of
  fault relative to the other
• A area of fault surface that ruptured

Rock k Basalt 2.381010 Pa Granite
1.38 Sandstone 1.81 Lithosphere 3.3 Water
0
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SEISMIC MOMENT, ENERGY, AND MAGNITUDE

• Energy release can be estimated from the seismic
  moment
• E in dyne-cm M0/20,000
• Mw 2/3(log10M0 in dyne-cm) - 16

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Bigger Faults Make Bigger Earthquakes
Length of Fault in