104_Lab_9_Earthquakes_Response_Form
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Lake Michigan College
Physical Geology 104/290
Name: ____________________
Lab Exercise 9 - Earthquakes
Background Information: The Loma Prieta Earthquake
As millions of Americans were tuned in to the beginning of the third game of the World Series
(championship baseball) at Candlestick Park, San Francisco, California, on October 17, 1989, a large
earthquake occurred. It was the largest to strike the San Francisco Bay region, home to almost 6 million
people, since the great earthquake of 1906, when fewer than a million people lived there. The
earthquake was felt over 400,000 square miles and resulted in 67 deaths; 12,000 people were left
homeless and it caused $6 billion in property damage. In terms of cost, it was one of the most expensive
natural disasters ever in the United States, and it was the most costly earthquake since 1906. The following problems illustrate how the epicenter was located, how the magnitude and intensities were
determined, how the intensity was influenced by the nature of the surficial material, and how energy from
the earthquake could be used to view the interior of the Earth. In each case, the scientific method is used
to answer the basic problems. Problems:
1. Geoscientists and engineers commonly simulate earthquakes in the laboratory to observe their effects
on models of constructions sites, buildings, bridges, and so on. Soil in broad terms for foundation
engineering is the ground supporting a structure. Soil is considered to be any loose sedimentary deposit,
such as gravel, sand, silt, clay or a mixture of these materials. Coarser grained, well-sorted materials are
more permeable to water and will shift when dry or saturated. Damp sand grains will stick together (as
when building sandcastles), but when subjected to an earthquake will be subject to liquefaction.
Watch the two videos below:
Example of Liquefaction (
https://www.youtube.com/watch?v=afBqD8Hm2Ak
)
Liquefaction animation (
https://www.youtube.com/watch?v=536xSZ_XkSs
)
a.
What happens to buildings on bedrock (equivalent to poorly-sorted, consolidated soils) during an
earthquake?
b.
What happens to buildings on poorly consolidated soils during an earthquake?
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Physical Geology 104/290
c.
What happens to buildings on saturated soils sediment during an earthquake?
d.
Which kind of Earth material is more hazardous to build on in earthquake prone regions: compacted
sediment or uncompacted sediment? Explain your reasoning.
e.
Consider the moist, compacted beach sand in the first video. Do you think this material would be a
choice to build on? Explain your reasoning.
f.
Write a brief statement that summarizes how water in a sandy substrate beneath a home can be
beneficial or hazardous. Justify your reasoning by referencing the videos you viewed.
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Physical Geology 104/290
2. Look at the figure of the three simulated seismograms from the October 17, 1989, Loma Prieta
Earthquake (Fig. 9- next page). Each was recorded at a different seismic station. Show your work for full
credit. a. Specifically, where
was the epicenter?
(1) Find the differences in arrival time between P and S using Fig. 1. Next determine the distance to the epicenter from the three stations using the travel-time curves in Fig. 2. Distance from epicenter of: Station A: ______________
Station B: ______________
Station C: ______________
(2) Plot the locations of the stations on Fig. 3. See the caption for Fig. 3 for station locations. (3) Locate the epicenter of the earthquake on Fig. 3 from intersecting circles centered at the three
stations. Note that your epicenter and another student’s may not be the same. Why?
b. When did the earthquake occur?
(1) Use the travel time curves and the time values indicated on the seismograms to determine the
time of occurrence.
(2) The time you determined is Universal Coordinate Time. What time would it have been in the Pacific Daylight time zone
?
c. How big was the earthquake? Determine the Richter magnitude
from the seismograms using the method shown in Fig. 4 below.
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Figure 1. Three seismographs for Problem 2. Vertical tick marks on the seismographs are one minute apart. The
Universal Coordinate Times given below the lower seismograph also apply to the upper to seismograms (e.g., the P
wave arrived at Station A between 00:05 and 00:06). Because Universal Coordinate Time is based on a 24-hour day,
00:05 is the same as 12:05 am. Determine the differences in arrival times between the P and S waves at the recording
stations. We will use these values in Fig. 2 to obtain the distances to the epicenter.
Station A: ___________
Station B: ___________
Station C: ___________
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Figure 2. Travel time curves for P and S waves. The enlargement of Fig. 4 has the same time scale as the seismograms
in Fig. 1 and is to be used for Problem 2. Record the distances from the epicenter to the recording stations.
Station A ________
Station B _________
Station C _________
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Figure 3. Map for Problem 1. Plot stations A (lat 40º08’N, long 121º30’W), B (lat 34º08’N, long 118º00’W), and C (lat
41º00’N, long 114º30’W). Then locate the epicenter using the distances from the epicenter obtained from Fig. 2.
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Figure 4. Scale for determining Richter magnitude of an earthquake.
3. Questionnaires designed for assigning Modified Mercalli scale intensities to the October 17
earthquake were returned from the locations shown on Fig. 5. The responses were divided into five
groups (see Table below). Using these responses, assign Modified Mercalli Scale intensities to the five
groups, using Table 1 (on the last page of this lab) as a guide. Next, draw isoseismals on the map of San
Francisco, based on intensity distribution. How does the area of highest intensity compare with the
epicenter you located in Problem 2?
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Earthquake Intensity Survey
Questionnaire Category Descriptions
Modified Mercalli
Scale
Equivalents
A.
Felt by all, and people were extremely frightened. Drivers had great difficulty controlling
vehicles. Some buildings collapsed, others very seriously damaged. Concrete bridge collapsed. Smell of gas in some areas. Ground cracked. B.
Felt by all, most people were frightened. Drivers had some difficulty steering. Some reasonably well-built buildings partially collapsed, poorly built buildings badly damaged.
Some water towers damaged. Large trees had tops broken. Some reports of furniture and people being thrown in air. Cracks in ground. C.
Felt by all, some people knocked down. Noticed by drivers. Damage to buildings moderate, expect for those of poor construction. Windows broken, thinks knocked off shelves, plaster walls damaged. Trees and bushes shook. Caving noticed on some steep hillsides and along stream banks. D.
Felt by all, some had trouble walking. Not noticed by most drivers. Damage to structures slight, but windows broken, things knocked off shelves, pictures knocked off walls. Plaster walls cracked. E.
Felt by most people. Water in swimming pools disturbed. Some reported broken glassware. Pendulum clocks stopped. 8
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Figure 5. Map of San Francisco Bay area showing locations for which intensities have been assigned. Letters A through
E correspond to earthquake intensities described in the above table. For use with Problem 3.
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Table 1. Modified Mercalli Scale.
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