ESS 8 Final Review

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ESS 8 Final Review

• Book all chapters.
• Appendices
• A quakes in world
• B quakes in US
• C Mercalli intensity scale
• Quiz p.356 in Bolts book
• Glossary
• WWW pages

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Map of major plates
Press, 20-3
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Some facts

• There are 10 to 15 major plates
• The US is split, mostly on the North American
  Plate, but a western sliver on the Pacific Plate
• The boundaries between plates are faults
• but most faults are not plate boundaries
• Earthquakes are essentially the moving plates
  rubbing together

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Plates are not continents

• And continental boundaries usually are not plate
  boundaries
• For example, east coast of US is far from the
  edge of the North American plate
• Continental lithosphere lighter silicates
• Oceanic lithosphere heavier basaltic rocks
  (olivine, pyroxene and plagioclase feldspar rich
  in iron, magnesium, calcium and aluminium)

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Plate boundaries

• Divergent - plates pull apart Mantle material
  rises to fill the space between the separating
  plates.
• Example Pacific Atlantic Indian oceanic ridges
• Lot of small earthquakes, shallow.
• Convergent - plates collide One plate get
  pushed down while the other stays on top.
• Example Pacific rim subduction zones
• Lot of earthquakes, any magn. Down to 600km
  depth.
• Transform - plate rub against each other Both
  plates stay on surface and move sideways.
• Example San Andreas transform fault
• A lot of small events, some major ones, shallow
  (20km)

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Mid-ocean ridge
Press, 20-4a
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Rift valley
Press, 20-4b
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Normal fault - divergence
Press, 18-12a,b
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Oceanic crust over oceanic crust
Press, 20-6b
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Continental crust over oceanic crust
Press, 20-6a
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Continental collision
Press, 20-6c
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Thrust fault - convergence
Press, 18-12a,c
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Transform boundary
Press, 10-22
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Strike-slip fault - transform
Left-lateral
Press, 18-12a,d
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Other ways to deform rock
Press, 10-6
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Convection

• Heat a liquid from below, cool it on top
• Cooler material is more dense
• Hotter material is less dense
• So the cool stuff on top sinks, and the warms
  stuff on the bottom rises
• The liquid continually overturns, like a pot on a
  stove
• Convection is tending to make everything have a
  more similar temperature

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Convection in action

• Water on stove Tectonic plates on mantle

Press, 1-13a, 1-14a
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Core, mantle, and crust
Press, 1-6c
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Some facts

• Tectonic plates move 0 to 20 cm/year
• This is about 25 miles per million years
• The mantle is moving at similar velocities
• It takes about 100 million years for the mantle
  to overturn
• The outer core is a liquid, and it is also
  convecting, but much faster, creating the Earths
  magnetic field

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Where are quakes?

• Mostly near plate boundaries Interplate
• Greatest number at subduction zones
• But also plenty at ridges and transform zones
• A few near hot spots Intraplate
• Well discuss with volcanoes
• Some anomalies in US, also intraplate
• Failed rift - New Madrid
• Unloading after ice age? East coast

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Earthquakes Mgt5, 1963-1988
Keller, 1-5
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Why dont quakes extend deeper?

• Deeper in the Earth, it is hotter
• There is also more pressure, variations in
  composition, and changes in crystal structure,
  but these dont matter as much
• If material is within a few hundred degrees of
  its melting temperature, it quietly flows rather
  than suddenly cracks in an earthquake

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Tectonics of western N. Am.

• Pacific and North America are big plates
• Juan de Fuca, Cocos are smaller plates
• Mix of transform, ridge, and subduction
  boundaries
• Location of boundaries has evolved over past 30 My

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US and Mexico coast

• Three little plates subducting offshore Oregon,
  Washington, and B. Columbia
• Juan de Fuca Plate
• Gorda Plate
• Explorer Plate
• Transform fault San Andreas Fault CA.
• Spreading ridge splitting Gulf of California
• Oblique because ridges are combined with
  transform faults
• Cocos Plate subducting to the south

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N.Am.map
USGS Prof. Paper, 1-2
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Why do we believePlate Tectonics?

• Evidence that Atlantic ocean was once closed
• Same rocks fossils on matching coasts
• East and west Atlantic coasts line up
• Age dating of seafloor rocks
• Other evidence for motion of plates
• Magnetic lineaments on oceanic crust
• Earthquakes focal mechanisms, glacial signs
• Now, we can record plate motion directly

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History of plate tectonics

• 1660 - Francis Bacon, and probably many others,
  noticed similarity in coastlines, no idea what it
  meant
• 1912 Alfred Wegener noticed coastal fit and
  fossil and rock similarities, but very few others
  believe the theory (convection lacking)
• 1960s magnetic stripes, seafloor spreading and
  earthquake distributions convince scientific
  community

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Supercontinents

• Pangaea is name for supercontinent that existed
  about 200 Mya
• All major continents, N. America near equator
• Gondwana is name for supercontinent at about 750
  Mya
• S. America, Africa, India, Antarctica, Australia
• Near South Pole
• Joined by rest of continents about 300-200 Mya gt
  Pangaea
• Supercontinent cycle of 500 Mya

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More supercontinents

• Rodinia
• 1200 - 700 Mya
• Involved most continents
• But in different configuration than Pangaea
• Probably still more earlier supercontinents
• Age of the earth 4.5 billion years
• Age of the universe 15 billion years.

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Liquids versus Solids

• Liquids flow viscosity h resists.
• Solids deform elasticity G resists.
• Maxwell characteristic time t h/G
• h is viscosity coefficient and G is elastic
  modulus
• t 10-12 seconds for water
• t 106 years for earth crust
• Time scale of deformation lt t gt solid
• Time scale of deformation gt t gt liquid
• See experiment with silly-putty

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The earthquake cycle

• Before Loading After quake

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Strain accumulation

• Steady strain rate over many years
• Distributed across zone about 100 km wide
• Only top 20 km build strain in California
• Deeper rocks seem to flow due to higher temp.
• We see strain accumulate with GPS
• Global Positioning System
• If steady, no clues of coming quakes

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Elastic Rebound

• After 100 years of accumulating strain

old road
new road
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Elastic Rebound

• After earthquake

old road
new road
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Elastic Rebound

• Deformation during the earthquake cycle

time 0 yrs
width of deformed zone set by thickness of crust
time 100 yrs
after earthquake!
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Maximum size of quakes

• Subduction zones
• Some bigger than M9
• 1960 Chile quake was 9.5
• 1964 Alaska quake was 9.2
• Larger volume with cold rock
• Bigger cracks, thus larger magnitudes
• Transform and ridge quakes
• Biggest quakes we have seen are M8
• San Francisco 1906 was 7.9
• Most are smaller than M7

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Magnitudes and fault rupture sizes

• Magnitude 8 500 km
• Magnitude 7 70 km
• Magnitude 6 10 km
• Magnitude 5 1.5 km
• Magnitude 4 200 m
• Magnitude 3 30 m
• Magnitude 2 4 m

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How faults break

• Rupture begins
• place on fault at that time closest to breaking
• Spreads outward over fault surface from focus
• At about 3 km/sec (near shear-wave velocity)
• Vslip 1m/s ltlt Vrupture 3 km/s lt Vsurface lt VS lt
  VP
• Larger area implies larger magnitude, and longer
  duration of rupture earthquake

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Vocabulary

• Focus - point where the rupture started
• Hypocenter - location and time of quake beginning
• Epicenter - surface projection of hypocenter
• No clear pattern as to where hypocenter is on the
  fault plane
• Rupture - the sliding of one side of the fault
  against the other side

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Epicenter and hypocenter
Tarbuck 6-3
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Measuring earthquake size

• 1. Intensity
• Some famous old quakes
• 2. Magnitude
• 3. Seismic moment
• Some notable recent quakes

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Local or Richter magnitude

• If seismograph not 100 km from epicenter
• ML log10 (A) C(D) where
• A is the maximum seismic wave amplitude in
  microns (10-6 m) recorded on a standard
  seismograph
• C is a correction factor that is a function of
  distance D from the seismograph to the epicenter

P
S
surface
A
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Types of Magnitude

• ML - Local or Richter magnitude
• Original magnitude, developed by Charles Richter
  in 1930s
• uses S wave recorded within 300 km of epicenter
• mb - Body-wave magnitude
• uses P wave
• MS - Surface wave magnitude
• uses surface wave
• MW - Moment magnitude
• uses seismic moment

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Definition of Seismic Moment

• M0 ? S D F D seismic moment
• Units are force-length, Newton-meters
• Varies over many orders of magnitude
• ? is the rigidity of the rock
• D is the amount of slip or offset between the two
  sides of the fault
• S is the surface area that ruptured
• MW (2/3)(log M0) - 6.0 is now replacing other
  magnitude scales, such as Richter magnitude or
  surface wave magnitude, because it provides a
  consistent measure of size of earthquakes from
  the smallest microtremors to the greatest
  earthquakes ever recorded

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Rule of Thumb

• On average a magnitude m1 earthquake has
• 10 times greater peak amplitude of shaking than a
  magnitude m earthquake
• 30 times greater energy and moment release
• 3 (6-7) longer duration of slip for small (large)

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P waves

• Longitudinal - material moves back and forth
  (vibrates) in same direction that wave travels,
  produces compression/dilatation cycle
• Fastest type of wave, so arrives first
• termed Primary wave
• Typical velocities in crust 5 - 7 km/sec
• Travels through solids or fluids

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S waves

• Shearing - material moves back and forth
  perpendicular to the direction the wave travels
  in a twisting motion
• Slower than P wave, arrives second
• termed Secondary wave
• Typical velocities in crust 3-5 km/sec
• Travels through solids, but not fluids
• because there is no restoring force for the
  perpendicular motions

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Surface Waves

• Two types
• Love waves
• Rayleigh waves
• Travel a bit slower than S waves
• Largest amplitude waves
• Need a surface to travel along, which is the
  rock-air interface at the Earths surface
• Motion is strongest near the surface
• Most strongly generated by earthquakes near the
  surface
• Technically, have the names of Love and Rayleigh
  waves

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S wave
shadow
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ReviewProfile ofthe Earth
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Crust, Mantle, and Core

• Crust is thin veneer floating on mantle ,Rigid
  plates
• Layer of lighter composition than mantle (2.7
  g/cc ),
• Moho (Mohorovicic) is boundary between crust and
  mantle
• Thickness mapped by seismic work
• Thinner under oceans (4-6 km)
• Thicker under continents (25-80 km)
• Mantle is most of Earths mass, dense rock (3.3
  g/cc)
• Slowly flowing in convection
• Several phase changes in upper mantle
• Deeper rock is denser and stiffer due to
  increasing pressure
• Cores radius is about half of Earths radius
• Outer core is liquid iron (85), Convection leads
  to magnetic field
• Lower P velocity than mantle and no S waves
  allowed in liquid!
• Inner core is solid iron
• Inner core grows as outer core freezes Because
  Earth is cooling

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Bigger magnitudemore fault area, more offset
9.2
9.5
Kovach, p. 47
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Global counts of quakes
Rough numbers per year 2000 with M near 5 180
with M near 6 17 with M near 7 2 with M near
7.9 0.25 with M near 8.6
Note, there are several kinds of magnitudes and
we havent defined bins precisely
Kovach, p. 46
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Gutenberg-Richter power law distribution
Log N a - b Mw
7.3
6.7
8
Mw (2/3) log M0 -6.0
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Aftershocks

• smaller earthquakes following the largest
  earthquake of a sequence (the mainshock) near
  mainshock rupture zone
• follow almost all shallow earthquakes
• cover ruptured area
• can number in thousands
• can last for years or decades
• aftershocks of Northridge M 6.7 are still
  occurring

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Aftershocks
Seismicity rate in South California, 1987-2000

• non-stationary seismicity rate
• increase of the seismicity rate after large
  earthquakes aftershocks

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Numbers of aftershocks

• Northridge 13,523 aftershocks in 1994-1996
• Landers 65,380 aftershocks in 1992-1996
• 22 with M gt 5.0
• Most were M1 or 2
• There were many more too small to detect

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Distribution of sizes

• As for mainshocks, there are many more small
  aftershocks in a sequence than big aftershocks
• If mainshock has M 6
• 1 or 2 aftershocks with M 5 to 6
• 10s of M 4 to 5
• If mainshock has M 8, an M 7 aftershock is likely

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Probability of quake

• Find the faults
• Estimate how faults are segmented
• How does each segment behave
• Size of its quakes
• Time between quakes - recurrence interval
• Sum up risk from all segments of all faults
• (This exercise tells how much shaking)
• Then figure out expected damage

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Fault zone segmentation

• Characteristic earthquake model
• Only one segment breaks at a time
• Segments defined by
• Ends of fault traces
• Fault intersections
• Best guesses - segment defined from prior quakes
• Not clear whether the concept of fault
  segmentation is correct

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Wasatch Faultsegmentation
1
1
2
3
4
5
6
Keller, 8-21
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HistoryofWasatchsegments
Age of faulting events on the Wasatch Fault
Pinter workbook 10-4
Now
6000
(years)
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How does this apply to SoCal?

• Outline
• Segmentation of the San Andreas Fault
• Behavior of a segment on the San Andreas
• Probabilities for San Andreas segments
• Locations of all SoCal faults
• Total probability across SoCal

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San Andreassegmentation
1906-type events
creep
Four major segments
1857-type events
Keller, 8-20
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Big Onehistoryin SoCal
1857-type segment
Keller, 8-23
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From this history

• 10 events in 1300 years
• An event every 130 years, on average
• Last event 140 years ago
• We are overdue!
• But events are not regularly timed
• So better guess would be
• about 25 chance of this quake in next 30 years
• (thats 30 years / 130 year repeat time)

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30-yr probability of quakes in California
Parkfield
1906 repeat
1857 repeat
Yanev p. 39
1857-type is given 30 chance in 30 years
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Avoid living in fault zone

• Should be zoned for parks
• Or, at a minimum, roadways
• It's best to live 5 miles or more away from
  faults
• Often unrealistic
• Even creeping faults are bad news

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Relation of danger to faults

• Worst danger near faults
• Most damage within 50 km
• Occasional pockets of damage out to 100-200 km
  from rupture
• Usually due to very soft soil
• Shape of isoseismals
• M lt 6.5 form circular isoseismals
• Long rupture elongated isoseismals

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Landslides

• Landslide a chunk of ground, usually wet and
  weak, breaks loose, then slides down hill
• Earthquakes often trigger landslides
• Landslide most common on hillsides, steep slopes
• From both natural and man-made causes

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Summary of soft ground

• Landslides
• Can result from natural or man-made problems
• Biggest slides are natural
• Soft soils have several problems
• Liquefaction, landslide, settling, river banks
• Indications of various problems are similar
• Yanev (an engineer) says to consult an engineer
• Tsunamis can hit some of the same areas

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Tsunamis

• Sea wave triggered by undersea event that uplifts
  or downdrops ocean floor
• Undersea earthquake
• Undersea landslide
• Undersea volcanic eruption
• Very occasionally, big meteorite impact!!
• These events can displace lots of water,
  producing a wave

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Tsunami Hazards

• Several wave crests with 15 minutes between
  crests.
• Often water withdraws from shore first.
• People go to beach to look, gather clams, etc.
• Water returns very rapidly causing much damage.
• Avoid beach if you feel strong quake!

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Convergence direction

• Indo- Autralian plate underthrusts Asia
• 4 cm/year
• more oblique to north

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2004 Great Sumatra Earthquake map of Tsunami
energy propagation
Titov et al.
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Structural componentsof a building

• Distributing elements
• Horizontal components
• Floors and roof
• Resisting elements
• Vertical
• Walls, columns, bracing
• Foundation
• Connections

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Elements of a building

• Distributing elements
• Horizontal components
• Floors and roof
• Resisting elements
• Vertical
• Walls, columns, bracing
• Foundation
• Connections

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Building materials

• Wood and steel preferred over concrete and brick
  because
• Light, which lessens weight that walls must
  support
• Flexible so it can deflect without cracking or
  breaking.
• But too much flexibility is bad, making bracing
  necessary

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Wood-frame buildings

• If well-built, safest structures due to lightness
  and flexibility of wood
• May still have damage if
• On unstable ground
• Not well fastened to foundation
• Inadequate lateral bracing
• Poorly maintained
• Weak foundation

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Diagonal bracing

• Some wood frame houses have only diagonal
  bracing, as opposed to plywood sheathing on shear
  walls
• Weak, and better construction is not expensive
• Sheet rock sheathing has little strength

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Weak diagonal bracing
Soon covered by sheetrock Cut into boards
Yanev 89
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Properplywoodbracing
Lots of nails Big pieces of wood Attach to
foundation
Yanev 90
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