Lecture 9 Earthquakes

Homework Questions for Lecture 9
ES 1000
Use these questions to test your knowledge of Lecture 9.
A. Short answer:
1. Seismic velocities across the _________ - mantle boundary increase
dramatically. The difference is referred to as the Mohorovicic
2. Deeper than about 700 kilometers, higher temperatures and pressures
cause stressed rocks to deform ______________, rather than rupture
or shift.
3. Rocks under stress may crack, and expand in volume. This expansion
is called ____________, and the process causes many effects that may
be used to predict earthquakes.
4. S-waves can pass through _________ media only.
5. P-waves that pass through the center of the Earth arrive ________
than would be expected if the core were all liquid.
6. Fault segments that have not quaked recently may be locked in place
by friction. These seismic _________ are thought to be candidates for
major earthquakes.
B. Match the Terms.
1. Tsunami _____
a. Velocity discontinuity
2. Time delay P to S______
b. 80% of Earth’s seismic energy
3. Mohorovicic ____
c. Himalayas
4. Richter Scale ____
d. caused by underwater earthquake
5. Divergent Zone____
e. much slower than P waves
6. Benioff _____
f. distance to epicenter
7. Liquefaction _____
g. subduction zone foci
8. Pacific Rim Region___
h. amplitude reveals energy
9. Collision Zone earthquakes ____ i. foci <20 km depth
10. Shear (S) waves ____
j. makes “quick clay”
C. True or False? Circle the correct answer.
1. P waves travel faster through the liquid outer core compared to the
solid inner core. True or False?
2. Within a solid rock layer, rigidity increases with increasing depth.
True or False?
3. S (shear) waves travel readily through the liquid outer core. True or
4. P and S waves travel faster in the upper mantle than in the crust. The
change in velocity is called the Mohorovicic Discontinuity. True or
5. Benioff first observed that many coastal earthquake foci are
progressively deeper the farther inland they occur. True or False?
6. The further away an earthquake is, the greater the delay between
primary (P) and secondary (S) waves. True or False?
7. P waves result in alternating contraction and expansion of rocks in
the direction of propagation. True or False?
8. Most earthquakes are caused by the release of accumulated strain
energy in rocks undergoing plastic deformation. True or False?
9. An earthquake’s epicenter is in the exact center of the fault that
causes the earthquake. True or False?
10.If an earthquake occurs, the first waves to arrive at a seismograph are
S waves. True or False?
D. Multiple choice:
1) Earthquake waves that alternately cause rocks to compress and expand
along the line of travel are:
a) P waves.
b) S waves.
c) side-to-side surface waves.
d) rolling surface waves.
2) To locate the epicenter of an earthquake, a seismologist must determine
all of the following EXCEPT:
a) the velocity of both P waves and S waves.
b) the type of fault from which it originated.
c) the difference in travel times between P waves and S waves.
d) the distance from the epicenter to at least three different seismological
3) The lag between P- and S-wave arrival times can be converted to:
a) the distance between the earthquake epicenter and the seismograph
b) the intensity of the earthquake.
c) the magnitude of the earthquake.
4) Liquefaction during an earthquake primarily affects:
a) fine-grained clays and silts.
b) foliated metamorphic rocks.
c) thick sandstone rock layers.
5) All of the following are true of tsunami EXCEPT:
a) they can travel at speeds greater than 500 miles per hour.
b) they are caused by undersea earthquakes.
c) they are often responsible for the destruction of ships at sea.
d) they can often form waves more than 100 feet high.
6) When Benioff-Wadati zones are plotted on a map, they show:
a) elastic modulus values in rocks within a subduction zone.
b) semi-concentric zones of varying degrees of earthquake damage.
c) earthquake numbers decreasing away from areas of oceanic rifting.
d) earthquake foci becoming progressively deeper inland from ocean
7) Few earthquakes are recorded at depths below 450 miles (720 km)
a) they are too deep within the Earth to be detected by seismographs.
b) lithostatic pressure at these depths prevents rocks from shifting
relative to each other.
c) slabs that descend that far have been heated sufficiently to deform
8) Japan is the site of about 15% of the Earth’s seismic energy release
because it is located:
a) where the Eurasian plate descends beneath the Pacific plate.
b) where the Pacific plate descends beneath the Eurasian plate.
c) at the transform boundary between the Pacific and Eurasian plates.
E. Short answers
Discuss the three most damaging effects of earthquakes: Fires, tsunamis, and
liquefaction. Be sure to include:
a. What types of seismic waves cause the most damage to buildings and
other structures?
b. How are these waves related to body waves (P and S waves)?
c. Why are fires an especially great problem after an earthquake? Include the
role of gas mains, broken water mains, street rubble, etc.
d. How are tsunamis generated?
e. What causes liquefaction? Include the role of water pressure changes due
to seismic P-waves.
f. Free fact: dredge spoils are similar to pick-up-sticks. Question: why is this
an unstable configuration?
g. Should you build a house on a dredge landfill near a major fault?
Name _________________________
1. Last time you calculated the expected values for the number of trees in
five classes of soil moisture. We are ready to begin calculating Chi-square
values, a test statistic that compares your observations to the values
For eight of the ten data cells below I calculated:
You calculate the last two, and fill in the two boxes.
Tomorrow we will total the individual (O-E)2/E values in each cell to get our
chi-square statistic. Then we will count up the degrees of freedom, compare
it to values appropriate for a normal distribution, and see if our null
hypothesis is believable.