Transcript EarthquakesHnrs2

Earthquakes
Parkland High School
What is an Earthquake?

Vibration of the Earth produced by the
rapid release of energy or movement of
the fractures in the crust
 Focus---the place within the Earth where
the rock breaks, energy is released and
radiates in the form of seismic waves
 Epicenter---the point on the ground's
surface directly above the focus
Earthquakes and Faults
Vertical and horizontal movements
are associated with large fractures in
Earth’s crust called faults
 Movement along these faults is
explained by plate tectonics (faults
associated with plate boundaries
which strain rocks)
 Three types of faults
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Reverse Fault
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Caused by compression
• Stress that decreases the volume of
material
Shortens the crust
 http://www.calstatela.edu/faculty/acol
vil/struct/blind_animation.gif
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Normal Fault
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Caused by tension
– Stress that pulls a material apart
Extends the crust
 http://www.calstatela.edu/faculty/acol
vil/struct/normal_animation.gif
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Strike-Slip Fault
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Caused by shear
– Stress that causes a material to twist
Ex-San Andreas Fault
 http://www.calstatela.edu/faculty/acol
vil/struct/rightlateral_animation.gif
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Cause of Earthquakes
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Stress
– Force per unit area or pressure acting on a
rock
– When force exceeds strength of rock it
fractures
– Three types (compression, tension, and shear)
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Strain
– Deformation of materials in response to stress;
the result of the stress
Strain/Deformation
– Elastic rebound or deformation - when
stresses are removed, rock returns to
original shape
(think of a rubber band)
– Ductile or Plastic deformation - permanent
deformation; when stresses are removed,
rock stays bent (results in increase in size)
– Rupture or Failure - breakage and fracturing
of the rock, causing an earthquake
Cause Continued…
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Can also be looked at as in Figure 6.5
on pg 167 and compared to a limber
stick
– Original position
– Buildup of strain
– Slippage (earthquake)
– Strain released
Stress-Strain Curve
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Has two segments: straight and curved
– Low stress  Straight segment  Elastic
deformation
– High stress  Curved segment  Ductile
deformation
– Most brittle materials (glass, wood) fail before
much ductile deformation occurs, while most
ductile materials (rubber, metals) can undergo a
great deal of deformation before failure begins or
they never fail at all
– Most rocks are brittle under low temps in Earth's
crust and ductile under high temps at greater
depths
Foreshocks and Aftershocks
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Foreshocks
– Small earthquakes that precede a major
earthquake by days or possibly years
– Can be used in prediction
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Aftershocks
– Smaller earthquakes that occur after a
major earthquake as the fault adjusts
and moves
– Weaker but can still cause damage
Seismology
The study of earthquake waves
 Began with the Chinese about 2000
years ago
 A seismograph or seismometer
detects/records seismic waves
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Seismograph
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How it works:
– Suspended mass hanging from a support that
is attached to (and moves with) the ground.
– Inertia keeps the suspended mass stationary
while the ground moves below it.
– The movement is recorded on a rotating drum
or magnetic tape.
– Prints seismogram (shows that waves are
elastic energy)
Seismic Waves
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Types of waves
– Body
• Travel through Earth’s interior
• Primary or P waves
• Secondary or S waves
– Surface
• Travel along Earth’s outer layer
P Waves
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Also called compressional waves
Push and pull waves (compress and expand the
rocks in the same direction the wave is traveling)
Fastest of the seismic waves
Travel through solids, liquids, and gases
(temporarily change the volume of the material by
compressing and expanding)
http://www.eserc.stonybrook.edu/wise/WSE187sp
r2002/PrimaryWaves.html
http://wwwrohan.sdsu.edu/~rmellors/lab8/l8pwav2.htm
S Waves
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Move rocks from side to side or at right angles to
the direction the wave is traveling
 Travel through solids only (temporarily change
the shape of the material)
 http://www.eserc.stonybrook.edu/wise/WSE187sp
r2002/SecondaryWaves.html
 http://wwwrohan.sdsu.edu/~rmellors/lab8/l8swav2.htm
 http://www.classzone.com/books/earth_science/t
erc/content/visualizations/es1002/es1002page01.c
fm?chapter_no=visualization
Surface Waves
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Complex motion: up-and-down and side-toside (two directions)
Slowest
Causes damage to structures during an
earthquake
http://wwwrohan.sdsu.edu/~rmellors/lab8/l8rwav2.htm
http://wwwrohan.sdsu.edu/~rmellors/lab8/l8awav2.htm
Seismic Record
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P waves arrive first, followed by S waves, and
then surface waves
Results from their speeds
Velocity of P waves through granite is 6 km/s
Velocity of S waves through granite is 3.5 km/s
On average, P waves travel about 1.7 times faster
than S waves
Surface waves travel at 90% of the velocity of S
waves
Earth’s Interior
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Please read this section in text on pg. 182-187
and take notes/outline
 Layers defined by composition: crust, mantle,
core
 Layers defined by physical properties:
lithosphere, asthenosphere, mesosphere, inner
and outer core
 Moho
 Shadow zone
 Discovering Earth’s composition
 See figure 6.25 and 6.26
Locating an Earthquake
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The greater the interval measured on a
seismogram between the arrival of the
first P wave and the first S wave, the
greater the distance to the earthquake
source
 So, first examine the seismograph and
determine the elapsed time between the
arrival of the first P-wave and the first Swave
Locating an Earthquake Continued…
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Next, use a time-distance graph:
Knowing the S - P time, you can determine the
distance to the epicenter from the seismic station
– See figure 6.9 on pg. 172
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Then, on a map, draw a circle around the seismic
station
(Radius of circle = distance to epicenter)
Repeat this for two other seismic stations
(Triangulation)
The three circles will meet at a point; that point is
the epicenter.
Locations of Earthquakes
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Most occur along tectonic plate
boundaries:
– Around Pacific Ocean (Circum-Pacific Belt)
– Mediterranean-Asia Belt (Indonesia, Himalayan
region)
– Mid-ocean ridges
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Some occur far from plate boundaries
Measuring Earthquakes
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Two different types to describe the size
– Intensity
• Measure of the degree of the shaking based on
amount of damage
• Modified Mercalli Scale
– Magnitude
• Measure of the amount of energy released at the
source
• Relies on seismic records
• Richter and Moment Magnitude Scales
Modified Mercalli Scale
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Developed in 1902 by Guiseppe Mercalli
Intensity scale
Describes damage to structures.
Ranges from I (felt by only a few) to XII (total
destruction)
Modified using California buildings as its
standard so usable throughout US and Canada
Disadvantages: based on effects, so not only
ground shaking, but also population density,
building design, and the nature of surface
materials
Richter Scale
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Developed by Charles Richter in 1935 at CIT
 Based on amplitude of largest seismic wave (P, S,
or surface) recorded on the seismogram--measures magnitude
 Logarithmic scale: each number on the Richter
Scale is ten times greater in wave amplitude (an 8
on scale is 10 times larger than a 7 and 100 times
larger than a 6) and each number on the Richter
Scale involves an energy release about 32 times
as great
Richter Continued…
Wood-Anderson seismograph is the
standard recording device
 Largest magnitude ever recorded
was 8.9
 Saturated for large earthquakes
because they cannot distinguish
between the size of the events
 See Table 6.2 on pg. 175
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Moment Magnitude Scale
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Also measures magnitude of earthquakes
Takes into account the size of the fault
rupture (area), the amount of movement/
displacement along the fault, and the rocks’
stiffness/strength
Estimated from size of several types of waves
Better to describe very large earthquakes
Strongest on record---moment magnitude of
9.6 (1960 Chile)
Destruction
Seismic Vibrations
 Tsunamis
 Landslides and Ground Subsidence
 Fire
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Seismic Vibrations
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Structural damage from earthquake
waves depends on:
– Intensity
– Duration
– Nature of material upon which structure
sits
– Design of structure
Structural Damage
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Most damage occurs to unreinforced building
made of stone, concrete, etc.
Wooden structures are resilient and sustain
less damage
High-rise, steel-frame buildings are often
reinforced and sustain less damage
Buildings may rest on rubber structures to
absorb vibrations
Soft sediments amplify vibrations more than
solid bedrock
Liquefaction: soil turns into a fluid (saturated
with water)
Tsunamis
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Earthquakes under the ocean caused by
seismic sea waves (caused by
displacement of sea floor along a fault)
Waves travel at 500 - 950 km/hr
Fast, high energy waves, but not tall (less
than 1 m) in deep sea
Not distinguishable in open ocean (height
less than 1 m)
When they enter shallow water, they slow
down, the water stacks up (30 m)
Tsunamis Continued…
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First sign = withdrawal of water from coast
 5 - 30 min later, a BIG wall of water arrives
(100 ft)…can extend hundreds of meters
inland
 Each surge is followed by a retreat of
water
 Waves are separated by intervals of 10-60
minutes
Tsunami Warning System
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Put in place by the U.S. Coast and
Geodetic Survey for coastal areas of the
Pacific after 1946 tsunami struck Hawaii
 Large earthquakes reported to the
Tsunami Warning Center in Honolulu and
scientists use tidal gauges to determine if
tsunami has formed
 Within an hour a warning is issued
Landslides and Ground Subsidence
Caused by liquefaction
 Can cause great disaster
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Fire
Begin when gas and electrical lines
sever
 Can be compounded when water
lines are broken as well
 Can be contained by creating a fire
break: buildings are dynamited along
a boulevard
 Can cause destruction and death
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Earthquake Prediction
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Short-range
– Goal: to provide a warning of the
location and magnitude of a large quake
within a narrow time-frame (hours or
days)
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Long-range
– Goal: to predict the probability of a
certain magnitude quake within a time
scale of 30-100 years or more
Short Range
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Efforts occurring in countries with
greatest risk: US, Japan, China, and
Russia
 Must be accurate and reliable to be
utilized for evacuation
 Monitor possible precursors
– Ground tilt (lasers, creep meters, tilt meters,
strain gauges, etc.) on rocks near faults
– Animal behavior
– Foreshocks
Long Range
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Based on the idea that quakes are cyclical or
repetitive (as soon as one is over, the plates begin to
build strain on rocks again)
 Monitor to look for patterns
– Parkfield, California had earthquakes every 22
years
– In anticipation of the next one, seismographs were
put into place, but it has not yet occurred (overdue
by 16 years since last one was in 1966)
 Seismic gaps: sections of active faults that have not
experienced significant earthquakes for a long period
of time
– San Andreas---San Francisco will probably
experience an earthquake of magnitude 7 or higher
in next 30 years (1988 to 2018)