Exercise 7
QUESTIONS 7, PART A1
1. a. Describe the geographic orientation (compass direction) of the faults shown in Figure 7.1.
b. What is the large-scale geological process that is occurring in this part of California that
accounts for fault orientation and motion?
2. One theory for the prediction of earthquakes suggests that new events will fill in gaps where
earthquakes have not been recently recorded. The gaps are presumed to be zones where stress is
accumulating, which will be released in a future earthquake. In areas along faults with frequent
earthquakes, stress is released in each event.
a. Describe the differences in locations of epicenters for earthquakes that occurred before the
Loma Prieta earthquake (Figure 7.2a) and the main shock and aftershocks (Figure 7.2b and
shaded zone in Figure 7.1).
b. Do the Loma Prieta earthquake and its aftershocks fit the pattern of new events filling gaps?
3. Figure 7.3 shows a map and an insert with a cross-section view along line B-B’; of the Loma
Prieta earthquake and its aftershocks.
a. Looking at the map view, would you infer that all of the earthquakes occurred along the San
Andreas Fault?
No; it seems to be a more diffuse zone.
b. Compare the map view with the cross-section view. What would you infer from the cross-
section view about the number of faults?
That there are at least two major faults involved. Most of the events occur along the San
c. What is the evidence for your inferences?
The earthquakes occur in two clusters in the cross section view. Most earthquakes form a zone
QUESTIONS 7, PART A2
1. Describe the general relationship between the recorded acceleration at different sites and the
epicentral area, which is indicated by shading.
2. a. Draw a contour line around the sites with 20% g in Figure 7.4. 20% g is a value for
moderate damage.
b. What is the general shape of the contour?
c. How does its shape compare with the orientation of the faults that are shown in Figure 7.1?
3. What geological factors contribute to the difficulties in drawing the irregular 20% g isoseismal
line?
Students should refer to material on page 102 and Figure 7.12 on page 112 for context. The
4. From the data presented in Figure 7.4, summarize the role of distance from the epicenter in
attenuating earthquake waves.
rather large differences between adjacent sites.
5. Compare the data shown in Figure 7.5, which is an isoseismal map showing areas of similar
damage to structures as measured by the Modified Mercalli Intensity scale, with the acceleration
Greater acceleration causes more intense impacts.
QUESTIONS 7, PART B 1
Read and compare the maps in Figures 7.6, 7.7, and 7.8 to answer the following questions.
1. a. What two geologic units in San Francisco and Oakland have the maximum predicted
intensities for earthquakes?
b. What are the geologic conditions that have lead these areas to have high predicted maximum
intensities?
2. Compare Figure 7.6 with Figure 7.8.
a. What geologic conditions (materials and setting) led to the maximum intensities during the
1906 earthquake?
b. What geologic conditions led to the minimum intensities during the 1906 earthquake?
3. a. Using data from all three maps, determine typical geologic conditions that are likely to
create areas with maximum seismic impacts. Explain these conditions below.
b. Using data from all three maps, determine typical geologic conditions that are likely to create
areas with minimum seismic impacts. Explain these conditions below.
QUESTIONS 7, PART B2
1. What are the ages of fills in the Marina District (Figure 7.9)? Was any fill emplaced in the last
50 years? How well engineered for seismic response do you think the fill is? Explain your answer.
2. a. What is the relationship between the age and kinds of fills?
b. How do you interpret the term hydraulic?
c. What is the coastal natural landform that existed at “A” (Figure 7.9) prior to 1912?
A beach and dunes (these materials are adjacent to the 1912–1917 fill, and may be interpreted to
3. Sand boils are sites of liquefaction (where the ground changes from behaving as a solid to
behaving as a fluid, like quicksand, when shaken). Use Figure 7.11 and explain the relationship of
sand boils to areas of fill. Why does this relationship exist?
4. Figure 7.12 presents three seismic responses. The larger squiggles on the figure represent
greater velocities when the sites were shaken. What is the relationship of velocity to types of
geologic deposit? If you wanted to build near the Marina district, yet be relatively safe from
earthquake damage (assuming that you were building an appropriate, seismic-resistant structure),
where would be a safer place to build? Mark the site on Figure 7.9 and explain your choice.
Sand deposits and fill have far more shaking than bedrock. The optimum building sites would be
———