Name: ____________________________________
Geology 2 – Physical Geology Lab
Lab #14 – Earthquakes
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)
1
10
100
10
10
10
Distance (km)
100
100
100
5
50
500
Richter Magnitude
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?
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?
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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.
5) Describe the type of motion each of these three wave types is transmitted through the earth.
P-wave:
S-Wave”
L-Wave:
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?
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?
8)
What is the motion along these faults?
9)
Which fault is considered the boundary between the Pacific and North American plates?
Why?
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.
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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?
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)
NA
12) Where does Qm crop out most of the time? Why is this so?
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.
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Figure 5: Base Map for San Francisco Bay Seismic Hazard Map.
Part IV: Geomorphology of the San Andreas Fault
On the map provided by the instructor, identify the San Andreas Fault considering the
morphology and topography of the region. Show your instructor evidence of faults in the area
and identify as many portions of the San Andreas Fault as you can. Try to find
 San Gregorio fault
 Hayward fault
 Calaveras fault.
Instructor initials____________________________
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