What is an Earthquake?

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What is an Earthquake?
Earthquakes occur along (within) faults. These
represent fractures (zones of weakness) within
the brittle rocks that makeup the crust of our
planet. It is the abrupt rupture or movement
along these structures that triggers an
earthquake. These phenomena rarely last even 1
minute yet are responsible for the greatest
number of lives lost (and property damage) due to
a natural event!
TYPES OF FAULTS
Strike-slip Thrust Normal The
form of faulting depends upon the type of plate
motion and nature of rocks.
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Where do earthquakes occur? Is it random? Is
any place safe or free of earthquakes?
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HOW COMMON ARE EARTHQUAKES? Magnitude (Richter
Scale) Number per Year 0 3.4 gt800,000 4
.9 5.4 1400 5.5 6.1 500 7.0
7.3 15 gt8 1 every 10 years
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Earthquakes create seismic waves which shake the
ground as they pass. Earthquakes create waves
just like waves of water moving across the ocean
and waves of air moving across a field of wheat.
Consider what happens when a drop of rain hits
a pond of water. The drop disturbs the flat
surface of the water and creates waves that
travel outward in all directions from the
disturbance. These waves travel on the surface of
the pond, along the interface between the water
and the air.
When an earthquake occurs, the energy is
dissipated into the surrounding medium through
seismic waves.
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HOW IS THE ENERGY DISPERSED SPREAD INTO THE
SURROUNDING MEDIUM?
Seismic waves are the waves of energy caused by
the sudden breaking of rock within the earth or
an explosion. They are the energy that travels
through the earth and is recorded on
seismographs. Types of Seismic Waves There are
several different kinds of seismic waves, and
they all move in different ways. The two main
types of waves are body waves and surface waves.
Body waves can travel through the earth's inner
layers, but surface waves can only move along the
surface of the planet like ripples on water.
Earthquakes radiate seismic energy as both body
and surface waves.
BODY WAVES P-waves also known as primary
waves or pressure waves S-waves also referred
to as secondary or shear waves SURFACE
WAVES Love and Raleigh waves
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SURFACE WAVES
(Theses produce all the damage!)
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HOW IS EARTHQUAKE INTENSITY MEASURED?
There are currently two measuring schemes used
to quantify or qualify the destructiveness or
power of an earthquake. 1) The Mercalli
Intensity scale (it is qualitatively based) 2)
The Richter Magnitude scale (it is quantitatively
based)
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The following is an abbreviated description of
the 12 levels of Modified Mercalli intensity. I.
Not felt except by a very few under especially
favorable conditions. II. Felt only by a few
persons at rest, especially on upper floors of
buildings. III. Felt quite noticeably by persons
indoors, especially on upper floors of buildings.
Many people do not recognize it as an earthquake.
Standing motor cars may rock slightly. Vibrations
similar to the passing of a truck. Duration
estimated. IV. Felt indoors by many, outdoors by
few during the day. At night, some awakened.
Dishes, windows, doors disturbed walls make
cracking sound. Sensation like heavy truck
striking building. Standing motor cars rocked
noticeably. V. Felt by nearly everyone many
awakened. Some dishes, windows broken. Unstable
objects overturned. Pendulum clocks may stop.
VI. Felt by all, many frightened. Some heavy
furniture moved a few instances of fallen
plaster. Damage slight. VII. Damage negligible
in buildings of good design and construction
slight to moderate in well-built ordinary
structures considerable damage in poorly built
or badly designed structures some chimneys
broken. VIII. Damage slight in specially
designed structures considerable damage in
ordinary substantial buildings with partial
collapse. Damage great in poorly built
structures. Fall of chimneys, factory stacks,
columns, monuments, walls. Heavy furniture
overturned. IX. Damage considerable in specially
designed structures well-designed frame
structures thrown out of plumb. Damage great in
substantial buildings, with partial collapse.
Buildings shifted off foundations. X. Some
well-built wooden structures destroyed most
masonry and frame structures destroyed with
foundations. Rails bent. XI. Few, if any
(masonry) structures remain standing. Bridges
destroyed. Rails bent greatly. XII. Damage
total. Lines of sight and level are distorted.
Objects thrown into the air.
The Mercalli Intensity Scale
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Richter Equivalent energy yield Magnitude in
TNT Example -1.5 6 ounces Breaking a rock
on a lab table 1.0 30 pounds Large Blast
at a Construction Site 1.5 320 pounds
2.0 1 ton Large Quarry or Mine Blast 2.5
4.6 tons 3.0 29 tons 3.5 73 tons
4.0 1,000 tons Small Nuclear Weapon 4.5
5,100 tons Average Tornado (total energy)
5.0 32,000 tons 5.5 80,000 tons Little
Skull Mtn., NV Quake, 1992 6.0 1 million
tons Double Spring Flat, NV Quake, 1994 6.5
5 million tons Northridge, CA Quake, 1994
7.0 32 million tons Hyogo-Ken Nanbu, Japan
Quake, 1995 Largest Thermonuclear Weapon
7.5 160 million tons Landers, CA Quake, 1992
8.0 1 billion tons San Francisco, CA Quake,
1906 8.5 5 billion tons Anchorage, AK
Quake, 1964 9.0 32 billion tons Chilean
Quake, 1960 10.0 1 trillion tons
(San-Andreas type fault circling Earth) 12.0
160 trillion tons (Fault Earth in half through
center, OR Earth's daily receipt of solar
energy)
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A rapidly expanding "solar quake" on the Suns
surface depicted here by the Michelson Doppler
Imager (MDI). It immediately followed a solar
flare and spread out like a ripple in water more
than 100,000 km at the solar surface. Scientists
have shown that solar flares produce seismic
waves, and gigantic seismic quakes, in the Sun's
interior.
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Comparision of seismic effects between the 1906
San Francisco Quake and the New Madrid Quake of
1811-1812.
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LABORATORY ACTIVITYDetermination of Earthquake
Epicenters and their Magnitudeshttp//www.scien
cecourseware.com/VirtualEarthquake/
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SECONDARY EFFECTS ASSOCIATED WITH EARTH QUAKES
It is these phenomena that actually kill people,
i.e., ground-shaking does not usually harm an
individual (maybe their pants!), it is what the
shaking does to the surrounding environment that
results in bodily harm! -Ground shaking
(structural collapse, large fissures,
etc.) -Fires (in urban settings broken gas
lines, etc.) -Landslides -Ground
liquefaction -Seismic sea waves (Tsunamis)
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Northridge, CA 1994 (Mag. - 6.7, 10 sec.
duration)
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Northridge, CA 1994 (Mag. - 6.7, 10 sec.
duration)
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Loma Prieta, CA 1989 (Mag. 7.1, 7-15 sec.
duration
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Kobe, Japan 1995 (Mag. 7.2)
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Kobe, Japan 1995 (Mag. 7.2)
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Landers, CA 1992 (Mag. - 7.3)
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Mexico City, 1985 (Mag. 7.9)
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DEMONSTRATION/LAB ACTIVITYThe Natural
Frequency of Buildings
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Ground Liquefaction Niigata, Japan 1964 (Mag.
7.5)
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The Government Hill Elementary school in
Anchorage was torn apart by subsidence of the
graben at the head of the Government Hill
landslide. The south wing of the school dropped
about 30 feet (9 m) the east wind split
lengthwise and collapsed. The playground became a
chaotic mass of blocks and fissures.
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Good Friday Quake, Anchorage, AK 1964 (Mag. 8.6)
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This slide shows the Turnagain Heights landslide
in Anchorage. Seventy-five homes twisted,
slumped, or collapsed when liquefaction of
subsoils caused parts of the suburban bluff to
move as much as 2,000 feet (606 m) downward
toward the bay forming a complex system of ridges
and depressions.
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Loma Prieta, CA 1989 (7.1, 7-15 seconds)
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DEMONSTRATION/LAB ACTIVITYSoils and
Earthquakes(Ground Liquefaction)
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This slide shows a fire at Valdez, Alaska, The
tank fire was triggered by failure of oil storage
tanks at the Union Oil tank farm. By 1030 p.m.
about 5 hours after the quake, the whole
waterfront was burning furiously. Some buildings
along Front Street and Standard Oil's pumping
control station also caught fire. The Union Oil
tank farm continued to burn for two weeks.
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This view, taken a few months after the
earthquake, is looking north along the waterfront
at Seward. Note the "scalloped' shoreline left by
the underwater landslides, the severed tracks in
the railroad yard which dangle over the landslide
scarp, and the heaps of railroad cars and other
debris thrown up by the tsunami waves. This
massive submarine landslide destroyed a large
portion of the Seward waterfront during the quake.
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Living spruce trees up to 24 inches in diameter
and between 88 and 101 feet above sea level were
broken and splintered near Shoup Bay by the
surge-wave generated by an underwater landslide
in Port Valdez, Prince William Sound.
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Tsunami Generated by Earthquake of March 27,
1964, Prince William Sound, Alaska. Surge wave
left 2 x 12 in. plank in truck tire at Whittier,
Alaska. One of the waves, probably the same one
that caused the major damage in Whittier, reached
a height of 31.7 m above low tide.At Whittier
the waves destroyed two saw mills the Union Oil
Company tank farm, wharf and buildings the
Alaska Railroad depot numerous frame dwellings
and the railroad ramp handling towers at the army
pier. They also caused great damage to the small
boat harbor. The tsunami killed thirteen people
at Whittier, a community of 70 people.
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Aftermath of the Chilean tsunami in the Waiakea
area of Hilo, Hawaii, 10,000 km from the
generating area. Parking meters were bent by the
force of the debris-filled waves. Note the
scattered debris and the gutted foundation. The
earthquake off the coast of central Chile
generated this tsunami that affected the entire
Pacific Basin. One of the most seriously affected
areas was Hilo, Hawaii, where 61 deaths and 23
million in damage occurred.
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Faults of Southern California
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A SHAKE MAP of the region surrounding the
Northridge quake, 1994 (a suburb of Los Angeles).
The colors reflect the degree/intensity of
ground motion experienced on the surface of the
earth.
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Bigger faults (longer) make bigger earthquakes
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Bigger earthquakes last longer
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FACTORS EFFECTING THE DESTRUCTIVENESS OF AN
EARTHQUAKE
Magnitude
The more energy released, the greater potential
for damage, regardless of all other factors (and
greater areal extent).
Distance
What Controls the Level of Shaking?
Shaking decays with distance
Local soils and bedrock geology
Soil characteristics may amplify the
shaking. (seasonal climatic variations can impact
this) Complex geology tends to dampen waves
Building codes and materials
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• Strategies of Earthquake Prediction
• or
• Flip a Coin
• Lengthen Historical Data Base
• Historical Records
• Paleoseismology
• Short-term Prediction
• Precursors
• Increased frequency of micro-tremors
• Ground bulging or dilation
• Abrupt changes in ground-water levels
• Animal behavior
• Long-term Prediction
• Seismic Gaps
• Risk Levels

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The deadliest earthquakes on record (in terms of
lives lost).
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Assigned exercises and/or demonstrations
Earthquake location (epicenter) and magnitude
determination Virtual Earthquake internet
exercise http//www.sciencecourseware.com/Virtual
Earthquake/ Ground liquefaction (demonstration
and lab activity) The Natural Frequency of
Buildings (lab activity/demonstration) Making
your own P and S waves (demo)