Download PDF file of Chapter 11 lecture - Earthquakes

Dr. James Wittke
[email protected]
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Vibrations produced by
rapid release of energy
(rupture and slippage
along faults)
Origin of break at focus
(hypocenter)
Epicenter located directly
above focus
Energy in form of seismic
waves radiates in all
directions from source
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Most of motion along
faults can be explained
by plate tectonics theory
◦ Extension (normal)
◦ Compression (reverse,
thrust)
◦ Shear stress (transform)
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Type of fault can be
determined from
earthquake data
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Faults kept from moving by
friction  build up elastic strain
Earthquakes occur when
stresses overcome friction 
rocks on both sides “rebound”
Break starts at focus and rupture
spreads outward across fault
surface (travels at 2-3 km/s)
Rupture reaches surface 
surface breaks
Final displacement called fault
slip (typically 1 m, maximum
20 m)
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Surface breaks
(ruptures) can be
100s km long
◦ San Francisco
(1906) break ran
430 km
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Some faults do
not reach
surface, e.g.,
Northridge (1994)
Boys adjacent to young surface break due to
relatively recent, but probably prehistoric,
earthquake (near Minjar River, Russia).
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Most studied fault system in
world
Displacement along discrete
segments 100 to 200 km
long
Some sections show slow,
gradual displacement (no
earthquakes  fault creep)
Other segments slip regularly
causing small earthquakes
Other segments store energy
for 100s of years before
rupturing in great
earthquakes
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Small foreshocks
may precede major
earthquake by days
or as much as
several years
Adjustments after
major earthquake
can cause smaller
earthquakes called
aftershocks
Sumatra 2004 Aftershocks (star
indicates location of main earthquake)
Alaska 2002 Aftershocks
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Seismometers record ground movement caused by body
(P, S) and surface waves
Different types of seismometers needed to record
vertical and horizontal ground motion
Records obtained called seismograms (above)
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Travel through Earth’s
interior
Two types based on how
they propagate…
◦ Primary (P) waves
◦ Secondary (S) waves
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P waves faster than S
waves
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Travel through solids, liquids, and gases
Push-pull (compress and expand) motion, changing
volume of intervening material
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Shaking motion at right angles to their direction of
travel
Travel only through solids
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Travel along surface of Earth
Greatest amplitude and slowest
velocity
Cause greatest destruction
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Three station recordings
needed to locate
epicenter
Each station determines
time interval between first
P wave arrival and first S
wave arrival
Travel-time graph used to
determine each station’s
distance to epicenter
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Circle with radius
equal to distance
to epicenter
drawn around
each station
Point where all
three circles
intersect is
earthquake
epicenter
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Intensity
◦ Qualitative measure damage and effects at a
given location
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Magnitude
◦ Quantitative measure of energy released at
source of earthquake
◦ Based upon amplitude of largest seismic wave
recorded
◦ Accounts for decrease in wave amplitude with
distance from epicenter
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Qualitative report of earthquake's intensity or destructiveness
Roman numerals (I to XII) describe what people felt or saw
Damage/experiences may not reflect earthquake’s size accurately
◦ Affected by how close to focus, local geology, type of earthquake, etc.
Northridge 1994
Japan 1925
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Introduced by Charles
Richter in 1935
◦ Based on amplitude of largest
seismic wave
◦ Accounts for decrease in
amplitude with distance
◦ Intended for local shallow
southern CA earthquakes
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Magnitudes <2.0 not felt
by humans
Each unit of magnitude
increase…
◦ 10x increase in amplitude
◦ About 32x increase in energy
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Developed because Richter
scale does not adequately
indicate size of very large
earthquakes
Total strain energy based
upon…
◦ Displacement along fault
◦ Strength of rock
◦ Area of rupture surface
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Examples…
◦ Alaska 1964: ML = 8.3, MW =
9.2
◦ San Francisco 1906: ML = 8.3,
MW = 7.9
◦ Chile 1960: MW = 9.5 (largest
known)
About 95% of energy released by earthquakes originates in few
relatively narrow zones that wind around globe
Earthquakes
with magnitudes
>7.5 since 1990
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Shallow: 0-70 km deep
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Intermediate: 70-300 km
deep
◦ In crust (most destructive)
◦ Divergent plate boundaries
◦ Convergent plate margins
(subduction)
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Deep: >300 km
◦ Convergent plate margins
(subduction)
◦ Almost all in circum-Pacific
belt (landward of deepocean trenches)
Earthquake foci at subduction margins form WadatiBenioff zone (depths increase away from trench)
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Damage depends on...
◦ Magnitude
◦ Duration of vibrations
◦ Nature of material upon
which structure rests
◦ Type and design of
structures
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Deaths depend on…
◦ Amount of structural
damage
◦ Population density
◦ Time of day
Damage in
Anchorage due to
Alaska 1964
earthquake
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Amplification
depends on
nature of ground
Soft sediments
amplify waves
more than
bedrock
Predicted
amplification (Los
Angeles area)
Unconsolidated materials
saturated with water turn
into mobile fluid
Liquefaction hazard in Alameda, Berkeley, Emeryville,
Oakland, and Piedmont for magnitude 7.1 earthquake
on the Hayward fault
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Sudden vertical displacement along ocean-bottom
fault, or
Large undersea landslide triggered by earthquake
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Want: warning of location and magnitude of
earthquake within narrow time frame
Methods: monitor phenomena before
forthcoming earthquake
◦ Ground uplift or subsidence
◦ Gas releases
◦ Strain (deformation) in rocks
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No reliable method exists
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Based on idea that earthquake occurrence is
repetitive or cyclical
Provides a probability of certain magnitude
earthquake occurring on long time scale (30100+ years)
Probability of earthquake of listed size
occurring between 1988 and 2018
Earthquake
Prediction:
Geographic
Progression