Geology 2 – Physical Geology Lab
Lab #14 – Earthquakes - KEY
Learning Outcomes:
 Determine Richter magnitude given an earthquake’s amplitude and distance data.
 Be able to sketch and label the P-, S-, and surface waves on a simplified seismogram.
 Understand the differing seismic hazards of different geologic rock units.
 Experience creating a seismic hazard map
 Recognize the topographic expressions and names of four major faults in the San
Francisco-Monterey Bay area using topographic maps
 Understand the meaning of and be able to use the following terms
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earthquake
seismic waves
fault
focus
Richter scale
magnitude
amplitude
epicenter
P-wave
S-wave
S-P lag time
San Gregorio Fault
San Andreas Fault
Hayward Fault
Calaveras Fault
seismic hazard map
Materials:
 Lab 13 worksheet, colored pencils, rulers, textbook
Part I – Determining Richter Magnitude of Earthquakes
1. Using the earthquake magnitude diagram (Fig. 1) on the next page, determine the Richter
magnitude for the earthquake data given in the table below.
Amplitude (mm)
Distance (km)
Richter Magnitude
1
100
3
10
100
4
100
100
5
10
5
2.3 – 2.4
10
50
3.6 - 3.7
10
500
5.6 – 5.7
2) Looking at the first three earthquakes in the table above, what is the effect on magnitude of a
ten-fold increase in maximum seismic-wave amplitude? Why does this occur?
A ten-fold increase in seismic wave amplitude increases Richter magnitude by 1. Therefore,
an increase in Richter magnitude represents a ten-fold increase in earthquake size.
3) Looking at the last three earthquakes in the table above, what is the effect on magnitude of a
ten-fold increase in the distance between the recording station and the epicenter? Why does this
occur? The last three data show that if amplitude is equal at different distances, then the
magnitude of the earthquake must vary. Same amplitude at increasing distances means larger
earthquakes. It takes a larger earthquake to feel the same amplitude at a farther distance
because amplitude dissipates with distance.
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Figure 1: Earthquake magnitude determined from seismic wave amplitude and
distance from epicenter.
This diagram shows how to determine the magnitude of an earthquake given the amplitude of the
maximum seismic wave and the distance from the seismometer to the epicenter. The distance to
the epicenter may be given directly in distance (km), or as the S-P lag time (seconds). The
magnitude is determined by drawing a line between the maximum wave amplitude (85 mm in the
example) as shown on the right-hand column and the epicentral distance (300 km, or 34 S-P lag
time seconds, in the example) on the left-hand column. The magnitude of the earthquake is
given by the intersection between this line and the magnitude column in the center of the
diagram. In this example, the magnitude of the earthquake is 6. (After Bolt, 1976.)
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4) Sketch a simple seismogram showing the P-waves, S-waves, and surface waves. Draw the
waves approximately to scale and label them.
See figure 11.10 on page 336 of the text.
5) Describe the type of motion each of these three wave types is transmitted through the earth.
P-wave: push-pull particles of matter in the direction of their path of travel;
compressional waves. Similar to sound waves in motion. P waves travel faster than Swaves..
S-Wave: shear waves because they push material at right angles to the direction of
travel. Shear waves do not exist in gases or liquids.
L-Wave: Also known as surface waves. Motion is a rolling, elliptical motion or a
shaking sideways with no vertical motion.
6) On the diagram on the following page, note that distance correlates with “S-P lag time” (the
difference in arrival time between the S-wave and the P-wave). Why does distance correlate with
the S-P lag time?
P-waves travel faster than S-waves, so P-waves always arrive at the seismometer before the Swave. With distance from the epicenter, the amount of time between P-wave and S-wave
arrival at the seismometer increases.
Part II – Major Faults in the San Francisco-Monterey Bay Area.
A map on a following page shows the major faults in the SF-Monterey Bay Area and a profile of
earthquakes that have occurred along the San Andreas Fault. You should know the location and
something about the San Gregario, the San Andreas, the Hayward, and the Calaveras Faults.
7) What is the orientation of the faults? Faults trend northwest-southeast
8) What is the motion along these faults? Right lateral strike-slip or transverse
9) Which fault is considered the boundary between the Pacific and North American plates?
Why? The San Andreas Fault is considered the boundary between the Pacific and North
American plates because substantially different sequences of rocks occur on either side
of the fault due to fault motion displacement. The rock units on the adjacent sides of the
other faults are not as offset, so they have not had as much displacement as the San
Andreas Fault.
10) One theory for the prediction of earthquakes holds that new events will fill in gaps where
earthquakes have not recently occurred. The gaps are presumed to be zones where stress is
accumulating and will be released in a future earthquake. In areas with frequent
earthquakes, less stress builds up. Using the map and graphs (Fig. 2) on the next page for
clues, does the location of the Loma Prieta earthquake and its aftershocks fit the pattern of
new events filling gaps? Explain. The data from the Loma Prieta quake fit the prediction
very well. The quake occurred where little quake activity had occurred for a long time.
The Loma Prieta data filled in the seismic gap previously very well. Overlay the Loma
Prieta data in the lower box on top of the previous quake data shown in the upper box.
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Figure 2: Distribution of earthquakes in the San Francisco-Monterey Bay area by distance
and depth of focus.
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Part III: Predicting Seismic Hazard
The map below (Fig. 3) shows a geologic map for the East Bay of San Francisco. Descriptions
of the geologic units are listed in the Table 1 below the map. Seismograms from the Loma Prieta
earthquake associated with the three rock units also are shown in Figure 3. During the 1989
Loma Prieta earthquake, the Cypress Structure, shown as a dashed line between A and B in
Figure 3, collapsed. The freeway that collapsed was an elevated structure for much of the
distance shown on this map. Closely examine the geologic map and seismograms.
11) Why did the freeway collapse exactly where it did? Why did it not collapse along other
portions of its length?
The Cypress structure collapsed where it did because that area was built on Qm, Holocene
mud and artificial fill, which is the weakest material to build upon and the most susceptible to
liquefaction and seismic shaking. See discussion in the text.
Kjf bedrock
Qm Bay Mud
Qal Alluvium
Figure 3: Geologic Map of East Bay of San Francisco, California with associated rock
descriptions and seismograms.
TABLE 1: Geologic Descriptions of Rock Units in East Bay of San Francisco
Geologic Unit
Rock Description
Qm
Holocene mud deposited in bays, lagoons, or tidal flats. Also
includes bay fill. Water saturated and unconsolidated soil.
Qal
Quaternary alluvium. Sediments of various origins including
beaches, sand dunes, rivers and landslides.
KJf
Franciscan rocks; bedrock observed on NE side of San Andreas
Fault system.
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Seismic Hazard Map for Downtown San Francisco.
With your experience from your MPC geology class, you’ve been asked to create a seismic
hazard map for downtown San Francisco, using only a geologic map for reference and your class
notes. Read all the instructions and look at the maps before you begin.
Step 1: First, color the geologic map (Fig. 4) using the color key given in Table 1 for the
following geologic units. (If you can’t find exact colors, use something close.)
TABLE 2: Geologic Descriptions of Rock Units in the San Francisco Bay Area
Geologic
Color
Rock Description
Unit
Qm
red
Qal
orange
Qts
KJf
yellow
green
R
blue
Holocene mud deposited in bays, lagoons, or tidal flats.
Also includes bay fill. Water saturated and unconsolidated
Quaternary alluvium. Sediments of various origins
including beaches, sand dunes, rivers and landslides.
Consolidated sedimentary rocks; up to 60 million years old.
Franciscan rocks; bedrock observed on NE side of San
Andreas Fault
Water bodies, including Lake Merced.
Shaking
Risk
(Step 2)
Highest
risk -1
2
3
Lowest
risk - 4
NA
12) Where does Qm crop out most of the time? Why is this so?
Qm crops out mostly in the low-lying areas near the bay because it is Holocene mud or bay
fill. Modern society has brought in the bay fill and Holocene mud was deposited near the
shores of present-day San Francisco Bay.
Step 2: Decide which one of the rock units will shake the most and which rock unit will shake
the least, everything else being equal. Mark your educated guesses for the 4 rock types in the last
column of Table 1 above.
Step 3: Draw lines on the map (Fig. 5) on the last page of the handout that separate regions of
differing intensities of shaking according to the Table 2. Use the vulnerability to shaking of each
rock unit that you determined in Step 2, and consider the distance from the San Andreas Fault, to
draw lines on your map that separate the regions of differing shaking. Notice that Figures 4 and
5 cover different areas of the San Francisco Bay area.
TABLE 2: Shaking Intensity Scale
Shaking
Intensity Scale
Very violent
X-XII
Strong
IV-IX
weak
I-III
The final product is a map that would be of interest to homeowners and potential homeowners in
San Francisco.
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Figure 4: Geologic Map of San Francisco Bay.
Your geologic map should look something like this one, which was drawn by Andy Hunter,
Fall ’99.
Figure 5: Base Map for San Francisco Bay Seismic Hazard Map.
This map shows the
intensities for the 1906
earthquake in San Francisco.
Your hazard map should look
somewhat similar to this one,
but you were asked to draw
only three levels of risk, not 5
as shown here. The area
nearest the San Andreas
Fault (southwest corner of
the map) should have the
highest amount of hazard.
The areas underlain by
Holocene mud (Qm) should
also have a high hazard. The
most stable areas are those in
the hills underlain by the
Franciscan bedrock.
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