Powerpoint Presentation Physical Geology, 10/e

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Big Bend in the San Andreas Fault
Many faults in Southern California other
than San Andreas (SAF) Most are right-lateral
(like SAF) but many in So-Cal are not
(thrust, normal, blind thrust) Right-lateral
strike slip motion common in northern
California but things are different in LA at
the Big Bend Bend in SAF causes intense
compression and formation of the transverse
ranges (San Gabriels, Santa Susanas...)
Pressure causes rocks here to deform and break
alleviating stress and creating non-uniform
slip events.
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Big Bend in the San Andreas Fault
Right-lateral strike-slip motion along the SAF
becomes compressional at a bend in the fault
line.
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Northridge Earthquake, 1994
- January 17, 1994, 430 am - Magnitude 6.9 -
Blind thrust fault - Difficult to find or
predict - Duration 10-20 seconds
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EarthquakesChapter 16
1989, Loma Prieta earthquake, Mw 7.2
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Historic Earthquakes
1906 San Francisco (7.8) - 280 miles of
displacement - Shaking 1 minute - Damaged
water mains, fires spread and caused many deaths
(3000).
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Historic Earthquakes
1989 Loma Prieta, Mw 7.2 - Shaking for 15
seconds - Death toll 63
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Historic Earthquakes
1964 southern Alaska (Mw 9.2) - Shaking for
3 minutes - Rupture 350,000 square miles -
Death toll from quake 15 (remote area) -
Tsunami, landslides 100 more
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Historic Earthquakes
1994 Northridge - 1.8 miles from CSUN -
Shaking 40 seconds - Damaged all 53 CSUN
buildings - Damaged 300 other schools - Lower
story buildings collapsed - 4 interstate hwy's
closed for months (Golden State, Santa
Monica fwys) - 15000 people live in tents for
days - 10000 without water, elect, gas -
Arid climate did not cause liquifaction of soils
shaking minimized (compared to 1964 Alaska
and 1989 Loma Prieta). - Landslides in Santa
Susana, Santa Monica, San Gabriel Mtns
blocked roads and traffic, damaged water lines
and homes in Palisades - Sylmar Olive View
Hospital rebuilt from 1971 to code stayed
intact. - Aftershocks 5.9 1 min after main
shock, 5.6 11 hrs after.
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Historic Earthquakes
2002 inland Alaska Denali Fault - Propagated
east 7000 miles/hr - Offset streams,
glaciers, landslides - Trans-Alaska Oil
Pipeline no serious damaged - pre-engineered
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Historic Earthquakes
2004 Sumatra Andaman E.Q. - Mw 9.3 -
Second largest recorded on Earth - Major
damage in Sumatra - Tsunami damage spread far
to Indonesia, Thailand, Sri Lanka,
India, East Africa - Death toll 220,000 from
force of tsunami wave
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Faults and Crater Offsets on the Moon
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Moon Quakes
Moon quakes were recorded by a seismometer from
the Apollo 16 mission
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INSIGHT Mission to Mars
The INSIGHT missionwill deploy one seismometer
to measure Mars quakes! Do you think this is
a good use of our research dollars ? Why or Why
not ?
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Earthquakes

• An earthquake is a trembling or shaking of the
  ground caused by the sudden release of energy
• Tectonic forces produce stresses on rocks that
  eventually exceed their elastic limits, resulting
  in brittle failure
• Energy is released during earthquakes in the form
  of seismic waves
• Elastic rebound theory - earthquakes
• are a sudden release of strain stored in
• rocks that bend until they finally break
• and move along a fault

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Seismic Waves

• Focus (or hypocenter) - the point of initial
  breakage and movement along a fault, where
  seismic waves originate
• Epicenter - point on Earths surface directly
  above the focus
• Two types of seismic waves are produced during
  earthquakes
• Body waves - travel outward from the focus in all
  directions through Earths interior
• Surface waves - travel along Earths surface away
  from the epicenter

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Body Waves

• P wave - compressional body wave in which rock
  vibrates back and forth parallel to the direction
  of wave propagation
• Fast (4 to 7 kms/sec) wave that is the first or
  primary wave to arrive at recording station
  following earthquake
• Pass through solids and fluids
• S wave shearing body wave in which rock
  vibrates back and forth perpendicular to the
  direction of wave propagation
• Slower (2 to 5 km/sec) wave that is the
  secondary wave to arrive at recording
  station following earthquake
• Pass through solids only

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Surface Waves

• Surface waves are slower than
• body waves but have the larges
• amplitude
• Love waves - side-to-side motion of the ground
  surface
• Cant travel through fluids
• Rayleigh waves - ground to moves in an elliptical
  path opposite the direction of wave motion
• Extremely destructive to buildings

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Measuring Earthquakes

• Seismometers - used to measure seismic waves
• Seismographs - recording devices used to produce
  a permanent record of the motion detected by
  seismometers

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Measuring Earthquakes

• Seismograms - permanent paper (or digital)
  records of the earthquake vibrations
• Used to measure the earthquake strengths

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Locating Earthquakes

• P- and S-waves leave E.Q.
• focus at the same time
• P-wave gets farther and farther ahead of the
  S-wave with distance and time from the earthquake
• Travel-time curve - used to determine distance to
  focus
• based on time between first P- and S-wave
  arrivals

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Locating Earthquakes

• One station gives distance to E.Q. but not
  direction
• Plotting distances from 3 stations on a map, as
  circles with radii equaling the distance from the
  quake. Point where 3 circles overlap locates the
  earthquake epicenter
• Depth of focus beneath Earths surface can also
  be determined
• Shallow focus 0-70 km deep
• Intermediate focus 70-350 km deep
• Deep focus 350-670 km deep

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Measuring the Sumatra Andaman Earthquake
A Global Seismic Network (GSN) is maintained
around the Earth to monitor earthquake
activity as well as nuclear testing. Earthquake
locations and depth can be determined within
minutes of an earthquake using this GSN network.
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Measuring the Size of Earthquakes

• Earthquake size measured two ways - intensity
  and magnitude
• Intensity - a measure of the effects an
  earthquake produces (on both structures and
  people)
• Modified Mercalli scale

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Measuring the Size of Earthquakes

• Magnitude is a measure
• of the amount of energy
• released by an earthquake
• Richter scale
• Body waves
• Surface waves
• Moment magnitude - more objective measure of
  energy released by a major earthquake Mo m
  U A
• Uses rock strength, surface area, fault rupture
  distance
• Smaller earthquakes are more common than large
  ones

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Earthquake Magnitudes How Big is Big ?
Earthquake magnitudes are logarithmic M 10 R
(M ground movement, R Richter scale) If
R 1, then M 10
If R 2, then M
100 So this means that a magnitude 7 earthquake
will have 10 times as much ground movement as a
magnitude 6 earthquake.
Change by 1
Change by 10
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Location and Size of Earthquakes in the U.S.

• Earthquakes occur throughout the U.S., but are
  much more common in the western states and Alaska
• Largest seismic risks or hazards exist near the
  plate boundary along the U.S. Pacific coast
  (e.g., San Andreas fault), and around New Madrid,
  Missouri
• Seismic risk determined based on the
  assumption that large future earthquakes will
  occur where they have occurred in the past

Earthquake locations since 1977
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Effects of Earthquakes

• Earthquakes produce several types of effects, all
  of which can cause loss of property and human
  life
• Ground motion is the familiar trembling and
  shaking of the land during an earthquake
• Can topple buildings and bridges
• Fire is a problem just after earthquakes because
  of broken gas and water mains and fallen
  electrical wires
• Landslides can be triggered by ground shaking,
  particularly in larger quakes
• Liquefaction occurs when water-saturated soil or
  sediment sloshes like a liquid during a quake
• Permanent displacement of the land surface can
  also occur, leaving fractures and scarps

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World Earthquake Distribution

• Most earthquakes occur in narrow geographic belts
  - plate boundaries
• Most important concentrations in circum-Pacific
  and Mediterranean-Himalayan belts
• Shallow-focus earthquakes occur at
  mid-oceanic ridges

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World Earthquake Distribution

• Nearly all intermediate- and deep-focus
  earthquakes occur
• in Benioff zones
• - inclined seismic activity associated with
  descending oceanic plate at subduction
• E.Q.'s are caused by plate interactions along
  tectonic plate boundaries
• Plate boundaries are identified and defined by
  earthquakes

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Earthquakes Tectonics
Earthquakes occur at each of the three types of
plate boundaries -At divergent boundaries,
tensional forces - shallow-focus quakes on
normal faults -At transform boundaries, shear
forces - shallow-focus quakes along strike-slip
faults -At convergent boundaries, compressional
forces produce shallow- to deep-focus quakes
along reverse faults
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First Motion of Ground Motion Observed on a
Seismogram
Extension (normal faults)
Compression (thrust faults)
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Activity
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Building Codes for Earthquakes
Best materials are -Strong -Flexible -Light G
ood examples Steel Wood Reinforced concrete
(rebar steel core) Poor examples Simple
concrete Brick chimneys often fall (in
moderate E.Q.'s) Heavy roofs (Tile) Strong
liquefaction can supersede all these
precautions...
1999, Izmit Turkey
1964 Niigata, Japan - liquifaction
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Buildings and Bridges...
1995 Kobe, Japan Elevated highway knocked over
by strong horizontal jolt. Damage to bridge and
88,000 buildings costs exceeded 400 billion
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Tsunami Waves Created by Earthquakes
Tsunami waves are generated by submarine
earthakes Long low waves are formed above
displaced seafloor that travel for miles along
the base of the seafloor. Displacement of
seawater surface can be as little as a few
centimeters in the middle of the
ocean. Displacements increase dramatically as
they approach a continental shelf
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Tsunami Travel time in the Pacific Ocean
Earthquake in Alaska may form a tsunami in the
Pacific
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Monitoring Tsunami Waves
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Tsunami in Sumatra, 2004