EESA06 Final Exam Ch3 Summary + 40 MCQ/T or F

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19 Apr 2012
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PHYSICAL GEOLOGY AND THE ENVIRONMENT
(2ND CANADIAN EDITION)
Chapter 3: Earthquakes
Chapter Summary:
Earthquakes usually occur when rocks break and move along a fault to release strain that has
gradually built up in the rock. Volcanic activity can also cause earthquakes. Deep quakes may be
caused by mineral transformations.
Seismic waves move out from the earthquake's focus. Body waves (P waves and S waves) move
through Earth's interior, and surface waves (Love and Rayleigh waves) move on Earth's surface.
Seismographs record seismic waves on seismograms, which can be used to determine an
earthquake's strength, location, and depth of focus. Most earthquakes are shallow–focus quakes,
but some occur as deep as 670 kilometres below Earth's surface.
The time interval between first arrivals of P and S waves is used to determine the distance
between the seismograph and the epicentre. Three or more stations are needed to determine the
location of earthquakes.
Earthquake intensity is determined by assessing damage and is measured on the modified
Mercalli scale.
Earthquake magnitude, determined by the amplitude of seismic waves on a seismogram, is
measured on the Richter scale. Moment magnitudes, determined by field work, are widely used
today and often are larger than Richter magnitudes.
The most noticeable effects of earthquakes are ground motion and displacement (which destroy
buildings and thereby injure or kill people), fire, landslides, and tsunamis. Aftershocks can
continue to cause damage months after the main shock.
Earthquakes are generally distributed in belts. The circum– Pacific belt contains most of the
world's earthquakes. Earthquakes also occur on the Mediterranean–Himalayan belt, the crest of
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PHYSICAL GEOLOGY AND THE ENVIRONMENT
(2ND CANADIAN EDITION)
the mid–oceanic ridge, and in association with basaltic volcanoes.
Benioff zones of shallow–, intermediate–, and deep–focus earthquakes are associated with
andesitic volcanoes, oceanic trenches, and the edges of continents or island arcs.
The concept of plate tectonics explains most earthquakes as being caused by interactions
between two plates at their boundaries. Plate boundaries are generally defined by bands of
earthquakes.
Divergent plate boundaries are marked by a narrow zone of shallow earthquakes along normal
faults, usually in a rift valley. Transform boundaries are marked by shallow quakes caused by
strike–slip motion along one or more faults.
Convergent boundaries where continents collide are marked by a very broad zone of shallow
quakes. Convergent boundaries involving deep subduction are marked by Benioff zones of
quakes caused by tension, underthrusting, and compression.
The distribution of quakes indicates subduction angles of a down–going plate. The subduction
angle is probably controlled by plate density and rate of plate convergence.
Determining the probability of an earthquake occurring uses the measurement of rock properties
near faults, slip rate studies, and paleoseismology investigations to determine the recurrence
interval of quakes along individual faults.
Questions:
1. The elastic rebound theory: (Page 68)
A. explains folding of rocks
B. explains the behaviour of seismic waves
C. explains the origins of earthquakes
D. none of these
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PHYSICAL GEOLOGY AND THE ENVIRONMENT
(2ND CANADIAN EDITION)
2. The point within the Earth where seismic waves originate is: (Page 72)
A. the epicenter
B. the fault scarp
C. the origin
D. the focus
3. P-waves are: (Page 72)
A. Transverse surface waves
B. Compressional body waves
C. Tensional surface waves
D. Shearing body waves
4. The minimum number of seismic stations needed to locate an earthquake is: (Page 75-76)
A. 8
B. 2
C. 3
D. 1
5. The Richter Scale is used to determine: (Page 76-77)
A. Intensity of earthquakes
B. The magnitude of earthquakes
C. The damage from earthquakes
D. The number of casualties in an earthquake
6. Benioff Zones are associated with: (Page 88)
A. mid-ocean ridges
B. ancient mountain chains
C. subduction zones
D. all of these
7. Most earthquakes at divergent plate boundaries are: (Page 88-90)
A. shallow focus
B. intermediate focus
C. deep focus
D. all of these
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