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Earthquakes!
Earthquakes are the most destructive natural disaster in terms of loss of life and property
damage. They are especially dangerous to urban areas where buildings are high and closely
spaced, population density is high, flammable substances are abundant, and the disruption of
vital services like water can be devastating. There is no way to stop earthquakes and methods of
prediction are virtually non-existent. Cities that experience infrequent earthquakes are most at
risk. There are no earthquake building codes, and preexisting buildings, bridges, and roads are
allowed to deteriorate to such conditions that even minor shocks could cause major damage.
An earthquake involves the release of stored energy from the earth in the form of strong
vibrations or waves. The sources of most earthquakes are active faults. In these active faults,
one block of crust grinds over or past another. This movement is not smooth but rather jerky.
Each jerk is an earthquake. Friction along the entire fault plane and asperities (roughness on the
surface) lock the fault at a single point and prevent movement. The stress on the fault builds up
continuously as the crustal blocks try to move until frictional forces are exceeded. The blocks
finally slip and an earthquake results. The amount of stress released by this movement of the
fault is the amount of energy in the earthquake, in other words, its magnitude.
Earthquake Waves
An Earthquake produces three types of waves: compressional (P) waves, shear (S)
waves, and Surface waves (Rayleigh and Love). All waves are produced at the same time by the
slip on the fault. The point of slip is the focus and the point on the land surface above it is the
epicenter. The waves travel in all directions in spherical fronts away from the focus. Picture the
way waves travel when a rock is thrown into a lake to understand seismic waves. The energy of
the earthquake is contained within those fronts. When the waves are close to the focus, there is a
high amount of energy per unit area of the wave front. Because this spherical front grows larger
with time as the waves travel away from the focus, the energy per unit area of the wave grows
smaller with time. Therefore the power or energy of an earthquake drops off quickly away from
the epicenter as does the damage caused by it.
Fig. 1) P- and S-wave motions.
Fig. 2) Surface wave motions.
The P-wave is the fastest wave and moves with a push-pull motion in the direction of
propagation. The S-wave is the second fastest and vibrates side-to-side as it moves forward.
The surface waves are much slower than the others and begin at the epicenter. The Raleigh wave
moves like an ocean wave showing a retrograde motion of the earth’s surface. Love waves move
like S-waves but on the surface. When a person “feels” an earthquake, they feel the P-wave first,
S-wave second, and finally the surface waves. The closer a person is to the epicenter, the closer
together they feel each of the waves.
The speed of an earthquake wave depends on the material through which it travels.
Waves travel fast in crystalline material but slow in unconsolidated sediments. Waves are also
weakened more by sediments than crystalline rocks. Therefore seismic waves are felt and
damage occurs at greater distances if they travel through crystalline rocks rather than through
unconsolidated sediments. On the other hand, surface waves are more damaging to structures in
unconsolidated sediments because the structures cannot be fixed to the earth’s surface as well
and the waves can readily deform the sediments as they pass through. Another problem in these
areas is liquefaction in which ground water is shaken so hard that it quickly moves to the
surface. The surface turns to mud or even quicksand. If the surface waves originate in the
ocean, they will travel as waves in the ocean. The waves are tsunamis or seismic sea waves
(erroneously called tidal waves) which can be up to 18 meters high and do considerable costal
damage.
Earthquake “Arrivals”
The arrival time of each of the earthquake waves (P, S, and surface) at a given location is
recorded on an instrument called a seismograph. The seismograph is a turning drum that is
isolated from the earth and a recording pen that is connected to the earth. When the earthquake
waves pass, the pen is moved by the waves and it marks the recording paper on the drum. The
first wave to arrive is the P wave followed by S and finally surface waves. The time delay
between the arrivals of each wave can be used to calculate the distance to the earthquake focus.
The earthquake focus is located using three seismographs (triangulation).
Fig. 3) Earthquake Seismograph.
Earthquake Magnitudes
The magnitude or power of an earthquake is determined by how far the pen on a
seismograph is moved by the earthquake. This movement of the pen reflects the amplitude of
the wave. Magnitude (M) of the earthquake using the Richter Scale is determined by the
equation:
M = log10(a/T) + c
Where: a = amplitude, T = the period of the wave (also from the seismograph), and
c = an attenuation factor based on the type of material that the wave has passed through.
Because the Richter Scale is a logarithmic function, each magnitude is ten times stronger
than the next down. An earthquake of magnitude 5 is ten times stronger than a 4 and one
hundred times stronger than a 3. Other scales include the Mercalli and Modified Mercalli scales,
which are based on the amount of damage done to structures.
The real control on the damage done by earthquakes, however, is the construction of the
buildings in the area. In 1989, the magnitude 6.9 Loma Prieta Earthquake killed 72 people in the
well-prepared San Francisco area. The next year, the magnitude 6.7 Armenia earthquake killed
40,000 people in a poorly prepared are. Weathered and crumbling buildings, bridges, and other
public structures fall apart in earthquakes. If private homes are not made to withstand
earthquakes, they won’t. Glass in buildings and statuary becomes deadly projectiles as they are
broken off of tall buildings. Rupturing of gas, oil, and electric transmission lines cause fires that
claim more lives than the earthquake itself.
Laboratory Procedure
Slope Stability1) With the empty plastic tray in place, measure the angle of repose for the provided
materials (sand, rounded and angular gravel) one at a time. After recording the initial
angle of repose, initiate a small earthquake, than re-measure the angle of repose.
Record your results below.
Angle Before EQ
Angle After EQ
a. Sand
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b. Round Gravel
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c. Angular Gravel
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2) After an earthquake, what happened to the angle of repose?
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3) What implications might your results have for earthquake prone areas?
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Liquefaction4) Place the model building in the large tray of moistened, unconsolidated sediment.
Initiate a moderate earthquake and observe what happens. Record your observations
below.
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5) Do you think the same thing would occur if the sediment wasn’t saturated (dry)?
Why?
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6) What if the sediments were consolidated (not loose)? Why?
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Earthquake Simulation7) Construct a building on the earthquake platform using the wooden blocks.
What happens to your building once an earthquake is generated?
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8) How might you prevent this from happening?
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9) With the large stabilizer in place, construct the same building as before.
What happens to the building once an earthquake is generated?
(Note: You may have to hold the stabilizer steady as the movement begins)
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10) In what instances might this type of motion stabilizer NOT work? Why?
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11) Place a large amount of plastic balls on the earthquake platform, than place one of the
plastic lids upside down on top of the balls, such that the lid can move freely.
Construct the same building as before on top of the plastic lid, and generate an
earthquake. What happened?
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12) Which stabilization mechanism do you think would be most effective in preventing
earthquake damage? Why?
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13) Observe the seismograph on the computer screen which is connected to the
earthquake simulator. What happens to the wave amplitude (height) and wave length
(distance between waves) as you generate larger earthquakes? What does this mean?
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