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EARTHQUAKES
Earthquakes occur along faults, planes of weakness in the crustal rocks.
Although earthquakes can occur anywhere, they are most likely along crustal
plate boundaries, such as the area called the “Ring of Fire” around the
Pacific Rim. At plate boundaries the pressures can be compressional,
tensional, or slip-strike faults where plates move past each other.
Compressional
Tensional
Slip-strike
For instance, the recent earthquake in the Los Angeles area was along a
compressional thrust fault deep in the earth, a “side effect” of the main San
Andreas slip-strike fault movements. The San Andreas Fault marks
movement of the North American plate against the Pacific plate, which
southern California and Baja California are attached to. These areas are
essentially on their way to Alaska, while the rest of the U.S. is moving
westerly.
Earthquakes start at a focus, a point in the earth where the break starts. We
calculate this point reflected on the earth’s surface as the epicenter, by
calculating the distance to the earthquake epicenter from three seismographs,
the epicenter being where the distance circles cross each other.
Earthquake waves (seismic waves) come as surface or body waves, that is
they travel on the earth’s surface or through the earth. Primary (P) waves, a
body wave, arrive first, followed by the Secondary (S) wave, and then the
surface waves on a seismograph.
Damage from earthquake waves is caused by the effect at your location of
the combination of the waves and the time the earthquake lasts. The longer
the quake, the more damage. The closer to the epicenter usually implies
more damage. The immediate damage is from shaking side to side, although
tall buildings will have a vertical acceleration component also. The safest
building is a single story frame house that does not have a tile roof.
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Other dangers from earthquakes are fires, ground failure, and the resultant
landslides and tsunamis near the ocean. In fact, fire and the inability to put
them out often causes the most loss of property from and earthquake.
Earthquake power is measured by the Richter scale, which measures the size
of the wave on a seismograph. On this scale, each whole number means a
10X greater amplitude and 30X greater release of energy. This is why a 5.0
is a minor shaking, and a 7.0 can cause major damage. (The 7.0 has a 100X
greater amplitude and a 900X greater energy release than the 5.0.)
We will now do a lab exercise to locate the epicenter of an earthquake and
time of occurrence by working with seismographic records and a time travel
graph.
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EARTHQUAKE RECORDS
The earth is continually undergoing change due to stresses that exist within.
At least a million times each year the earth suddenly fractures and the
seismic waves of an earthquake radiate in all directions from the focus.
Seismograph stations located throughout the world record these waves on
seismograms. In this exercise we will examine how seismograms are used
to determine the time of occurrence and location of an earthquake.
Examining Seismograms
The three basic types of waves produced by earthquakes are: P waves, S
waves, and Surface waves. P waves travel most rapidly, therefore, they
reach the seismograph station first. Examine Diagram 1, which is a
“typical” seismogram. The dots on this seismogram mark time intervals of
one minute.
Answer the following two questions by referring to Diagram 1.
1. How many minutes elapsed between the arrival of the first p-wave and
the arrival of the first S-wave?
_________________________
2. How many minutes elapsed between the arrival of he first S-wave and the
first surface wave?
__________________________
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Locating An Earthquake
The difference in the velocities of the P and S waves provides a method for
determining the distance to the location of the earthquake or epicenter. The
principle used is analogous to a race between two autos, one faster than the
other. The greater the distance of the race, the greater will be the difference
in the arrival times at the finish line.
1. From the seismogram in Diagram 1, you should have determined that the
arrival times of the first P and S waves differed by 4 minutes. Using the
travel-time graph, determine the distance to the epicenter in this instance.
This is done by finding the place on the graph where the vertical
separation between the P and S lines is equal to 4 minutes (one square on
the graph is one minute). Draw a vertical line at this location
extending to the bottom of the graph. Label the line “Bloomington.”
The distance to the earthquake in degrees can be read directly below your
line.
Answer = _____________ degrees (Have your answer checked before
you continue.)
2. Using your answer from number 1 above, calculate how many kilometers
to the earthquake epicenter.
(One degree = 111 km or 69 Miles)
___________________km
_______________________miles
3. The time travel graph can also be used to determine the time required for
seismic waves to reach a seismograph station. Note on the time-travel
graph that time is given on the vertical axis. Thus, for the example you
did above, the first P-wave required 5 minutes and 15 seconds to reach
the seismograph. This is determined by finding the place where the
Bloomington line crossed the P-wave curve, and reading this value from
the vertical axis.
How many minutes were required for the first S-wave to reach the
seismic station?
__________________________ minutes
4. In question #2 you determined the distance from an earthquake to a
seismograph station. However, all we know is that the earthquake
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occurred somewhere along a circle, a given number of kilometers away.
Records from three different seismograph stations are needed to more
accurately locate an earthquake. In the following exercises you will
interpret three seismograms in order to determine the location of an
earthquake.
a. On Diagram 3, label the arrival of the first P-wave and the first Swave on each seismogram.
b. In each case, how much time elapsed between the arrival of the first
P-wave and the first S-wave? (Determine to the nearest ½ minute.)
Nagpur
_______________ minutes
Darwin
_______________ minutes
Paris
_______________ minutes
c. Using the time-travel graph determine the distance to the epicenter
from each seismograph station. Draw a line on the graph representing
each situation and label each line with the city’s name.
Nagpur
DISTANCE
_________ degrees _________km __________miles
Darwin
_________ degrees _________km __________miles
Paris
_________ degrees _________km __________miles
d. Using the world map provided, determine the location of the
earthquake epicenter. Do this by drawing three circles (one from each
seismic station) using the distances from part “c.” Use the scale on
the side of the map and a drawing compass to make your circles.
In what country was the earthquake located? ___________________
What is the approximate latitude and longitude of the earthquake
epicenter?
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5. Next we will determine the time of origin of this earthquake. Each
seismogram has a time designated on it. (9:43 GMT means 9 hours, 43
minutes, Greenwich Mean Time.)
a. For Nagpur, India, note the time of the arrival of the first P-wave.
_________________________
b. Using the time-travel graph, determine the time required for the
first P-wave to travel from the epicenter to the seismograph.
_________________________
c. Determine the time of the occurrence of the earthquake by taking
the arrival time of the first P-wave minus the time required for the
wave to reach the recording instrument.
_________________________
6. Using the data for Darwin, determine the time of origin of the
earthquake.
a. For Darwin, note the time of arrival of the first P-wave.
_________________________
b. Using the time-travel graph, determine the time required for the
first P-wave to travel from the epicenter to the seismograph.
_________________________
c. Determine the time of the occurrence of the earthquake by taking
the arrival time of the first P-wave minus the time required fort he
wave to reach the recording instrument.
_________________________
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d. Does the time of the occurrence of the earthquake determined for
Darwin agree with that determined for Nagpur?
YES
NO
(circle one)
NO
(circle one)
Should they agree?
YES
EXTRA CREDIT
Determine the velocity of the wave from Darwin in feet per second and
meters per second. We already have a value in miles per minute derived
from how far the wave traveled and how long it took.
Velocity of wave ______________________________________m/sec
Velocity of wave ______________________________________ft/sec
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