Practice Questions for Lecture 10
Geology 1200
Use these questions to test your knowledge of Lecture10.
A. Short answer:
1. Seismic velocities across the _________ - mantle boundary increase
dramatically. The difference is referred to as the Mohorovicic
Discontinuity.
2. Deeper than about 700 kilometers, higher temperatures and pressures
cause stressed rocks to deform ______________, rather than rupture
or shift elastically. Thus there are no deeper earthquakes.
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. P-wave velocities in the upper mantle are ________ than in the crust.
7. P-wave velocities in the solid inner core are __________ than in the
liquid outer core.
8. Fault segments that have not quaked recently may be locked in place
by friction. These seismic _________ are thought to be candidates for
major earthquakes.
9. Mid-ocean ridge earthquakes are mostly less than ___ kilometers
down.
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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. In the lithosphere, where rigidity increases with depth, seismic
waves curve back toward the surface. True or False?
2. P waves travel faster through the liquid outer core compared to
the solid inner core. True or False?
3. Within a solid rock layer, rigidity increases with increasing
depth. True or False?
4. S (shear) waves travel readily through the liquid outer core.
True or False?
5. 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 False?
6. Benioff first observed that many coastal earthquake foci are
progressively deeper the farther inland they occur . True or
False?
7. The further away an earthquake is, the greater the delay
between primary (P) and secondary (S) waves. True or False?
8. P waves result in alternating contraction and expansion of rocks
in the direction of propagation. True or False?
9. Most earthquakes are caused by the release of accumulated
strain energy in rocks undergoing plastic deformation. True or
False?
10.An earthquake’s epicenter is in the exact center of the fault that
causes the earthquake. True or False?
11.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. Which of the following is NOT true of earthquake waves?
(a) P waves are body waves and act by compressing and expanding rocks.
(b) S waves are body waves and act on rocks in a shearing motion.
(c) Surface waves arrive at a seismograph first because they are not
impeded by rock.
(d) The combining of both types of surface waves is responsible for most
earthquake damage to rigid structures.
3. 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
stations.
4. The lag between P- and S-wave arrival times can be converted to:
(a) the distance between the earthquake epicenter and the seismograph
station.
(b) the intensity of the earthquake.
(c) the magnitude of the earthquake.
5. Liquefaction during an earthquake primarily affects:
(a) fine-grained clays and silts.
(b) foliated metamorphic rocks.
(c)thick sandstone layers.
6. 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.
7. When Benioff-Wadati zones are plotted on a map, they show:
(a) elastic modulus values in rocks within a subduction zone.
(b) semiconcentric 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
trenches.
8. Few earthquakes are recorded at depths below 450 miles (720 km)
because:
(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
plastically.
9. Major earthquakes are likely in all of the following tectonic settings
EXCEPT:
(a) divergent boundaries.
(b) transform boundaries.
(c) subduction zones.
(d) continental collision boundaries.
10. 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
1. Describe P and S seismic waves. What are their motions? How do
they differ in speed through solid rock, and through liquid?
2. Explain why earthquakes cause damaging fires, tsunamis and
liquefaction.