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9/19/2012
1
EARTHQUAKES
Definition of Earthquake
Ground trembling or shaking due to sudden
release of energy accumulated in deformed
rock.
Rocks at relatively shallow depths in the Earth’s
crust (cool and brittle) deform until they reach a
yield point and break forming faults.
At new ruptures or movement along older faults,
strain energy is released suddenly and the earth
quakes.
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Aftershocks and Foreshocks
Aftershock — a series of smaller quakes that
occur after a major earthquake as the crust
readjusts to the change in stress.
Aftershocks are usually smaller but are very
destructive.
Foreshocks — small earthquakes that may (or
may not) precede a major quake by days to
years.
It is hoped that monitoring foreshocks may be
useful
as a prediction tool.
Location of an Earthquake
Focus (Hypocenter): precise underground location at
which rocks begin to rupture.
Epicenter: point on the Earth’s surface directly above
the focus.
The energy of an earthquake is
released from the slippage along a
fault and radiates in all directions.
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Seismology
Seismology is the study of earthquakes.
Seismographs are instruments that detect and record earthquakes.
In the seismograph, inertia tends to
keep the suspended mass motionless
while the recording surface (on a
rotating drum) vibrates with the
bedrock.
Thus the seismograph measures the
displacement or movement of the
ground as seismic waves pass through
the station.
QuickTime™ and a Cinepak decompressor are needed to see this picture.
Typical seismographs consist of
rotating drums with recording
paper.
Most modern seismographs
now record data digitally and
are available in near real time
on the internet.
USGS
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Some Uses of the Seismograph
Measurement of the energy released by an earthquake (Richter Magnitude)
Measurement of the location of an earthquake (epicenter)
Interpretation of the Interior of the Earth (Chapter 11)
Detection of underground nuclear bomb testing
Numerous spin-off apps
Thousands of seismographs are
deployed in national and international
networks to record earthquakes.
Seismographs
This extensive network
permits us to determine
the location of an
earthquake and to
study the earth’s
interior.
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Seismograms record the arrival of seismic energy at the recording
station.
Careful analysis of seismograms indicates that several different types
of seismic waves are recorded.
•Body waves travel through the
Earth’s interior and provide useful
information about the earthquake
and the interior structure of the
Earth.
•Surface waves move along the
surface of the Earth. They tend to be
the most destructive.
Seismic Waves – Seismic Energy Traveling
How does seismic energy propagate through the Earth?
earthquake
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Two types of body waves:
P-waves - primary waves -
compress and extend
material in the direction of
wave travel.
S-waves - secondary
waves move the material in
a direction that is normal to
the direction of wave travel.
Body Waves: P-waves
P-waves travel ~6 km/sec. They are
compressional waves and particle
motion is in the travel direction.
QuickTime™ and a Cinepak decompressor are needed to see this picture.
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Body Waves: S-waves
S-waves travel in the crust ~3.6 km/sec
(slower than p-waves). They
propagate through the Earth by
displacing particles perpendicular to
the direction of travel.
QuickTime™ and a Cinepak decompressor are needed to see this picture.
P-waves travel ~1.7 times
faster than s-waves.
Hence, the farther the
seismograph is from the
location of the earthquake
the greater the difference
in arrival times between
the p-wave and s-wave.
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Seismograms
From the arrival times of the different types of waves, we can see that
VelocityP-waves > VelocityS waves > Velocitysurface waves
The amplitude shows relative destructive force(energy) of each wave:
Energysurface waves > EnergyS-waves > EnergyP-waves
•Note that the S-wave arrives at this seismograph station ~240
seconds after the arrival of the P-wave.
• The greater the difference between the arrival of the P- and S-waves
(the S-P interval), the more distant the earthquake from the recording
station.
S-P interval
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Locating the Distance to an Earthquake
We can determine the distance to the earthquake epicenter from the
seismograph by the lag time between the P-waves and S-waves.
Locating the Epicenter of an Earthquake
If we have the
distance to the
earthquake
determined for 3
different seismograph
stations, we can
locate the epicenter
by triangulation.
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Measuring the Size of an Earthquake
Richter Scale
Moment-Magnitude Scale
Modified Mercalli Intensity Scale
Earthquake Magnitude
The magnitude of an earthquake is related to the amount of energy
released during a seismic event.
The Richter scale is used to describe earthquake magnitude. It is
determined from deflections recorded on seismographs and uses a
logarithmic scale - each unit is a 10-fold increase in the amplitude of
the seismic waves and a 32-fold increase in energy.
Earthquakes with magnitudes less than ~2.0 are not commonly felt by
people. Although there is no upper limit to the Richter scale, the
largest earthquakes have magnitudes of ~8.9. The energy released by
an earthquake of this size is equal to the detonation of 1 billion tons of
TNT.
The Richter scale is not used to express damage like the Mercalli
scale.
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The Richter scale is quick-n-dirty
The Richter magnitude is
determined from the
maximum amplitude of
displacement measured on
seismogram at a known
distance from the
epicenter.
In this example, the
magnitude is 5.0 if the
max. amplitude is 23 mm
and s-p interval is 24
seconds (distance 215
km).
Earthquake wave amplitude decreases with distance from the epicenter.
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Earthquake magnitude describes the
size of an earthquake.
Earthquake Magnitude
Earthquake magnitude is calculated
on a logarithmic scale, so...
The difference in energy-release from
one magnitude to the next is very
large.
Magnitude 4 1x
Magnitude 5 32x
Magnitude 6 1000x
Magnitude 7 32,000x
Magnitude 8 1,000,000x
Relative amount of energy
released during earthquake:
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Moment-Magnitude Scale
Actually what is currently used.
Based on “seismic moment”
– Moment= total length of fault rupture
X (times) depth of fault rupture
X total amount of slip along rupture
X strength of rock
– Sources of measurements
Field observations
Location of foci of shock and aftershocks
Geologic cross sections
Earthquake Intensity
Earthquake intensity describes the
strength of ground shaking at a
location.
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The local intensity of an earthquake
depends on:
1. The magnitude of the earthquake
2. The distance from the epicenter
3. The type of ground
Earthquake Intensity
Earthquake Intensity and Magnitude
The intensity of an earthquake is based on the observed effects of the
earthquake — it is an assessment of the damage caused by an
earthquake at a specific location. Thus the intensity of an earthquake
depends upon the strength of the earthquake, but also on the distance
from the epicenter — it varies from place to place with respect to the
earthquake's epicenter.
The modified Mercalli intensity
scale is composed of 12 increasing
levels of intensity that range from
imperceptible shaking to
catastrophic destruction. It does not
have a mathematical basis but is
arbitrary and based on observed
effects.
Figure shows Mercalli intensity of Tamiskaming,
Quebec earthquake (M6.2) of 1935
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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.
Destructive Effects of
Earthquakes
Direct Hazards:
1. Ground shaking
2. Tsunamis
3. Fire
4. Landslides & Ground
Subsidence
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1. intensity
2. duration
3. nature of rocks and soil on
which a structure is located
4. structural design
Ground Shaking
Seismic vibrations damage structures. The
amount of structural damage is controlled by
Liquefaction is when soil turns to a mobile fluid, resulted in buildings
settling and collapsing under their own weight.
Buildings with foundations in bedrock are generally more earthquake
ready.
Much of the "flat lands" in the East Bay are susceptible to liquefaction.
Left photo from Marina District in S.F.
Above photo where bridge piers sink into
ground
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Tsunami
Tsunamis are giant water waves that usually result from the vertical
displacement of the seafloor during
an earthquake.
An earthquake can occur in one
area and the tsunami may inundate
another thousand of kilometers
away.
The tsunami that occurred as a result of
the M9.1 earthquake on Dec. 26, 2004
inundated coastlines around the Indian
Ocean resulting in ~275,000 fatalities.
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Fire
Although ground shaking can be
devastating, fires that occur after an
earthquake can be responsible for even
more deaths and damage.
The photos on the right show the fire
that occurred after the 1906 San
Francisco earthquake and burned down
500 blocks of the city.
The left photo shows the fire in
the Marina district after the
Loma Prieta earthquake (1989).
Landslides and Ground Subsidence
Earthquakes are a trigger for landslides - this is a particular hazard in
the S.F. Bay Area.
In addition, ground shaking and liquefaction can cause the ground to
subside - this is particularly damaging to buildings.
Landslide after 1999 Chi-Chi
earthquake in Taiwan blocking a
river.
Homes damaged in Alaska by ground
subsidence during an earthquake.